* * * * * +-----------------------------------------------------------+ | Transcriber's Note: | | | | Inconsistent hyphenation in the original document has | | been preserved. | | | | In the equations, superscripted characters are marked | | with ^'s, and subscripted characters are marked with | | _ (underscore) in front. | | | | Obvious typographical errors have been corrected. For | | a complete list, please see the end of this document. | | | +-----------------------------------------------------------+ * * * * * A STUDY OFRECENT EARTHQUAKES. BYCHARLES DAVISON, Sc. D. , F. G. S. AUTHOR OF"THE HEREFORD EARTHQUAKE OF DECEMBER 17TH, 1896. " WITH 80 ILLUSTRATIONS London and Newcastle-on-Tyne:THE WALTER SCOTT PUBLISHING CO. , LTD. 1905 PREFACE. The present volume differs from a text-book of seismology in givingbrief, though detailed, accounts of individual earthquakes rather thana discussion of the phenomena and distribution of earthquakes ingeneral. At the close of his _Les Tremblements de Terre_, ProfessorFouqué has devoted a few chapters to some of the principal earthquakesbetween 1854 and 1887; and there are also the well-known chapters inLyell's _Principles of Geology_ dealing with earthquakes of a stillearlier date. With these exceptions, there is no other work coveringthe same ground; and he who wishes to study any particular earthquakecan only do so by reading long reports or series of papers writtenperhaps in several different languages. The object of this volume isto save him this trouble, and to present to him the facts that seemmost worthy of his attention. The chapter on the Japanese earthquake is reprinted, with a few slightadditions, from a paper published in the _Geographical Journal_, and Iam indebted to the editor, not only for the necessary permission, butalso for his courtesy in furnishing me with _clichés_ of the blockswhich illustrated the original paper. The editor of _Knowledge_ hasalso allowed me to use a paper which appeared four years ago as thefoundation of the ninth chapter in this book. CHARLES DAVISON. BIRMINGHAM, _January, 1905. _ CONTENTS. CHAPTER I. PAGE INTRODUCTION 1 CHAPTER II. THE NEAPOLITAN EARTHQUAKE OF DECEMBER 16TH, 1857 7 CHAPTER III. THE ISCHIAN EARTHQUAKES OF MARCH 4TH, 1881, AND JULY28TH, 1883 45 CHAPTER IV. THE ANDALUSIAN EARTHQUAKE OF DECEMBER 25TH, 1884 75 CHAPTER V. THE CHARLESTON EARTHQUAKE OF AUGUST 31ST, 1886 102 CHAPTER VI. THE RIVIERA EARTHQUAKE OF FEBRUARY 23RD, 1887 138 CHAPTER VII. THE JAPANESE EARTHQUAKE OF OCTOBER 28TH, 1891 177 CHAPTER VIII. THE HEREFORD EARTHQUAKE OF DECEMBER 17TH, 1896, ANDTHE INVERNESS EARTHQUAKE OF SEPTEMBER 18TH, 1901 215 CHAPTER IX. THE INDIAN EARTHQUAKE OF JUNE 12TH, 1897 262 CHAPTER X. CONCLUSION 321 INDEX 349 LIST OF ILLUSTRATIONS. FIG. PAGE 1. Diagram to illustrate Simple Harmonic Motion 4 2. Isoseismal Lines of the Neapolitan Earthquake 9 3. Diagram to illustrate Wave-path and Angle of Emergence 12 4. Diagram to illustrate Mallet's Method of determining Position of Epicentre 14 5. Plan of Cathedral Church at Potenza 16 6. Fallen Gate-pillars near Saponara 17 7. Model to illustrate the Motion of an Earth-particle during an Earthquake 19 8. Plan of Directions of Fall of Overturned Stone-lamps at Tokio during the Earthquake of 1894 19 9. Meizoseismal Area of Neapolitan Earthquake 22 10. Distribution of Death-rate within Meizoseismal Area of Neapolitan Earthquake 24 11. Diagram to illustrate Mallet's Method of determining Depth of Seismic Focus 26 12. Vertical Section of Cathedral Church at Potenza 27 13. Diagram of Wave-paths at Seismic Vertical of Neapolitan Earthquake 29 14. Geological Sketch-map of Ischia 47 15. Isoseismal Lines of Ischian Earthquake of 1881 51 16. Isoseismal Lines of Ischian Earthquake of 1883 59 17. Diagram of Wave-paths at Seismic Vertical of Ischian Earthquake of 1883 62 18. Diagram showing connection between Depth of Focus and Rate of Decline in Intensity 68 19. Isoseismal Lines of Andalusian Earthquake according to Taramelli and Mercalli 79 20. Isoseismal Lines of Andalusian Earthquake according to Fouqué, etc. 81 21. Magnetograph Records of Andalusian Earthquake at Lisbon 83 22. Nature of Shock of Andalusian Earthquake 88 23. Diagram to illustrate Variation in Nature of Shock 89 24. Structure of Meizoseismal Area of Andalusian Earthquake 100 25. Isoseismal Lines of Charleston Earthquake 106 26. Curve of Intensity at Charleston 110 27. Flexure of Rails at Jedburgh 113 28. Epicentral Isoseismal Lines of Charleston Earthquake according to Sloan 118 29. Epicentral Isoseismal Lines of Charleston Earthquake according to Dutton 119 30. Planes of Oscillation of Stopped Pendulum Clocks at Charleston 121 31. Diagram to illustrate Dutton's Method of determining Depth of Seismic Focus 124 32. Diagram to explain Origin of Regions of Defective Intensity 136 33. Isoseismal Lines of Riviera Earthquake 144 34. Meizoseismal Area of Riviera Earthquake 148 35. Nature of Shock of Riviera Earthquake 152 36. Seismographic Record at Moncalieri 155 37. Distribution of Observatories at which Magnetographs were disturbed by Riviera Earthquake 158 38. Record of Tide-gauge at Nice 163 39. Record of Tide-gauge at Genoa 164 40. Distribution of Seismic Activity in the Riviera 172 41. Isoseismal Lines of Japanese Earthquake 178 42. Structure of Meizoseismal Area of Japanese Earthquake 180 43. Plan of Directions of Fall of Overturned Bodies at Nagoya 187 44. Map of Mean Directions of Shock and Isoseismal Lines in Central District of Japanese Earthquake 188 45. Meizoseismal Area of Japanese Earthquake 190 46. Fault-scarp near Fujitani 191 47. Fault-scarp at Midori 192 48. Displacement of Field-divisions by the Fault near Nishi-Katabira 193 49. Map of Swamp formed by Stoppage of River Toba by Fault-scarp 194 50. Shifting of Trees by Fault at Uméhara 195 51. Daily Frequency of After-shocks at Gifu and Nagoya 196 52. Monthly Frequency of After-shocks at Gifu 197 53. Distribution of Preliminary Shocks in Space 202 54. Distribution of After-shocks in Space, Nov. -Dec. 1891 203 55. Distribution of After-shocks in Space, Jan. -Feb. 1892 204 56. Distribution of After-shocks in Space, March-April 1892 205 57. Distribution of After-shocks in Space, May-June 1892 206 58. Distribution of Audible After-shocks in Space, Nov. 1891-Dec. 1892 208 59. Map of Adjoining Regions in which Seismic Activity was affected by Japanese Earthquake 210 60. Isoseismal and Isacoustic Lines of Hereford Earthquake 216 61. Nature of Shock of Hereford Earthquake 222 62. Coseismal Lines of Hereford Earthquake 228 63. Map of Minor Shocks of Hereford Earthquake 239 64. Geology of Meizoseismal Area of Hereford Earthquake 241 65. Isoseismal Lines of Inverness Earthquake 248 66. Diagram to illustrate supposed Fault-displacement causing Inverness Earthquake 256 67. Map of Epicentres of After-shocks of Inverness Earthquake 258 68. Isoseismal Lines of Indian Earthquake 263 69. Section of Tombs in Cemetery at Cherrapunji 270 70. Time-curve of Indian Earthquake 278 71. Seismographic Record of Indian Earthquake at Rocca di Papa 282 72. Seismographic Record of Indian Earthquake at Edinburgh 283 73. Displacement of Alluvium at Foot of a Hill 287 74. Twisting of Monument at Chhatak 294 75. Epicentral Area of Indian Earthquake 303 76. Plan of Chedrang Fault 305 77. Re-triangulation of Khasi Hills 313 78. Diagram of Thrust-planes 318 79. Seismographic Record of Tokio Earthquake of 1894 329 80. Time-curves of Principal Epochs of Earthquake-waves of Distant Origin 338 A STUDY OF RECENT EARTHQUAKES. CHAPTER I. INTRODUCTION. I propose in this book to describe a few of the more importantearthquakes that have occurred during the last half century. Injudging of importance, the standard which I have adopted is not thatof intensity only, but rather of the scientific value of the resultsthat have been achieved by the study of the shocks. Even with thisreservation, the number of earthquakes that might be included isconsiderable; and I have therefore selected those which seem toillustrate best the different methods of investigation employed byseismologists, or which are of special interest owing to the unusualcharacter of their phenomena or to the light cast by them on thenature and origin of earthquakes in general. Thus, the Neapolitan earthquake possesses interest from a historicalpoint of view; it is the first earthquake in the study of which modernscientific methods were employed. The Ischian earthquakes aredescribed as examples of those connected with volcanic action; theAndalusian earthquake is chiefly remarkable for the recognition of theunfelt earth-waves; that of Charleston for the detection of the doubleepicentre and the calculation of the velocity with which thevibrations travelled. In the Riviera earthquake are combined theprincipal features of the last two shocks with several phenomena ofmiscellaneous interest, especially those connected with its submarinefoci. The Japanese earthquake is distinguished from others by itsextraordinary fault-scarp and the very numerous shocks that followedit. The Hereford earthquake is a typical example of a twin earthquake, and provided many observations on the sound phenomena; while theInverness earthquakes are important on account of their connectionwith the growth of a well-known fault. The great Indian earthquakeowns few, if any, rivals within historical times, whether we considerthe intensity of the disturbance or the diversity and interest of thephenomena displayed by it--the widespread changes in the earth'scrust, both superficial and deep-seated, and the tracking of theunfelt pulsations completely round the globe. TERMS AND DEFINITIONS. Some terms are of such frequent use in describing earthquakes that itwill be convenient to group them here for reference, others morerarely employed being introduced as they are required. An earthquake is caused by a sudden displacement of the material whichcomposes the earth's interior. The displacement gives rise to seriesof waves, which are propagated outwards in all directions, and which, when they reach the surface, produce the sensations known to us asthose of an earthquake. The region within which the displacement occurs is sometimes calledthe _hypocentre_, but more frequently the _seismic focus_ or simplythe _focus_. The portion of the earth's surface which is verticallyabove the seismic focus is called the _epicentre_. The focus andepicentre are often spoken of for convenience as if they were points, and they may then be regarded as the centres of the region and area inwhich the intensity was greatest. This is not quite accurate, but toattempt a more exact definition would at present be out of place. An _isoseismal line_ is a curve which passes through all points atwhich the intensity of the shock was the same. It is but rarely thatthe absolute intensity at any point of an isoseismal line can beascertained, and only one example is given in this volume. As a rule, the intensity of a shock is determined by reference to the degrees ofdifferent arbitrary scales. These will be quoted when required. In every strong earthquake there is a central district which differsin a marked manner from that outside in the far greater strength andcomplexity of the phenomena. As this district includes the epicentre, it is sometimes referred to as the _epicentral area_, but the term_meizoseismal area_ is more appropriate, and will be employedaccordingly. The district over which an earthquake is perceptible to human beingswithout instrumental aid is its _disturbed area_. In like manner, thatover which the earthquake-sound is heard is the _sound-area_. A great earthquake never occurs alone. It is merely the most prominentmember of a group of shocks of greater or less intensity, and isknown as the _principal shock_ or _earthquake_, while the others arecalled _minor_ or _accessory shocks_, and _fore-shocks_ or_after-shocks_ according as they occur before or after the principalearthquake. When the sound only is heard, without an accompanyingtremor being anywhere perceptible, it is more accurately called an_earth-sound_, but is frequently for convenience numbered among theminor shocks. [Illustration: FIG. 1. --Diagram to illustrate simple harmonic motion. ] The movement of the ground during a vibration of the simplestcharacter (known as simple harmonic motion) is represented in Fig. 1. The pointer of the recording seismograph is here supposed to oscillatealong a line at right angles to AB, and the smoked paper or glass onwhich the record is made to travel to the left. The distance MP of thecrest P of any wave from the line AB represents the _amplitude_ of thevibration, the sum of the distances MP and NQ its _range_, and thelength AB the _period_ of the vibration. From the amplitude and periodwe can calculate, in the case of simple harmonic motion, both the_maximum velocity_ and _maximum acceleration_ of the vibratingparticles of the ground. [1] A few terms describing the nature of the shock are also in common useamong Italians and Spaniards. An _undulatory_ shock consists of one orseveral waves, the movement to and fro being along a nearly horizontalline; a _subsultory_ shock of movements in a nearly verticaldirection; while a _vorticose_ shock consists of undulatory orsubsultory movements crossing one another in different directions. ORIGIN OF EARTHQUAKES. Earthquakes are grouped, according to their origin, into threeclasses. The first consists of slight local shocks, caused by the fallof rock in underground passages; the second of _volcanic_ earthquakes, also local in character, but often of considerable intensity near thecentre of the disturbed area; while in the third class we have_tectonic_ earthquakes, or those directly connected with the shapingof the earth's crust, which vary in strength from the weakestperceptible tremor to the most destructive and widely felt shock. Ofthe earthquakes described in this volume, the Ischian earthquakesbelong to the second class, and all the others to the third. That tectonic earthquakes are closely connected with the formation offaults seems now established beyond doubt. They occur far from alltraces of recent volcanic action. Their isoseismal lines are elongatedin directions parallel to known faults, and this is sometimes the casein one and the same district with faults that occur at right angles toone another. Indeed, when several isoseismals are carefully drawn, itis possible from their form and relative position to predict theposition of the originating fault. [2] The initial formation andfurther spreading of the rent may be the cause of a few earthquakes, but by far the larger number are due to the subsequent growth of thefault. The relative displacement of the rocks adjoining the fault, which may amount to thousands of feet, occasionally even to miles, isthe result, not of one great movement, but of innumerable slips takingplace in different parts of the fault and spread over vast ages oftime. With every fault-slip, intense friction is suddenly brought intoaction by the rubbing of one mass of rock against the other; and, according to the modern view, it is this friction that gives rise tothe earthquake waves. In most earthquakes, the slip takes place at a considerable depth, perhaps not less than one or several miles, and the vertical slip isso small that it dies out before reaching the surface. But, in a fewviolent earthquakes, such as the Japanese and Indian earthquakesdescribed in this volume, the slip is continued up to the surface andis left visible there as a small cliff or fault-scarp. In these cases, the sudden spring of the crust may increase and complicate the effectsof the vibratory shock. FOOTNOTES: [1] If _a_ is the amplitude of the vibration and T its period, themaximum velocity is 2*pi*a/T and the maximum acceleration 4*pi^2a/T^2 [2] See Chapter VIII. , on the Hereford and Inverness earthquakes. CHAPTER II. THE NEAPOLITAN EARTHQUAKE OF DECEMBER 16TH, 1857. Half a century ago, seismology was in its infancy. On the Continent, Alexis Perrey of Dijon was compiling his earthquake catalogues withunfailing enthusiasm and industry. In 1846, Robert Mallet applied thelaws of wave-motion in solids, as they were then known, to thephenomena of earthquakes; and his memoir on the Dynamics ofEarthquakes[3] may be regarded as the foundation-stone of the newscience. During the next twelve years he contributed his well-knownReports to the British Association, [4] and prepared a series ofinstructions for the observation and study of earthquake-shocks. [5]The latter, it is worth noting, contains an outline, but hardly morethan an outline, of the methods of investigation which he developedand employed eight years afterwards in studying the Neapolitanearthquake. The history of Mallet's preparation for his great work is somewhatstrange. No one else at that time possessed so full a knowledge ofearthquake phenomena. It was, however, a knowledge that had little, if any, foundation in actual experience; for, when he was awakened bythe British earthquake of November 9th, 1852, he failed to recogniseits seismic character. Although this shock disturbed an area of about75, 000 square miles and was felt in all four parts of the kingdom, thepaucity of observations and the absence of durable records combined inpreventing the successful application of his new modes of study. [6]Nevertheless, with confidence unshaken in their power, he awaited theoccurrence of a more violent shock, but five years had to pass beforehis opportunity came towards the close of 1857. So destructive was the Neapolitan earthquake of this year (Malletranks it third among European earthquakes in extent and severity), that nearly a week elapsed before any news of it reached the outerworld. Without further loss of time, he applied for and obtained agrant of money from the Council of the Royal Society, and proceededearly in the following February to what was then the kingdom ofNaples. Armed with letters of authority to different officials, hevisited the chief towns and villages in the meizoseismal area; and, inspite of unfavourable weather and the difficulties of travelling in acountry so recently devastated, he completed his examination in littlemore than two months. It was a task, surely, that would have baffledany but the most enthusiastic investigator or one unspurred by thefeeling that he possessed the key to one of the most obscure ofNature's problems. Mallet's confidence in the accuracy of his methods was almostunbounded. His great report was published four years later; but heseems to have regarded it almost as a text-book of "observationalseismology" and the results of his Neapolitan work as mereillustrations. His successors, however, have transposed the order ofimportance, and rank his two large volumes as the model, if not theinspirer, of many of our more recent earthquake monographs. [Illustration: FIG. 2. --Isoseismal Lines of the Neapolitan Earthquake of 1857. (_Mallet. _)] ISOSEISMAL LINES AND DISTURBED AREA. The position of the meizoseismal area, to which Mallet devoted most ofhis time, is indicated by the small oval area marked 1 in Fig. 2, represented on a larger scale in Fig. 9. It is 40 miles long and 23miles wide, [7] and contains 950 square miles. Within this area, theloss of life was great and most of the towns were absolutelyprostrated. The next isoseismal, No. 2, which is also shown more clearly in Fig. 9, bounds the area in which the loss of life was still great and manypersons were wounded, while large portions of the towns within it werethrown down. Its length is 65 miles, width 47 miles, and area 2, 240square miles. The third isoseismal includes a district in whichbuildings were only occasionally thrown down, though none escaped someslight damage, and in which practically no loss of life occurred. Thiscurve is 103 miles long, 82 miles wide, and includes 6, 615 squaremiles. Lastly, the fourth isoseismal marks the boundary of thedisturbed area, which is 250 miles long, 210 miles wide, and containsnot more than 39, 200 square miles; an amount that must be regarded asstrangely small, and hardly justifying Mallet's estimate of theNeapolitan earthquake as the third among European earthquakes inextent as well as in seventy. DAMAGE CAUSED BY THE EARTHQUAKE. As regards destruction to life and property, however, the Neapolitanearthquake owns but few European rivals. Less favourable conditionsfor withstanding a great shock are seldom, indeed, to be found thanthose possessed by the mediæval towns and villages of the meizoseismalarea. In buildings of every class, the walls are very thick andconsist as a rule of a coarse, short-bedded, ill-laid rubble masonry, without thorough bonding and connected by mortar of slender cohesion. The floors are made of planks coated with a layer of concrete from sixto eight inches thick, the whole weighing from sixty to a hundredpounds per square foot. Only a little less heavy are the roofs, whichare covered with thick tiles secured, except at the ridges, by theirown weight alone. Thus, for the most part, the walls, floors, androofs are extremely massive, while the connections of all tothemselves and to each other are loose and imperfect. Again, the towns, for greater security from attacks in early times, are generally perched upon the summits and steep flanks of hills, especially of the lower spurs that skirt the great mountain ranges;and the rocking of the hill-sites, in Mallet's opinion, greatlyaggravated the natural effects of the shock. The streets, moreover, are steep and narrow, sometimes only five feet, and not often morethan fifteen feet, in width; and the houses, when shaken down, fellagainst one another and upon those beneath them. As Dolomieu said ofthe great earthquake in 1783, "the ground was shaken down like ashesor sand laid upon a table. " Of the total amount of damage, not even the roughest estimate can bemade. The official returns are clearly, and no doubt purposely, deficient, and obstacles were placed in Mallet's way when heendeavoured to ascertain the numbers of persons killed and wounded. Taking only the towns into account, he calculated that, out of a totalpopulation of 207, 000, the number of persons killed was 9, 589, and ofwounded 1, 343. [8] A few towns were marked by an excessively highdeath-rate. Thus, at Montemurro, 5000 out of 7002 persons were killedand 500 wounded; at Saponara, 2000 out of 4010 were killed; and, atPolla, more than 2000 out of a population of less than 7000. GENERAL OBJECTS OF INVESTIGATION. The principal objects of Mallet's investigation were to determine theposition of the epicentre and the depth of the seismic focus. If, inFig. 3, F represents the seismic focus (here, for convenience, supposed to be a point), the vertical line FE will cut the surface ofthe earth in the epicentre E. [9] The dotted lines represent circlesdrawn on the surface of the earth with E as centre and passing throughthe places P and Q. [Illustration: FIG. 3. --Diagram to illustrate wave-path and angle of emergence. ] When the impulse causing the earthquake takes place at the focus, twoelastic waves spread outwards from it in all directions through theearth's crust. The first wave which reaches a point P consists oflongitudinal vibrations, that is, the particle of rock at P moves in aclosed curve with its longer axis in the direction FP. Mallet supposesthis curve to be so elongated that it is practically a straight linecoincident in direction with FP. In the second or transversal wave, the vibration of the particle at P takes place in a plane at rightangles to FP. These vibrations Mallet, for his main purpose, neglects. Returning to the longitudinal wave, Mallet calls the line FP the_wave-path_ at P. The direction EP gives the azimuth of the wave-path, or its direction along the surface of the earth. The angle LPA, orEPF, he defines as the _angle of emergence_ at the point P. If Q befarther from E than P, the angle EQF is less than the angle EPF, orthe angle of emergence diminishes as the distance from the epicentreincreases. At the epicentre, the angle of emergence is a right-angle;at a great distance from the epicentre, it is nearly zero. Mallet argued that the direction of the wave-path FPA, or itsequivalents, the horizontal direction EPL and the angle of emergenceEPF, should be discoverable from the effects of the shock at P. Thecracks in damaged buildings, he urged, would be at right angles to thewave-path FPA; overturned monuments or gate-pillars should fall alongthe line EPL, either towards or from the epicentre according to theirconditions of support; loose or slightly attached bodies, such as thestone balls surmounting gate-pillars, should be projected nearly inthe direction of the wave-path FPA, and their subsequent positions, supposing the balls not to have rolled, should give the horizontaldirection EPL of the wave-path, and might, in some circumstances, determine the angle of emergence and the velocity with which they wereprojected. I shall return to details later on. For the present, it isclear that, in the destruction wrought by the earthquake, Malletexpected to find the materials most valuable for his purpose. Indeed, so obvious did this mode of examination appear to him, that he couldnot conceal his surprise at the blindness of his predecessors. Theyseem, he says, "to have been perfectly unconscious that in thefractured walls and overthrown objects scattered in all directionsbeneath their eyes, they had the most precious data for determiningthe velocities and directions of the shocks that produced them. " POSITION OF THE EPICENTRE. _Mallet's Method of Determining the Position of the Epicentre. _--Inmany cases the examination of a damaged building or of an overthrownbody served more than one purpose, providing materials forascertaining the depth of the seismic focus as well as the position ofthe epicentre. For the present, however, it will be convenient toconsider alone the method by which the latter object was to beattained. [Illustration: FIG. 4. --Diagram to illustrate Mallet's method of determining position of epicentre. ] Nothing could be simpler than the principle of the method proposed. The horizontal direction PL of the wave-path at any place P (Fig. 4), when produced backwards, must pass through the epicentre E; and theintersection of the directions at two places, P and Q, must thereforegive the position of the epicentre. In practice, it is of courseimpossible to determine the direction with very great accuracy, andMallet therefore found it necessary to make several measurements inevery place, and to visit all the more important towns within and nearthe meizoseismal area. In a ruined town there are many objects from which the direction maybe ascertained, the most important of all, according to Mallet, beingfissures in walls that are fractured but not overthrown. He regardedsuch fissures, indeed, as "the sheet-anchor, as respects direction ofwave-path, to the seismologist in the field, " and at least three outof every four of his determinations of the direction were made bytheir means. If the buildings are detached and large, simple andsymmetrical in form, well built and not too much injured, the fissuresin the walls should, he argued, occur along lines at right angles tothe wave-path, whether that path be parallel or inclined to theprincipal axis of the building. Cracks in the floors and ceilingsshould also be similarly directed, and provide evidence which Malletregarded as only second in value to that given by the walls. [Illustration: FIG. 5. --Plan of Cathedral Church at Potenza. (_Mallet. _)] No building showed the different kinds of evidence on which Malletrelied as clearly as the cathedral church at Potenza, the plan ofwhich is given in Fig. 5, and the vertical section along its axis inFig. 12. This is a modern work, nearly 200 feet long, with its axisdirected east and west. The walls are composed of fairly good rubblemasonry and brick; and the arches in the nave and transepts, thesemi-cylindrical roof and the central dome are made of brick. Thefissures represented in both diagrams were drawn to scale by thecathedral architect before Mallet's arrival, and, as the work of anunbiassed observer, are of special value. Most of those in the roof, it will be seen, were transverse to the axial line of the church; butthere were others parallel to this line, one in particular runningright along the soffit of the nave and chancel. There were alsonumerous small fissures in the dome, due to local structural causesand therefore of varying direction, and a large portion of the domeslipped westward, leaving open fissures of seven to eight inches inwidth. The mean direction of the wave-path, as deduced from nine setsof fissures, none of which differs more than four degrees from themean, is W. 2-1/2° S. And E. 2-1/2° N. , which corresponds preciselywith the direction of throw on the displaced portion of the dome. Thegreat east and west fissures in the arch of the nave and chancelMallet attributed to a second shock, of the existence of which thereis ample evidence. [Illustration: FIG. 6. --Fallen gate-pillars near Saponara. (_Mallet. _)] Next to fissures, Mallet made most use of overthrown objects, such asthe two gate piers near Saponara, represented in Fig. 6. They weremade of rubble ashlar masonry, three feet square and seven feet inheight. Both were fractured clean off at the level of the ground, themortar being poor, and fell in directions that were accuratelyparallel, indicating a wave-path towards S. 39-1/2°E. A fewobservations were also made on projected stones, fissures in nearlylevel ground, and the swinging of lamps and chandeliers; but theirvalue was small, except as corroboration of the more importantevidence afforded by fissures in the walls and roofs of buildings. _Remarks on Mallets Method. _--It would have been more difficult inMallet's day than it is now, to offer objections to his method ofdetermining the position of the epicentre. The focus, as he was wellaware, could not be a point, and, at places near the epicentre (thevery places where most of his observations were made), there must berapid changes of direction due to the arrival of vibrations fromdifferent parts of the focus. He records the occurrence of theso-called vorticose shocks at several places, though he attributesthem to another cause. Perhaps the best known example of such a shockis that which has been so well illustrated by the late ProfessorSekiya's model of the motion of an earth-particle during the Japaneseearthquake of January 15th, 1887. The motion in this case was socomplicated that the model was, for simplicity, made in three parts, the first of which alone is represented in Fig. 7. [10] It is clearthat in such an earthquake, Mallet's method would utterly fail ingiving definite results. While this shock was one of great complexity, another Japaneseearthquake, that of June 20th, 1894, was unusually simple incharacter. The movement at Tokio consisted of one very prominentoscillation with a total range of 73 mm. Or 2. 9 inches in thedirection S. 70° W. ; the vibrations which preceded and followed itbeing comparatively small. Most, if not all, of the damage caused bythe earthquake must have been due to this great oscillation; and yetthe cylindrical stone-lamps so common in Japanese gardens were foundby Professor Omori to have fallen in many different directions. Takingonly those which had circular bases, twenty-nine were overthrown indirections between north and east, sixteen between east and south, eighty-one between south and west, and fourteen between west andnorth. [11] Fig. 8 represents Professor Omori's results graphically, the line drawn from O to any point being proportional to the number oflamps which fell in directions between 7-1/2° on either side of theline. [Illustration: FIG. 7. --Model to illustrate the motion of an earth-particle during an earthquake. (_Sekiya. _)] [Illustration: FIG. 8. --Plan of directions of fall of overturned stone-lamps at Tokio during the earthquake of 1894. ] It will be seen from this figure that most of the stone lamps fell indirections between west and south-west, and it is remarkable that themean direction of fall is S. 70° W. , [12] which is exactly the same asthat of the great oscillation. Somewhat similar results were obtainedby this able seismologist at different places affected by the greatJapanese earthquake of 1891 (Figs. 43 and 44), and the study of theapparent directions observed during the Hereford earthquake of 1896leads to the same conclusion. It thus appears that an isolated observation may give a result verydifferent from the true direction. Indeed, if we may judge fromProfessor Omori's measurements in 1894, the chance that a singledirection may be within five degrees of the mean direction is about 1in 9. But, on the other hand, it is equally clear from these and otherobservations that the mean of a large number of measurements will givea result that agrees very closely with the true direction. One other point may be alluded to before leaving Professor Omori'sinteresting observations. It would seem, from the list that he gives, that he exercised no selection in his measurements, but continuedmeasuring the direction of every fallen lamp indifferently until hehad obtained sufficient records for his purpose. Now, if the number offallen lamps at his disposal had been small, say 12 instead of 144, the mean observed direction would probably have differed from thedirection given from the seismograph. [13] But, on the other hand, apreliminary survey without any actual measurements would have revealedat once the predominant direction of overthrow, and a fairly accurateresult might have been obtained by neglecting discordant directionsand taking the mean of those only which appeared to agree with thementally determined average. This, indeed, appears to have been the course followed, more or lessunconsciously, by Mallet in his Neapolitan work. "When the observer, "he says, "first enters upon one of those earthquake-shaken towns, hefinds himself in the midst of utter confusion. The eye is bewilderedby 'a city become an heap. ' He wanders over masses of dislocated stoneand mortar, with timbers half buried, prostrate, or standing stark upagainst the light, and is appalled by spectacles of desolation.... Houses seem to have been precipitated to the ground in every directionof azimuth. There seems no governing law, nor any indication of aprevailing direction of overturning force. It is only by first gainingsome commanding point, whence a general view over the whole field ofruin can be had, and observing its places of greatest and leastdestruction, and then by patient examination, compass in hand, of manydetails of overthrow, house by house and street by street, analysingeach detail and comparing the results, as to the direction of force, that must have produced each particular fall, with those previouslyobserved and compared, that we at length perceive, once for all, thatthis apparent confusion is but superficial. " [Illustration: FIG. 9. --Meizoseismal area of Neapolitan earthquake. (_Mallet. _)] _Mallet's Determination of the Epicentre. _--Within the thirdisoseismal line Mallet made altogether 177 measurements of thedirection of the wave-path at 78 places. These are plotted on hisgreat map of the earthquake; but, owing to the small scale of Fig. 9, it is only possible to represent, by means of short lines, the mean ormost trustworthy direction at each place. [14] Producing thesedirections backwards, he found that those at sixteen places passedwithin five hundred yards of a point which is practically coincidentwith the village of Caggiano; those at sixteen other places passedwithin one geographical mile (1. 153 statute miles) of this point; thedirections at sixteen more places within two and a half geographicalmiles; while those at twelve places passed through points not morethan five geographical miles from Caggiano. As the direction of theshock at places near the epicentre must have been influenced by themere size of the focus, this approximate coincidence is certainlyremarkable, and there can be little doubt, I think, that theepicentre, or, at any rate, _an_ epicentre must have been situated notfar from the position assigned to it by Mallet's laboriousobservations. _Existence of Two Epicentres. _--It is difficult, however, to realisethat the impulse at the focus corresponding to Mallet's epicentre wasthe origin of all the destruction of life and property that occurred. The position of the epicentre close to the north-west boundary of themeizoseismal area, the extraordinary extension of that area towardsthe south-east, and especially the great loss of life at Montemurroand the adjoining towns, can hardly be accounted for in this manner. Mallet himself recognised that these facts required explanation, andhe suggested that the situation and character of the towns were inpart responsible for their ruin, and the physical structure of thecountry for the course of the isoseismal lines. But the comparativeescape of places much nearer Caggiano, and the wide extent of themeizoseismal area, embracing many towns and villages of variedcharacter and site and many different surface-features, pointunmistakably to a different explanation. [Illustration: FIG. 10. --Distribution of death-rate within meizoseismal area of Neapolitan earthquake. ] One clue to the solution of the problem is afforded by the seismicdeath-rate of the damaged towns. From a table given by Mallet (vol. Ii. Pp. 162-163), we know the population before the earthquake of thedifferent communes in the province of Basilicata, and the loss of lifein each due to the shock; and from these figures we can find thepercentage of deaths at nearly every place of importance. As will beseen from Fig. 10, it varies from seventy-one at Montemurro and fiftyat Saponara down to less than one at all the places marked to whichfigures are not attached. There is thus a group of places, with itscentre near Montemurro, where the loss of life far exceeded that inthe surrounding country; and also a slightly less-marked group, withits centre near Polla, in the north-west of the meizoseismal area;while in the intermediate region the death-rate was invariably small. Too much stress should not be laid upon the exact figures, for therewere no doubt local conditions that affected the death-roll. But itseems clear that one focus was situated not far from Montemurro; whilethe north-westerly group of places, combined with Mallet'sobservations on the direction, point to a second focus near Polla, about twenty-four miles to the north-west. It will be seen in a latersection that the observations on the nature of the shock also implythe existence of a double focus. DEPTH OF THE SEISMIC FOCUS. _Mallet's Method of Determining the Depth of the Focus. _--Inascertaining the position of the epicentre, Mallet's work wasremarkable only for the novelty of the method employed by him; but, inhis attempt to calculate the depth of the seismic focus, he wasbreaking new ground. That the depth must be comparatively small hadalready been recognised, and was indeed obvious from the limited areadisturbed by nearly every earthquake. No one, however, had tried toestimate the depth in miles; and it is impossible not to sympathisewith Mallet while he accumulated his observations with feverishactivity and subjected them to the first rough examination even ifone cannot share his confidence that he had succeeded in measuring thedepth "in miles and yards with the certainty that belongs to anordinary geodetic operation. " The method employed by him for the purpose is no less simpletheoretically than that used for locating the epicentre. If theposition of the latter (E) is known, one accurate measurement of theangle of emergence EPF, at any other point P would be sufficient tofix the depth of some point within the focus F (Fig. 11). Here, again, Mallet relied chiefly on fissures in walls that were fractured but notoverthrown. In detail, these fissures are nearly always jagged orserrated, for they tend to follow the lines of joints rather thanbreak through the solid stone, though they sometimes traverse bricksand mortar alike. But the general course of the fissures, he urged, would be at right angles to the wave-path, and their inclination tothe vertical should be equal to the angle of emergence. [Illustration: FIG. 11. --Diagram to illustrate Mallet's method of determining depth of seismic focus. ] In obtaining measurements of this angle, the buildings to be chosenare those of large size, with few windows or other apertures, and withwalls made of brick or small short-bedded stones. The cathedral-churchat Potenza perhaps satisfies these conditions more closely than anyother structure examined by Mallet. The plan of the fissures in thewalls and roof has been given in Fig. 5, and Fig. 12 represents thefissures In the vertical section along the axial line and lookingnorth, as drawn by the cathedral architect. From these fissures Malletcalculated the mean angle of emergence at Potenza to be 23° 7'. Thedistance of Potenza from Caggiano being seventeen miles, and theheight of the former being 2, 580 feet, the depth of the focusresulting from this observation alone would be 6-3/4 miles below thelevel of the sea. [Illustration: FIG. 12. --Vertical section of Cathedral Church at Potenza. (_Mallet. _)] _Objection to Mallet's Method. _--The weakest point in Mallet's methodis probably his assumption that the wave-paths are straight linesextending outward from the focus. Even if the depth of the focus isnot more than a few miles, the waves must traverse rocks of varyingdensity and elasticity, and, at every bounding surface, they mustundergo refraction. If the rocks are so constituted that the velocityof the earth-waves in them increases with the depth, then thewave-paths must be bent continually outwards from the vertical, sothat the angle of emergence at the surface may be considerably lessthan it would have been with a constant velocity throughout. In thiscase, the actual depth will be greater, perhaps much greater, than thecalculated depth. For instance, if the angle of emergence at Potenzawere diminished only 5° by refraction, the calculated depth of thefocus would be too small by 1-3/4 miles. _Mallet's Estimate of the Depth of the Focus. _--Mallet measured theangle of emergence at twenty-six places, the mean angle (_i. E. _ themean of the greatest and least observed angles) varying from 72° atVietri di Potenza and 70° at Pertosa, which are about two miles fromthe calculated epicentre, to 11-1/2° at Salerno, distant about 40miles. Fig. 13 reproduces part of the diagram on which he plotted themean angle of emergence at different places. The horizontal linerepresents the level of the sea, and the vertical line one passingthrough the epicentre and focus, called by Mallet the "seismicvertical. " The lines on the left-hand side represent the commencingwave-paths (assumed straight) to the observing stations situated tothe westward of the meridian through the epicentre, those on theright-hand side corresponding to places to the eastward of the samemeridian. Small horizontal marks are added to indicate the depth inmiles below the level of the sea. [Illustration: FIG. 13. --Diagram of wave-paths at seismic vertical of Neopolitan earthquake. (_Mallet. _)] It will be seen, from this diagram, that all the wave-paths start fromthe seismic vertical at depths between three and nine miles; but thepoints of departure are clustered thickly within a portion, the lengthof which is about 3-1/2 miles and the mean depth about 6-1/2 miles. Sogreat was Mallet's confidence in these calculations that he assignsthe diverging origin of the wave-paths to different points of thefocus, and thus concludes that, while the mean depth of the focus wasabout 6-1/2 miles, its dimensions in a vertical direction did notexceed 3-1/2 miles. How far Mallet's results should be accepted as correct, it isdifficult to say in our ignorance of the constitution of the earth'sinterior. There can be no doubt that the focus was of considerablesize, and that, in consequence, the wave-paths would diverge fromdifferent points of it. But that each wave-path should actuallyintersect the focus, and so enable its magnitude to be determined, would surely involve an approach to some law connecting the directionof a wave-path with the depth of its own origin, and no such law seemsto be ascertainable. Nor can the limitation of these apparent originsbetween certain depths be held to argue that the focus, or any part ofit, was equally confined, for the wave-paths would to a great extentbe similarly refracted. I fear that the only conclusions that we canwith safety draw from Mallet's admirable work are that his figuresindicate the order of magnitude both of the vertical dimensions and ofthe mean depth of the focus. NATURE OF THE SHOCK. It is not easy to form any precise image of the earthquake as itappeared to the terrified witnesses within the meizoseismal area. Tominds unbalanced by the suddenness of the shock and by the crash offalling houses, actuated too by the intense need of safety, the meresuccession of events must have presented but little interest. Theinterval of two months that elapsed between the occurrence of theearthquake and its investigation was also unfavourable to thecollection of accurate accounts from a wonder-loving people. Only onefeature, therefore, stands out clearly in the few records given byMallet--namely, the division of the shock into two distinct parts. In the central district, this division is perhaps less apparent thanelsewhere. At Polla, for instance, which lies close to the north-westepicentre, the first warning was given by a rushing sound; almostinstantly, and while it was yet heard, came a strong subsultory orup-and-down movement, succeeded after a few seconds, but without anyinterval, by an undulatory motion. At Potenza, which is not far fromthe same epicentre but a few miles outside the meizoseismal area, theseparation was more pronounced. According to one observer, the firstmovement was from west to east; and, within a second or twoafterwards, there was a less violent shock in a transverse direction, followed immediately by a shaking in all directions, called by theItalians vorticose. Naples lies sixty-nine miles from the north-westepicentre, and here more accurate observations could be made. Dr. Lardner, well known fifty years ago as a writer of scientific works, describes the first movement felt there as "a short, jarring, horizontal oscillation, that made all doors and windows rattle, andthe floors and furniture creak. This ceased, and after an intervalthat seemed but a few seconds was renewed with greater violence, and, he thought, with a distinctly undulatory movement, 'like that in thecabin of a small vessel in a very short chopping sea. '" In five other earthquakes studied in this volume, the separation ofthe shock into two parts was a well-marked phenomenon. In theNeapolitan earthquake, the separation was so distinct that Mallet tooksome pains to account for its origin. He regarded it in every case asdue to the reflection or refraction of the earth-waves by underlyingrocks, though he does not explain why the reflected or refracted waveshould be more intense than that transmitted directly. I shall referto the subject in greater detail when describing the Andalusian, Charleston, Riviera, and Hereford earthquakes. For the present, it maybe sufficient to urge that the double shock cannot have been due tothe separation of the original waves by underground reflection orrefraction, for then the second part should have been generally theweaker; nor to the succession of longitudinal and transverse waves, for, in that case, every earthquake-shock should be duplicated. Theonly remaining supposition is that there was a second impulseoccurring either in the same or in a different focus. Which alternative should be adopted, the evidence on the nature of theshock is too scanty to determine. The defect is, however, supplementedby Mallet's observations on the direction of motion; for, at manyplaces within and near the meizoseismal area, he met with the clearestsigns of a double direction. Sometimes this was apparent to the sensesof the observer; in other cases, damaged buildings presented two setsof fissures. At La Sala and near Padula, the first movement was roughlyeast and west, the second north and south. At Moliterno, there wasevidence of a subordinate shock at right angles to the chief one; inthe neighbourhood of Tramutola, its direction was from about E. 30° S. In these and other cases, Mallet saw the effects of earthquake-echoes;but the underground reflection of earth-waves would give rise to thesecond part of the shock, not the first as at La Sala and Padula. Moreover, the secondary directions, though they are seldom recordedaccurately, point nearly to an epicentre not far from Montemurro. Theobservations on the nature and direction of the double shock thusconfirm the conclusion, derived from the distribution of the seismicdeath-rate, that there were two detached foci, one near Polla and theother near Montemurro. This seems to be the best explanation of the facts recorded by Mallet. There is, however, a possible difficulty that should not beoverlooked--namely, the apparently slight influence of the Montemurrofocus on the mean direction of the shock (Fig. 9). At a few places, ofcourse, the mean direction passes through both epicentres; at someothers, as we have seen, one of the two observed directions pointstowards the Montemurro epicentre. It is not impossible, also, thatMallet, after the first few days' work, may occasionally have quiteunconsciously selected and measured those fissures from the mazepresented to him which agreed most closely with his early impressionsobtained from the neighbourhood of Polla. But, for places nearer Pollathan Montemurro (and these form the majority of those visited byMallet), the probable explanation of the difficulty is that theMontemurro focus was not so deep as the Polla focus. This, as willappear more fully in the next chapter, would account for thecomparatively great intensity in the immediate neighbourhood ofMontemurro and for its rapid decline outwards; and it receives somesupport from an isolated reference by Mallet to two angles ofemergence at Padula, one of 25° from the north, and the other of 8° or10° in the perpendicular walls. ELEMENTS OF THE WAVE-MOTION. The elements of the wave-motion, as mentioned in the introductorychapter, are four in number, namely, the period, amplitude, maximumvelocity, and maximum acceleration. If any two of these are known foreach vibration--and the first two are now given by every accuratelyconstructed seismograph--the others can be determined if thevibrations follow the law of simple harmonic motion. [15] _Amplitude. _--To ascertain the amplitude, Mallet had to rely chieflyon the fissures made in very inelastic walls. If the parts into whichsuch a wall are fractured are free to move, and yet, being inelastic, obliged to remain in the farthest position to which they are carriedby the wave, the distance traversed by the centre of gravity of one ofthe displaced parts should give a "rude approximate measure" of thehorizontal amplitude of the earth-wave. At Certosa, near Padula, hethus found the amplitude to be about 4 inches, at Sarconi about 4-3/4inches, and at Tramutola about 4-1/2 inches. From somewhat similarevidence, the amplitude at Polla appears to have been about 2-1/2 or 3inches; and, from the oscillation of a suspended clock or watch on arough wall, about 3-1/2 inches at La Sala and 1-3/4 inches atBarielle. With the exception of Barielle, these places lie nearly on astraight line passing through Mallet's epicentre, and he gives thefollowing table, showing an increase in amplitude with the distancefrom the epicentre:-- Polla. La Sala. Certosa. Tramutola. Sarconi. Distance in miles 4. 0 13. 4 19. 0 23. 8 30. 8Amplitude in inches 2-1/2 3-1/2 4 4-1/2 4-3/4 The existence of the Montemurro focus must, however, complicate anyrelation that may connect these two quantities. _Maximum Velocity. _--The means at Mallet's disposal for determiningthe maximum velocity were more numerous than those available for theamplitude. From the dimensions of a fallen column of regular form weshould be able, he remarks, to find an inferior limit to the value ofthe maximum velocity; while a superior limit at the same place may beobtained from some other regular solid which escaped being overthrown. If a loose body is projected by the shock at a place where the angleof emergence is known, the horizontal and vertical distances traversedby the centre of gravity will give the velocity of projection. Or, iftwo such bodies are projected at one place, the same measures for eachwill as a rule give both the angle of emergence and the velocity ofprojection. A third method depends on the fissuring of walls, supposing that we know the force per unit surface which, when suddenlyapplied, is just sufficient to produce fracture. Sometimes more thanone method must be applied to the same object. The two gate-pillarsnear Saponara (illustrated in Fig. 6) for example required ahorizontal velocity of 5. 48 feet per second to fracture them, and anadditional velocity of 5. 14 feet per second to overthrow them. The well-known seismologist, Professor Milne, urges very forcibly thatmeasurements obtained from the projection or fall of columns areunreliable, for the earlier tremors might cause the columns to rock, and their overthrow need not therefore measure accurately the maximumvelocity of the critical vibration. [16] There can be no doubt thatMallet was alive to this difficulty, though he may not haveappreciated it at its full value. Thus, at the Certosa de St. Lorenzo, a monastery near Padula, a vase projected from the summit of a slendergate-pier implied a velocity of 21-3/4 feet per second; and the excessof about 8-1/4 feet per second above the velocity determined by othermeans is attributed by him to the oscillation of the pier itself. Howfar this source of error enters into other observations it isimpossible to say; but it is worth noticing how closely the velocitiesobtained by different methods agree with one another. Thus, fromprojection only, we have velocities of 11. 5 feet per second at theCertosa, 11. 8 at Moliterno and Monticchio, 14. 8 at Tramutola, and 9. 8feet per second at Sarconi; from overthrow alone, 11. 0 feet per secondat Viscolione, near Saponara, and 11. 6 at Barielle; from overthrow andprojection, 13. 2 feet per second at Polla and 12. 9 at Padula; fromfracture and overthrow, 12. 3 feet per second at Potenza and 15. 6 atSaponara. The comparatively high values at Tramutola and Saponara, Mallet imagined might be due to the oscillation of the hills on whichthese towns are built. He therefore omits them in calculating the meanmaximum velocity, which he finds to be twelve feet per second, avelocity less than that with which a man reaches the ground when hejumps off a table. With the same omissions, Mallet gives the following table, showing ageneral decrease in the maximum velocity as the distance from hisepicentre increases:-- Polla. Padula. Certosa. Moliterno. Viscolione. Sarconi. Distance inmiles 4. 0 19. 0 19. 0 29. 4 30. 0 30. 8 Max. Vel. Inft. Per sec. 13. 2 12. 9 11. 5 11. 8 11. 0 9. 8 On the north side of the epicentre we have:-- Potenza. Monticchio. Barielle. Distance in miles 17. 3 27. 1 28. 2Max. Vel. In ft. Per sec. 12. 3 11. 8 11. 6 It is not impossible that the high calculated velocities at Tramutolaand Saponara were partly or entirely due to the impulse from theMontemurro focus. If we take 4 inches for the amplitude of the largest variation, and 12feet per second for the maximum velocity, and assume the motion tohave been of a simple harmonic character, the period of a completevibration would be less than one-fifth of a second. [17] Now, we knowfrom seismographic records that this is roughly the period of thesmall tremors that form the commencement of an earthquake-shock, whilethe period of the largest vibrations may amount to as much as one ortwo seconds. We may therefore conclude either that the assumption ofsimple harmonic motion is incorrect, or that the maximum velocity istoo great, or more probably perhaps that the amplitude is toosmall. [18] SOUND-PHENOMENA. Mallet was one of the first seismologists to realise the significanceof the earthquake-sound; and he attended closely to the subject, though finding the sound even more elusive of precise observationthan the shock. The chief result obtained by him was the comparative smallness of thearea over which the sound was heard. He estimates it at little morethan 3, 300 square miles, or about one-twelfth of that over which theshock was felt. It extends north and south from Melfi to Lagonegro, and east and west from Monte Peloso to Duchessa and Senerchia. Thesound was thus confined to the region in which the shock attained itsmost destructive character. Towards the north and south ends of the sound-area all observersdescribed the sound as a low, grating, heavy, sighing rush, lastingfrom twenty to sixty seconds, some adding that it was also of arumbling nature. Near the centre and the east and west boundaries, thesound was distinctly more rumbling; it was shorter in duration, andbegan and ended more abruptly. The earthquake, Mallet remarks, "began everywhere with tremors; thesounds generally arrived at the same time; the apparent direction ofmovement of the tremulous oscillations appeared rapidly to change, andstill more rapidly to increase in amplitude; then the great _shove_ ofthe destructive shock arrived, in some places rather before, in some alittle after, the moment of loudest sound, and it died away suddenly(_i. E. _, with extreme rapidity) into tremors again, but differing indirection from that of the great shock itself. "[19] The earthquake-sound will be described more fully in the chapterdealing with the Hereford earthquake of 1896, in which it will befound that the phenomena recorded by Mallet are equally characteristicof the slighter shocks felt in this country. VELOCITY OF THE EARTH-WAVES. In 1857 little was known about the velocity of earthquake-waves. Experiments had been made by Mallet himself in 1849 in theneighbourhood of Dublin. These gave 825 feet per second for thevelocity in dense wet sand, 1, 306 feet per second in discontinuousgranite, and 1, 665 feet per second in more solid granite. [20] The onlyearthquake for which the velocity had been calculated was the Rhenishearthquake of 1846, the value ascertained by Schmidt being 1, 376French feet, or 1, 466 English feet, per second. The accurate public measurement of time, which, as Mallet remarks, isone of the surest indications of advancing civilisation, was, however, unknown in the kingdom of Naples; and his attempt was thereforefettered by the rarity of precise estimates of the time of occurrence. Throughout the whole disturbed area only six good records could beobtained, and three of these (at Vietri di Potenza, Atella, andNaples) were derived from stopped clocks, witnesses of rather doubtfulvalue. At Montefermo and Barielle the time was at once read from awatch, and at Melfi from an accurate pocket chronometer. The timesgiven vary from 9h. 59m. 16s. P. M. (Naples mean time) at Vietri diPotenza to 10h. 7m. 44s. At Naples. Allowing for the supposed changeof direction by refraction at the Monte St. Angelo range on the wayto Naples, Mallet finds the mean surface velocity to be 787 feet persecond. Omitting the Naples record, and taking account of thecalculated depth of the focus, the mean velocity becomes 804 feet persecond. MINOR SHOCKS. A great earthquake rarely, if ever, occurs without some preparation inthe form of a marked increase of seismic activity. Perrey recordsseveral shocks during the two years 1856-57 that were felt at placesas far apart as Naples, Melfi, and Cosenza. On December 7th, 1857, aslight shock, with a report from beneath like the explosion of a mine, was felt at Potenza. Then came the great earthquake on December 16th, at about 10 P. M. This was followed by numerous after-shocks--how numerous it isimpossible to say, for the records are of the scantiest description. For some hours the ground within the meizoseismal area is said to havetrembled almost incessantly. At Potenza many slight shocks, bothvertical and horizontal, were felt during the night, and for a monthor more they were so frequent as to render enumeration difficult. Mallet's last record is dated March 23rd, 1858, when four slightshocks were felt at La Sala and Potenza, but occasional tremors werereported to him until May 1859. The most important of all these after-shocks was one felt about anhour after the principal earthquake. Everywhere far less powerful, itwas yet strong enough to shake down many buildings at Polla that hadbeen shattered by the great shock. Towards the south at Moliterno, and towards the north at Oliveto and Barielle, it evidently attractedvery little attention. So far as can be judged from the evidence givenby Mallet, the disturbed area seems to have been approximately of thesame form and dimensions as the meizoseismal area, and elongated inthe same direction, but concentric with the north-west focus. On the other hand, if we may rely on too brief evidence, severalafter-shocks recorded only at Montemurro, Saponara, Viggiano, orLagonegro, were probably connected with the south-east or Montemurrofocus. ORIGIN OF THE EARTHQUAKE. Mallet's theories have suffered perhaps more than any other part ofhis work from the recent growth of our knowledge. From a historicalpoint of view, some reference to his explanation of the origin of theNeapolitan earthquake seems desirable, and his own conscientious workdemands it. On the other hand, his conclusions are, for the present atany rate, superseded, and it will therefore be sufficient to describethem briefly. Most of the wave-paths, as we have seen, pass within three miles of apoint almost coincident with the village of Caggiano. Of theremainder, six traverse a spot about two miles farther to thesouth-west, and three cross another about two miles farther to thenorth-east. Neglecting other points of intersection, but takingaccount of the observed emergences at Vietri di Potenza, Auletta, Polla, etc. , Mallet infers that the horizontal section of the focuswas a curve (indicated by the dotted line in Fig. 9) not less than tenmiles in length, and passing from near Balvano on the north, close toVietri di Potenza, Caggiano, and Pertosa, to a point about two and ahalf miles west of Polla. Again, he remarks, the observed emergencesat places near the epicentre indicate that the vertical section of theseismic focus was either more or less curved, or more probably asurface inclined towards the south-east. He concludes, therefore, thatthe seismic focus was a curved fissure, 10 miles long and 3-1/2 milesin height, and with its centre at a depth of 6-1/2 miles below thelevel of the sea. The production of this great fissure, accompanied, perhaps by theinjection into it of steam at high pressure, was regarded by Mallet asthe cause of the principal earthquake. He imagines that the rent wouldstart at or near the central point of the focus and then extendrapidly outwards in all directions. In the initial stage, vibrationsof very small amplitude would alone be transmitted, and these wouldgive rise to the early sounds and tremors. As the rending proceeded, the vibrations would increase in strength up to a certain point whenthey produced the shock itself. After this, they would decrease; and, in the final stage, would give place to the small vibrationscorresponding to the sounds and tremors that marked the close of theearthquake. The rush of steam at high pressure into the focus Mallet does not seemto have considered essential, though he evidently regarded it aspossible, indeed probable; and he suggests that it may have been inpart the cause of the earthquake which occurred an hour later. Thoughfeeling sceptical as to the existence of any general law of increaseof underground temperature, he assumes it, for the sake ofillustration, to be 1° F. For every 60 feet of descent. This wouldgive a temperature of 339° F. At the upper limit of the focus, 643° F. At its central point, and 884° F. At its lower margin. If the focuswere filled with steam at each of these temperatures, thecorresponding pressures on its walls would be 8, 149, and 684atmospheres, respectively. As the steam may be supposed to be admittedsuddenly and to be unlimited in supply, Mallet infers that it mightexist at the tension due to the highest of these temperatures, inwhich case it would be capable of lifting a column of limestone 8, 550feet in height (or about one-half the depth of the upper margin of thefocus), and would exert a pressure on the walls of the focus of 4. 58tons per square inch, or of more than 640, 528 millions of tons uponits whole surface. So many pages have already been given to this interesting earthquakethat I must sketch still more briefly my own view as to its origin. There were, I believe, two distinct foci with their centres abouttwenty-four miles apart along a north-west and south-east line, and itwas to this arrangement that the elongation of the meizoseismal areawas chiefly, though not entirely, due. The evidence is insufficient todetermine whether the earthquake was caused by fault-slipping; it isin no way opposed to this view, but if the Neapolitan earthquake stoodalone, we should hardly be justified in drawing any further inference. Relying, however, on knowledge obtained from the study of more recentshocks, it seems to me probable that the two foci formed parts of onefault with a general north-west and south-east direction. The slipcausing the first part of the double shock apparently took placewithin the south-east focus, and was followed after a few seconds byone within the north-west focus, greater in amount as well as moredeeply seated. In consequence of these displacements there were localincreases of stress, causing numerous small slips within or near bothprincipal foci; and, if we may judge from some slight shocks felt atLa Sala, accompanied also by other minor slips in the intermediateregion of the fault. REFERENCE. MALLET, R. --_The Great Neapolitan Earthquake of 1857: The First Principles of Observational Seismology_, etc. 2 vols 1862. FOOTNOTES: [3] _Irish Acad. Trans. _, vol. Xxi. , 1848, pp. 51-105 (read Feb. 9, 1846). [4] _Brit. Assoc. Reports_, 1850, pp. 1-87; 1851, pp. 272-330; 1852, pp. 1-176; 1853, pp. 117-212; 1854, pp. 1-326; 1858, pp. 1-136. [5] _A Manual of Scientific Enquiry_, edited by Sir J. F. W. Herschel, 1849, pp. 196-223. [6] _Irish Acad. Trans. _, vol. Xxii. , 1855, pp. 397-410. [7] The linear dimensions of the isoseismal lines are obtained bymeasurements from Mallet's maps. The areas are given by him ingeographical square miles. [8] Mallet, by some accident, omitted the losses at Polla andneighbouring towns from this estimate. Mercalli (_Geologia d'Italia_, pte. 3, p. 324) gives the number of killed as more than 12, 300. [9] Mallet does not make use of the term _epicentre_; he speaks of theline FE as the _seismic vertical_. The modern and accepted terms areused above [10] _Japan Seismol. Soc. Trans. _, vol. Xi. , 1887, pp. 175-177. [11] _Ital. Seismol. Soc. Boll. _, vol. Ii. , 1896, pp. 180-188. [12] Professor Omori gives the mean direction as S. 71° W. , but thiswas obtained from observation on lamps with square, as well as withcircular bases. [13] Twelve measurements chosen at random from Professor Omori's listgave a mean direction of S. 78° W. [14] When the accuracy of all the observations seemed equallyprobable, he adopted the mean of the two extremes as the truedirection. [15] If _a_ be the amplitude of a simple harmonic vibration, _T_ itscomplete period, _v_ its maximum velocity, and _f_ its maximumacceleration, we have v=2*pi*a/T and f=4*pi^2*a/T^2 [16] _Earthquakes and other Earth Movements_, pp. 81-82. [17] Obtained from the formula: T=2*pi*a/v=2*pi*x*(1/3)/12 [18] If we take the maximum velocity to be 12 feet per second, and theperiod to be one second, the amplitude would be about 11-1/2 inches. [19] Vol. Ii. , p. 299. The punctuation of the original is not followedin the above extract. [20] _British Association Report_, 1851, pp. 272-320. CHAPTER III. THE ISCHIAN EARTHQUAKES OF MARCH 4TH, 1881, AND JULY 28TH, 1883. Separated from Italy by a distance of not more than six miles, Ischiaand the intermediate island of Procida strictly form part of thePhlegræan Fields, the well-known volcanic district to the north ofNaples. Ischia, the larger of the two islands, is six miles long fromeast to west, and five miles from north to south, and contains an areaof twenty-six square miles. In 1881, the total population was 22, 170, that of Casamicciola, the largest town, being 3, 963. VOLCANIC HISTORY OF ISCHIA. The central feature of Ischia is the great crater of Epomeo (_a_, Fig. 14). On the south side, and partly also on the east, the crater-wallhas been broken down and removed; the portion remaining is about 1-1/2mile in diameter from east to west, and reaches a height of 2, 600 feetabove the sea-level. All the upper part of the mountain is composed ofa pumiceous tufa, rich in sanidine and of a characteristic greenishcolour. At two points, to the west near Forio and to the north betweenLacco and Casamicciola, this tufa is seen reaching down to the sea;but, in all other parts, it is covered by streams of trachitic lava, by more recent tufas, or by a deposit of marly appearance, which isregarded by Fuchs as resulting from the decomposition of the Epomeantufa. There are two distinct periods in the geological history of Ischia. The first, a submarine period, probably began with the dawn of thequaternary epoch, for all the marine fossils of the island belong toexisting species. About this time, Epomeo seems to have originated ineruptions occurring in a sea at least 1, 700 feet in depth--eruptionsthat preceded the formation of Monte Somma and were eithercontemporaneous or alternating with those that gave rise to the oldesttrachitic tufas of the Phlegræan Fields. The destruction of the southwall may have occurred much later through some great eruptiveparoxysm, but more probably, as Professor Mercalli suggests, throughearly marine erosion and subsequent subaerial denudation. To thesubmarine period must also be assigned the formation of the trachiticmasses which compose Monti Trippiti, Vetta, and Garofoli (_b_, _c_, _d_, Fig. 14), on the east side of Epomeo; and, in part only, those ofMonte Campagnano and Monte Vezza (_f_, _g_). At or near the close of the elevation, many violent eruptions occurredon the south-west of Epomeo, during which was formed the south-westcorner of the island, including Monte Imperatore and Capo Sant' Angelo(_h_, _i_). In the second or terrestrial period, when the island had practicallyattained its present altitude, the eruptive activity was almostconfined to the eastern and northern flanks of Epomeo. At thebeginning Monte Lo Toppo (_j_) was formed by a lateral eruption. Inthe north-west corner of the island, Monte Marecocco and Monte Zale(_k_ and _l_) owe their origin to a gigantic flow of sanidinictrachite, issuing probably from the depression which now existsbetween them. Lastly, towards the north-east, are the recent lateralcraters of Rotaro, Montagnone, Bagno, and Cremate (_m_, _n_, _p_, _s_), the first two being the most regular and best preserved in theisland. [Illustration: FIG. 14. --Geological sketch-map of Ischia. (_Mercalli. _)[21]] The earliest eruption of the historic, or rather human, period appearsto have taken place from Montagnone, and probably also at about thesame time from the secondary crater of Porto d'Ischia (_u_), about thebeginning of the eleventh century B. C. The eruptions of Marecocco andZale are referred to about B. C. 470; and those of Rotaro and Tabor(_q_) to between the years 400 and 352 B. C. Another eruption is saidto have occurred in B. C. 89, but the site of it is unknown; and threeothers are recorded on doubtful authority about the years A. D. 79-81, 138-161, and 284-305. The last outburst of all took place after theseries of earthquakes in 1302 from a new crater, that of Cremate(_s_), which opened on the north-east flank of Epomeo, and from whicha stream of lava, called the Arso (_t_), flowed down rapidly and, after a course of two miles, reached the sea. After the first eruptions to which it owed its origin, the centralcrater of Epomeo apparently remained inactive. All the later eruptionsoccurred either on the external flanks of the mountain or on radialfractures of the cone. [22] Trippiti, Lo Toppo, Montagnone and the Lagodel Bagno (_b_, _j_, _n_, _p_) lie in one line, Vetta and Cremate(_c_, _s_) on another, and Garofoli and Vatoliere (_d_, _e_) on athird, all passing through a point near the town of Fontana, whichoccupies the centre of the old crater of Epomeo. Professor Mercalli points out that the lateral eruptions of Epomeodiffer in one respect from those of Etna and Vesuvius. In thesevolcanoes the lava ascends to a considerable height in the centralchimney, and by its own weight rends open the flanks of the cone. InEpomeo, it appears to traverse lateral passages at some depth, perhapsfar below the level of the sea, and to rend the mountain by means ofthe elastic force of the aqueous vapour, etc. , which it contains. Itwill be seen how important is the bearing of this difference on theoccurrence of the Ischian earthquakes. The eruptions that have taken place during the last three thousandyears agree in several particulars. They either occurred suddenly, or, at any rate, were not preceded by a stage of moderate Strombolianactivity; they were always accompanied by violent earthquakes; and allsucceeded intervals of long repose. As the eruption of 1302 happenedafter at least a thousand years of rest, the lapse of six morecenturies does not justify us in concluding that Epomeo is at lastextinct. We seem, on the contrary, to be drawing near another epoch ofactivity. During the four and a half centuries that followed theeruption of 1302, we have no record of Ischian earthquakes. [23] Then, suddenly, on the night of July 28-29, 1762, Casamicciola was visitedby sixty-two shocks, some of which were very strong and damagedbuildings. On March 18th, 1796, another severe shock took place, butdestructive only in the neighbourhood of Casamicciola, where sevenpersons were killed. On February 2nd, 1828, the area of damage, thoughconcentric with the former, enlarged its boundaries; 30 persons werekilled and 50 wounded. On March 6th, 1841, and during the night ofAugust 15-16, 1867, further shocks injured houses at Casamicciola, butwithout causing any loss of life. Slight tremors occurred at variousdates in 1874, 1875, 1879, and 1880, leading up to the disastrousearthquakes here described, those of March 4th, 1881, when 127 personswere killed, and July 28th, 1883, which resulted in the death of 2, 313persons and the wounding of many others. EARTHQUAKE OF MARCH 4TH, 1881. The Ischian earthquakes have been fortunate in their investigators. Inthe spring of 1881, Dr. H. J. Johnston-Lavis, the chronicler for manyyears of Vesuvian phenomena, was residing in Naples. Impressed by arecent perusal of Mallet's report on the Neapolitan earthquake, andwishing to test the value of the methods explained in the lastchapter, he crossed over to Ischia on March 5th; and to his unweariedinquiries extending over more than three weeks and lasting fromthirteen to sixteen hours a day, we are indebted for most of what weknow about the earthquake of 1881. On March 4th, at 1. 5 P. M. , the great shock occurred abruptly, withoutany warning tremors. Its effects were aggravated by the faultyconstruction of the houses. The walls are of great thickness, looselyput together, and connected by mortar of the poorest quality. Thechimneys and roofs also are massive, and the rafters are so slightlyinserted in the walls that they were drawn out with the rocking of thehouses. In such cases, the destruction was often so complete that nofissures were left available for measurement. ISOSEISMAL LINES AND DISTURBED AREA. The isoseismal lines as drawn by Dr. Johnston-Lavis are represented bythe curves in Fig. 15. The isoseismal marked 1 bounds the area ofcomplete destruction; it is about 1 mile long from east to west, 2/3of a mile broad, and contains an area of not more than half a squaremile. The next isoseismal (2) marks the area of partial, but stillserious, destruction; this is nearly 2 miles long from east to west, 1-1/4 miles broad, and 2 square miles in area. Within the isoseismal3, buildings were more or less slightly damaged. The course of thiscurve is somewhat doubtful, but, as drawn, it is about 3 miles long, 2 miles wide, and 5 square miles in area. [Illustration: FIG. 15. --Isoseismal lines of the Ischian earthquake of 1881. (_Johnston-Lavis. _)] Outside the last curve, the shock diminished rapidly in intensity. AtMonte Tabor and Bagno, it was very slight; in the town of Ischia, onlyabout half the people were conscious of any movement; and at Capella, a small village to the south, it was not felt at all. Again, the shockwas perceptible, though only faintly, in the neighbourhood ofCampagnano, at Serrara to the south of Epomeo, and at Panza near thesouth-west corner of the island. On the other hand, at Fontana, whichoccupies approximately the centre of the crater of Epomeo, there wereevidences of a distinctly stronger shock. No house actually fell, andside walls were but little injured; but the roofs, which are of greatweight, suffered considerable injury. In the adjacent island of Procida, the shock was felt distinctly bymany people, and by some, though slightly, at Monte di Procida, Misenum, and Bacoli, on the coast of Italy. No record whatever wasgiven by the seismographs in the university of Naples and theobservatory on Vesuvius. We have of course no means of estimating theexact size of the disturbed area, but in this respect, disastrous asthe earthquake was in the neighbourhood of Casamicciola, it wasclearly inferior to all but the very weakest earthquakes felt in theBritish Islands. POSITION OF THE EPICENTRE. In determining the position of the epicentre, Mallet's method wasclosely followed. Fissures in buildings were used for the most part, in two out of every three cases; and occasional measurements weremade from objects overthrown, projected, or shifted, and also from thepersonal experiences of observers. The attempt to apply the methodwas, however, fraught with difficulties. The heterogeneous structureof the island was no doubt responsible for many divergent azimuths;the irregularity of the buildings both in form and material and theirvariety of site furnished other sources of error; even the smallnessof the area was a disadvantage in lessening the number of trustworthyrecords. Measurements were made at 55 places altogether, but in most cases theywere the results of isolated observations, not the means of several ateach place. On this account, I have not reproduced in Fig. 15 theazimuths shown in Dr. Johnston-Lavis's map of the earthquake. A largenumber of them clearly converge towards an area lying to the west ofCasamicciola; and, from their arrangement, Dr. Johnston-Lavisconcludes, though the evidence does not seem to me quite strong enoughfor the purpose, that they emanated from a fracture running from alittle west of north to a little east of south. This conclusion is, however, justified by other evidence. In thecentre of the injured district, Dr. Johnston-Lavis has traced ameizoseismal band, in which the shock must have been nearly or quitevertical. "The damage inflicted on buildings included within this bandwas, " he says, "very characteristic of the nature of the shock; thewalls having received but slight injury, whilst almost every floor andceiling had been totally destroyed. In fact, " he adds, "many houseswould have required no other repairs than the replacing of thedivisions between the different storeys. " The shaded central area inFig. 15 represents this band, passing in a nearly north and southdirection from a point midway between Campo and the upper part ofLacco on the north, through the west part of Casamenella and Campo, toa point near Frasso on the south; the length of the band being thusabout two-thirds of a mile. If the central line of this band is produced towards the south, asindicated by the dotted line, it grazes the west side of Fontana, where, as we have seen, there was a second meizoseismal area, muchsmaller than the other and surrounded by a district in which houseswere almost uninjured. That the shock in this town was vertical ornearly so, is shown by the nature of the damage (p. 52) and also bythe testimony of the inhabitants. I will give Dr. Johnston-Lavis'sexplanation of this detached meizoseismal area when discussing theorigin of the Ischian earthquakes; but the evidence seems to me tofavour either the existence of two distinct foci or, more probablyperhaps, the extension of the fissure to the south with an increasedimpulse beneath the centre of Epomeo. DEPTH OF THE SEISMIC FOCUS. At nine places, Dr. Johnston-Lavis was able to make measurements ofthe angle of emergence, in every case from fissures in buildings, andtherefore liable to sources of error already referred to. On the otherhand, owing to the small depth of the focus, there would probably beless general refraction of the wave-paths than in the Neapolitanearthquake. The depths indicated by these observations vary betweenabout 615 and 2, 885 feet, a difference that is no greater than mightbe expected, as the size of the focus was no doubt comparable withthat of the district in which observations were made. The mean depthDr. Johnston-Lavis finds to be about 1, 700 feet, or a little less thanone-third of a mile. NATURE OF THE SHOCK. The limited depth of the focus is also evident from the nature of theshock. It was only within the actual meizoseismal band that the shockwas subsultory or vertical throughout; at a short distance from theepicentre, the movement was both subsultory and undulatory; while nearthe third isoseismal, and in most of the region outside, the movementwas entirely undulatory or lateral. An observer at Perrone (which lies1-2/3 miles east of the epicentre) gives the following account of theshock:--"I was standing on my balcony (this faces Casamicciola)admiring the scene ... When I felt the house rock, feeling at the sametime as if something was rolling along beneath the ground. Thismovement was accompanied by a sound like this, Boob, boob -- boob ---- boob -- -- -- boob -- -- -- -- boob. Both noise and movement seemed tocome from Casamicciola.... In a few seconds, in the distance over thetown arose a terrific cloud of white dust, so that I imagined the townon fire.... I felt hardly any, if any, subsultory movement, but as Ileant upon the balcony rails, I was alternately pressed against themand then drawn away. " At Fontana, however, the undulatory shock was replaced by a verticalone. This was the universal experience, though one or two persons felta slight lateral movement immediately after. At Valle (near Barano)and Piejo, both places about a mile from Fontana, the verticalcomponent was also perceptible. AFTER-SHOCKS. The after-shocks were few and of slight intensity. Dr. Johnston-Lavisgives the following dates: March 7th, 12. 5 A. M. And midday; March11-12, 15-16, 17-18, 27 (?), April 5th and 6th, and July 18th, 8. 30P. M. The only shock of the series marked as strong occurred atmidnight on March 15-16 at Casamicciola. The last of all, that of July18th, consisted of a rumble and slight shock, and was most perceptibleat Fango. EARTHQUAKE OF JULY 28TH, 1883. Undeterred by the experience of 1881 or by the warnings ofseismologists, Casamicciola was rebuilt, only to suffer more completedisaster. On July 28th, 1883, at 9. 25 P. M. , occurred the mostdestructive earthquake of which we have any record in Ischia. Theshock lasted about fifteen seconds, and before it was over clouds ofdust were rising above the ruins of Casamicciola, Lacco, and Forio;1, 200 houses were destroyed, 2, 313 persons were killed, nearly 1, 800in Casamicciola alone, and more than 800 seriously wounded. "No betteridea, " says Dr. Johnston-Lavis, "of the absolute destruction ofbuildings could be conceived than what was actually realised atCasamicciola and Campo. Looking, on the following Monday, over thefield of destruction, I could discover (with few exceptions) thewall-stumps only remaining. " Dr. Johnston-Lavis again spent about three weeks in the island, examining the effects of the new shock with equal zeal and widerexperience. His monograph is now our chief work of reference onIschian earthquakes. Inquiries were also made by several Italianseismologists, among others by Professor M. S. De Rossi, the organiserof earthquake-studies in the peninsula; by Professor L. Palmieri, thefounder of the Vesuvian observatory; and especially by Professor G. Mercalli, whose valuable memoir supplements the report of Dr. Johnston-Lavis in some important particulars. PREPARATORY SIGNS. The interval between July 18th, 1881, when the last shock of that yearwas felt, and July 28th, 1883, was one of almost complete quiescence. Early in March 1882, a few slight shocks were noticed at Casamicciola. On July 24th, 1883, a watch hanging from a nail in a wall was seen toswing at 6 A. M. And 9 A. M. , and, on the same morning, at about 8. 30, aslight shock, accompanied by a rumbling sound, was felt atCasamicciola. Again, on the 28th, about a quarter of an hour beforethe great shock, one observer at Casamicciola states that anunderground noise was heard, and that some persons in consequence lefttheir houses. Many assertions have been made with regard to variations witnessed aday or two before the shock in the hot springs, such as an increase offlow or temperature and changes in their volume and purity. Fumarolesare alleged to have burst out with violence, and even flames to havebeen seen. The statements, though widely quoted, can hardly be saidto rest on satisfactory evidence. On the other hand, Dr. Johnston-Lavis arrived in the island within twenty-four hours afterthe shock, and, before another day had elapsed, he had examined mostof the places where the phenomena were said to have occurred, butcould find no remarkable change nor any signs of such having takenplace. It is also known, as he remarks, that the temperature of theIschian springs and fumaroles sometimes varies considerably withoutany earthquake following, that of the water of Gurgitello occasionallychanging by as much as 30° or 40°. We may therefore, I think, concludethat, except for one or two shocks and underground noises too slightto cause general alarm, there were no decisive heralds of the greatearthquake. ISOSEISMAL LINES AND DISTURBED AREA. The curves in Fig. 16 represent the isoseismal lines as drawn by Dr. Johnston-Lavis. As in the earthquake of 1881, they bound respectivelythe areas of complete destruction, partial destruction and slightdamage to buildings, the course of the outer line being to a greatextent conjectural owing to the small extent of land traversed by it. The first isoseismal is about 2-1/2 miles long, 1-1/2 miles broad, and3 square miles in area; the second about 4 miles long, 3-1/2 milesbroad, and 11 square miles in area; and the third about 6-1/2 mileslong, 6 miles broad, and 30 square miles in area. The curve drawn byProfessor Mercalli (Fig. 14) coincides nearly with the second of theselines. At Fontana, the damage exceeded that in the surrounding country, though the difference was of course less marked than on the previousoccasion. [Illustration: FIG. 16. --Isoseismal lines of the Ischian earthquake of 1883. ] Outside Ischia, the shock was felt distinctly in all the island ofProcida and in Vivara; on the mainland, by some as far as Pozzuoliand by several persons in Naples, which is twenty miles fromCasamicciola. The seismograph at the university of this cityregistered two small shocks, the first at 9. 10 P. M. , and the secondand stronger at 9. 25 P. M. ; and De Rossi states that at about 9. 30 P. M. The seismographs at Ceccano, Velletri, and Rome recorded a shockconsisting of very slow undulations. There are again no materials forestimating the size of the disturbed area, but there can be no doubtthat it was much less than that of a moderately strong Britishearthquake. POSITION OF THE EPICENTRE. Owing to the limited size of the disturbed area, time-observations, even had they been available, would not have sufficed to determine theposition of the epicentre, and both Dr. Johnston-Lavis and ProfessorMercalli therefore had recourse to Mallet's method, the former relyingchiefly, as before, on fissures in damaged buildings, and the latteron the overthrow or displacement of columns and other objects. Dr. Johnston-Lavis measured the azimuth of the wave-paths atsixty-five places, and at about one-third of these was able to maketwo or more observations. The azimuths converge towards the sameregion as in 1881, but the area covered by their intersections islarger. The meizoseismal band of maximum vertical destructionindicated by shading in Fig. 16 is also of the same form and slightlygreater extent, reaching from the upper part of Lacco to a littlesouth of Frasso, and being therefore nearly a mile in length. Thecentre of maximum impulse was in the same position as in 1881, orpossibly a little more to the south. Professor Mercalli's observations were made at forty-eight places, andin only six cases were they the same as those used by his predecessor. He also notices that most of the azimuths converge towardsCasamenella, and intersect within an elongated area. This area runs inthe same direction as Dr. Johnston-Lavis's meizoseismal band, but isless elongated, and situated a short distance farther to the south, though on the whole the agreement between the two areas is remarkablyclose. There was again apparently a second epicentre at Fontana. In thistown, according to Dr. Johnston-Lavis, there were two distinct typesof damage. As in 1881, there was evidence of a vertical blow, the onlyone that absolutely ruined houses; but, in addition, there was anotherindependent set of fissures, quite as widely distributed as theothers, though evidently caused by a less violent movement. Theseindicated a wave-path with a low angle of emergence coming frombetween north and north-north-west, or almost exactly in the line ofmeizoseismal band. To the south of Fontana, however, there is a groupof places, including Panza, Serrara, Barano, etc. , where the azimuthsdiverged rather widely from the epicentre at Casamenella. Theseazimuths are twelve in number, and it is worthy of notice that theyall intersected the crater of Epomeo, while half of them passed withina few hundred yards of Fontana. DEPTH OF THE SEISMIC FOCUS. Measurements of the angle of emergence were made by Dr. Johnston-Lavisat twenty-four places, and in every case from fissured walls. Thegreater part of the diagram on which his results are depicted isreproduced in Fig. 17. The horizontal line, as in Fig. 13, representsthe level of the sea, the longer vertical line one passing through theepicentre, and the shorter another through Fontana. The short lineson the left of the former show the incipient wave-paths to placeslying east of the epicentre; those on the right, with one exception, represent the wave-paths to places west of the same meridian. Smallhorizontal marks are inserted on the vertical lines to show the depthin tenths of a mile below the level of the sea. [Illustration: FIG. 17. --Diagram of wave-paths at seismic vertical of Ischian earthquake of 1883. (_Johnston-Lavis. _)] The six angles of emergence that would give the greatest depth belowthe epicentre were all measured at places in the south of the islandclose to the line joining Panza and Barano, and it will be noticedthat five of these apparent depths are much greater than thoseobtained from the other wave-paths. Excluding these observations, theremaining eighteen give depths ranging from about 450 to about 3, 350feet, and a mean depth of 1, 730 feet, [24] or nearly one-third of amile, that is, almost exactly the same as the mean depth found fromthe earthquake of 1881. The six exceptional angles of emergence come from the district ofdivergent azimuths to the south of Epomeo. Three of the correspondingazimuths pass within one-quarter of a mile from the centre of Fontana, and none of the other three more than three-quarters of a mile fromthe same point. Though disbelieving in a subsidiary focus below thistown, Dr. Johnston-Lavis has calculated its mean depth, supposing itto exist, and found it to be about 1, 560 feet below the sea level, aresult which is remarkably close to the calculated mean depth of thefocus near Casamenella. NATURE OF THE SHOCK. In the meizoseismal band, preliminary tremor and rumbling sound werealike absent. So sudden, indeed, was the onset of the earthquake, thatthe survivors generally found themselves beneath the ruins of theirhouses before they were conscious of any shock. The destruction, practically instantaneous, was wrought by four or five vertical blows, so powerful that, according to some observers, Casamicciola seemed tojump into the air. Then followed undulations, not noticed by all, thatappeared to come from every direction. The shock lasted altogetherfifteen seconds or more, [25] and was accompanied by a rumbling noise, in the midst of which were detonations as of thunder or of great blowsgiven upon an empty barrel. In the immediate neighbourhood of the meizoseismal area, at Perrone, Pennella, and Lower Lacco, the subsultory movement was still the moreprominent; but, farther away, as at Panza, Testacchio, Barano, Ischia, and Bagno, the subsultory motion was followed by distinctly horizontalundulations, while outside the island of Ischia only slow undulatorymovements were perceptible. LANDSLIPS. The dotted areas in Fig. 16 indicate the sites of the only landslipsof importance that were precipitated by the earthquake of 1883. Two ofthese occurred on the north slope of Epomeo, and the third on thewest flank of Monte Rotaro. The materials of the Epomean landslips hadevidently been separated for some time by shallow fissures from theadjoining rock, for the surfaces of the fissures were discoloured byfumarolic action. Immediately after the earthquake a cloud of dust wasseen to rise from the spots; the masses, already detached laterally, were merely set in motion by the shock; and they continued to slidedown during the following days either through the action of theafter-shocks or of the heavy rains that followed. All over the island, however, fissures and minor landslips occurred. At two places on the north coast the steep cliffs of incoherent tufawere so much damaged that, according to Dr. Johnston-Lavis, "largequantities of their materials were thrown into the sea. The water thensorted out the pieces of pumice, which in many cases were of verylarge size, and were seen floating about in the neighbourhood for somedays, " giving rise to the supposition that a submarine eruption hadtaken place to the north of the island. AFTER-SHOCKS. The after-shocks in 1883 were much more numerous than in 1881. Between9. 25 P. M. On July 28th and noon on August 3rd, twenty-one slightshocks were recorded at Casamicciola. At 2. 15 P. M. On August 3rd, aviolent shock occurred that caused further damage at Forio, and evenat places so far from the epicentre as Fiaiano, Barano, and Fontana, and increased the displacements of the landslips on Epomeo. Thisshock was also registered at the observatory on Vesuvius. After this the shocks became less frequent and slighter, twelve beingfelt at Casamicciola during the remainder of the year, and six in thefirst half of 1884. Several shocks and rumbling noises were alsoobserved in other parts of the island. Among them may be mentionednoises heard at Fontana on August 12th and 15th, and a slight shock atthe same place on August 17th; also on September 4th, at 10. 30 and10. 40 A. M. , slight shocks at Barano, Serrara, and Forio. On March27th, 1884, at 2. 7 P. M. , another strong shock occurred; strongest atSerrara, where the shock was subsultory and accompanied by noise; andless strong, though still subsultory, at Ciglio, Panza, Forio, Fiaiano, and Casamicciola, and very slight at Ischia. The series seemsto have ended during the following summer, with a slight shock atCasamicciola on July 21st, and a stronger one on July 23rd, felt fromCasamicciola on the north to Serrara on the south. Most of the after-shocks must have originated in the neighbourhood ofCasamicciola, but it is worthy of notice that more than one centre wasin action. Several were recorded at Ischia only. Others, as mentionedabove, affected chiefly the south part of the island, and especiallythe small towns of Serrara and Fontana. CHARACTERISTICS OF ISCHIAN EARTHQUAKES. After the eruption of 1302, there succeeded a period of comparativerepose in Ischia. The revival of activity dates from 1762, and, sincethat year, there have been four great earthquakes, namely, those of1796, 1828, 1881, and 1883. In every respect but that of increasingintensity, these earthquakes were apparently identical; each, asProfessor Mercalli says, was merely a replica on a different scale ofthose that preceded it. The principal features in which they resembleone another, and differ from the average tectonic earthquake, are thecoincidence of the epicentres, the small depth of the foci, and thesudden onset of the principal shock. 1. _Coincidence of Epicentres. _--In Fig. 14, which is copied fromProfessor Mercalli's map, are shown the areas in which buildings wereseriously damaged by these four earthquakes. The curves for 1796, 1828, and 1881 are approximately concentric. In 1796, the shock wasdisastrous only to the west of Casamicciola; in 1828, according toCovelli, "the ground most injured was not precisely the region ofCasamicciola, but that which lies between the district called Fangoand that known as Casamenella, situated to the west of Casamicciola, and a short distance from it. "[26] The epicentres may have variedslightly in size, but, in position, it is clear that all four werenearly or quite coincident. The meizoseismal bands in 1881 and 1883were also similar in form and elongated in the same direction. In the last two earthquakes there was, as we have seen, very distinctevidence of a secondary meizoseismal area surrounding Fontana, and itis remarkable that this was also noticeable in the earthquake of 1828. "Besides the centre of vibration in the district of Fango, " saysCovelli, "another less powerful centre showed itself in the localityof Fontana; this made itself felt more heavily than in surroundinglocalities; as if another centre of movement had taken place from thatpart, independent of the former. " 2. _Small Depth of the Foci. _--Mallet's method, as noted above, cannotbe trusted to yield accurate estimates of the focal depth, or toindicate more than its order of magnitude. But it is remarkable thatthe depths calculated by Dr. Johnston-Lavis for the last twoearthquakes are both only a little less than a third of a mile, and itis probable that the actual depth did not differ very greatly fromthis amount. The nature of the shock, vertical or nearly so close tothe epicentre and horizontal at a short distance from it, is merelypersonal testimony of the same character as fissures in masonry, andof course points to the same result. [Illustration: FIG. 18. --Diagram showing connection between depth of focus and rate of decline in intensity. ] But the most conclusive evidence on which we have to rely is theextraordinary intensity of the shock at the centre of a very smalldistributed area. In Great Britain, an earthquake felt over a districtof equal size would hardly at the centre exceed the trembling producedin a station platform by a passing train. The curves in Fig. 18 showhow the rate of decline in intensity depends on the depth of thefocus. They are drawn on the supposition that the intensity at anypoint on the surface varies inversely as the square of its distancefrom the focus; the curves _a_, _b_, _c_ corresponding to focisituated at depths of one-third of a mile, one mile, and two milesrespectively, and the figures below the horizontal line denoting thedistance in miles from the epicentre. Thus, the rapid decline ofintensity from the epicentre outwards shows that, in each of the fourgreat Ischian earthquakes, the depths of the focus must have been verysmall. 3. _Suddenness of the Shocks. _--In 1796, we have no record ofpreparatory shocks, but the evidence is scanty; in 1828 and 1881, noneare mentioned; in 1883, one or two tremors and underground noises, possibly of seismic origin, gave warning to a few. Fore-shocks, forall practical purposes, were conspicuous by their absence. Still more remarkable is the sudden advent of the great shocks. Therewere no preliminary tremors or rumbling sound, no animals showed signsof uneasiness and no birds fluttered screaming from trees or ground. The shock of 1828, says Covelli, "was announced by three powerfulblows coming almost vertically, from below upwards;" and the samewords apply equally well to the earthquakes of 1881 and 1883. Thedestruction of houses in every case was practically instantaneous, andcoincident with the first vibration. In all respects, tectonic earthquakes differ widely from the Ischianshocks. The epicentres of successive earthquakes are rarelycoincident, but show a distinct tendency to migration along certainlines; the decline in intensity outwards from the epicentre is nearlyalways very gradual, and therefore indicative of a comparativelydeep-seated focus; they are almost invariably preceded either by aseries of slight shocks and rumbling sounds, or, in an unstabledistrict, by a marked increase in their frequency. Distinctions, sogreat as these are, evidently remove the Ischian shocks from thecategory of tectonic earthquakes. ORIGIN OF THE ISCHIAN EARTHQUAKES. On the other hand, the Ischian earthquakes possess several featureswhich connect them closely with true volcanic earthquakes. 1. They originate beneath the northern slope of Epomeo--a volcano thatwe have no reason to consider absolutely extinct, but rather as onesubject to eruptions at long intervals of time--in a region as yetunoccupied by parasitic craters, but having the same relation to thecentral cone of Epomeo as those in which the recent craters of MonteRotaro, Montagnone and Cremate are situated. 2. In both the earthquakes of 1881 and 1883, the epicentre is anelongated band, the axis of which, if produced, would pass through thecentre of the old crater of Epomeo. Along the line of this band, occurthe fumaroles of Monte Cito and Ignazio Verde and the thermal springsof the Rita and Capitello. These facts, as Professor Mercallisuggests, lead us to believe that the foci of the earthquakes coincidewith a radial fracture of the volcano, the course of which, as tracedby him, is represented by the continuous line in Fig. 14. [27] 3. Except in their relations with actual eruptions, the Ischianearthquakes resemble closely the true volcanic earthquakes which fromtime to time shake the flanks of Etna. These are marked by greatintensity of the shock at the centre of a comparatively smalldisturbed area, epicentres often elongated radially to the cone, frequent repetition with similar characters in the same districts; andas a rule they precede by a short interval, but sometimes accompany orfollow, volcanic eruptions. [28] Two other phenomena may be referred to as probably indicating someconnection between Ischian earthquakes and the structure and historyof Epomeo. We have seen that, in the three earthquakes of 1828, 1881, and 1883, there is distinct evidence of a second meizoseismal area at Fontana, within which the shock was mainly subsultory. Dr. Johnston-Lavis, though recognising the possibility of the existence of two epicentres, prefers another explanation. [29] But the wide extension of thesouthern boundary of the area of destruction in 1883, and thelimitation of several of the after-shocks to the south of the island, seem to me to favour the existence of a second focus beneath thecrater of Epomeo, though, it may be, not entirely detached from thechief focus beneath Casamenella. Again, as Professor Mercalli remarks, all historic eruptions on theflanks of Epomeo were accompanied by very violent earthquakes; while, previously to 1302, only one disastrous earthquake, so far as known, occurred in the island without being attended by an eruption. Itshould be noticed also that the principal shocks during the recentrevival of activity (_i. E. _, since 1762) show a continual increase inintensity, whether this be measured by the damage to buildings, theloss of life, or the extent of the area of destruction (Fig. 14). It therefore seems legitimate to conclude that, in the recent Ischianearthquakes, we have merely so many unsuccessful attempts to force anew volcanic eruption. The passages once existing through Epomeo andits parasitic craters having become blocked, the highly heated magmabeneath is compelled to find a new outlet. Its tension slowlyincreasing, the crust above is at last rent, or an incipient rent isenlarged, the fluid rock is injected almost instantaneously with greatforce into the open fissure, and its sudden arrest by the containingwalls is the ultimate cause of an earthquake. With the expansion ofthe magma, its tension is at once correspondingly reduced, and sometime must elapse before it can again reach the critical point at whicha further rupture, resulting in a second shock, takes place. [30] Thus, with each great Ischian earthquake, we are, I believe, advancinga step nearer the time, which may be close at hand or may be veryremote, when the fracture will at last reach the surface, and abovethe site of Casamenella a new parasitic cone will rise, from which, asfrom Cremate in 1302, a stream of lava may flow down towards the sea. REFERENCES. 1. BALDACCI, L. --"Alcune osservazioni sul terremoto avvenuto all' Isola d'Ischia il 28 luglio 1883. " _Ital. Com. Geol. Boll. _, vol. Xiv. , 1883, pp. 157-166. 2. DAUBRÉE, A. --"Rapport sur le tremblement de terre ressenti à Ischia le 28 juillet, 1883; causes probables des tremblements de terre. " Paris, _Acad. Sci. _, _Compt. Rend. _, vol. Xcvii. , 1883, pp. 768-778. 3. DU BOIS, F. --"The Earthquakes of Ischia. " _Japan Seism. Soc. Trans. _, vol. Vii. , pt i. , 1883-84, pp. 16-42. 4. ---- "Farther Notes on the Earthquakes of Ischia. " _Ibid. _, vol. Viii. , 1885, pp. 95-99. 5. JOHNSTON-LAVIS, H. J. --_Monograph of the Earthquakes of Ischia_ (1885). 6. MERCALLI, G. --_Vulcani e fenomeni vulcanici in Italia_ (vol. Iii. Of _Geologia d'Italia_, by G. Negri, A. Stoppani, and G. Mercalli), 1883, pp. 46-50, 331-332. 7. ---- _L'Isola d'Ischia ed il terremoto del 28 luglio 1883_ (Milano, 1884). 8. PALMIERI, L. , E A. OGLIALORO. --"Sul terremoto dell' Isola d'Ischia della sera del 28 luglio 1883. " Napoli, _R. Accad. Atti_, vol. I. , 1884, pp. 1-28. 9. ROSSI, M. S. DE. --"Il terremoto di Casamicciola del 4 marzo 1881. " _Bull. Del Vulc. Ital. _, anno viii. , 1881, pp. 5-12. (In the same volume are brief notices by different writers on pp. 22, 38-42, 52-53, 67-68, 70-74. ) 10. ---- "Raccolta di fatti, relazioni, bibliografie sul terremoto di Casamicciola del 28 luglio 1883, con brevi osservazioni. " _Bull. Del Vulc. Ital. _, anno xi. , 1884, pp. 65-172. 11. ---- "Intorno all' odierna fase dei terremoti in Italia e segnatamente sul terremoto in Casamicciola del 4 Marzo 1881. " _Ital. Soc. Geogr. Boll. _, 1881. 12. SERPIERI, A. --"Sul terremoto d'Ischia il 28 luglio 1883. " _Scritti di Sismologia_, Pte. Ii. , pp. 207-216. 13. ---- "Sul terremoto dell' Isola d'Ischia il 28 luglio 1883. " _Ibid. _, pp. 217-232. FOOTNOTES: [21] The shaded areas indicate the principal trachytic masses, thebroken lines represent the boundaries of the craters that are stillrecognisable, and the dotted lines the boundaries of the areas withinwhich buildings were damaged by the earthquakes of 1796, 1828, 1881, and 1883 (according to Mercalli). The continuous curved line shows theposition of the radial fracture with which the earthquakes wereprobably connected. The trachytic masses and craters are denoted bythe following tables:-- _a. _ Epomeo. _b. _ Trippiti. _c. _ Vetta. _d. _ Garofoli. _e. _ Vatoliere. _f. _ Campagnano. _g. _ Vezza. _h. _ Imperatore. _i. _ C. St. Angelo. _j. _ Lo Toppo. _k. _ Marecocco. _l. _ Zale. _m. _ Rotaro. _n. _ Montagnone. _p. _ Bagno. _q. _ Tabor. _r. _ P. Castiglione. _s. _ Cremate. _t. _ Arso. _u. _ Porto d'Ischia. [22] It is possible that Monte Campagnano may form an exception tothis statement. [23] Shocks were felt in the island in 1559 and 1659, but one at leastwas of external origin. [24] Prof. Mercalli, from the five estimates of the angle of emergencewhich he considered most reliable, found the mean depth to be about3, 280 feet. [25] Professor de Rossi estimated the mean duration as not muchexceeding ten seconds. Dr. Johnston-Lavis, on the other hand, considers the general estimate of fifteen seconds as far too low. Inone case, at Casamicciola, he ranks it as high as thirty-one seconds. [26] Quoted from the useful translation of Covelli's memoir given byDr. Johnston-Lavis. [27] Baldacci supposes that the thermal springs and fumaroles ofForio, Stennecchia, Montecito, Casamicciola, and Castiglione lie alonga tangential fracture starting from Forio and passing by Casamicciolato near Punta di Castiglione. Mercalli, however, argues forciblyagainst this inference. [28] Professor Mercalli adds, as a fourth point of contact betweenIschian earthquakes and volcanic phenomena, the changes in thefumaroles and hot springs which preceded or accompanied or followedthe earthquakes of 1828, 1881, and 1883. [29] "Fontana, " he says, "occupies the centre of the great crater ofEpomeo... , and therefore lies immediately over the ancient chimney, which in all probability is filled by an old plug of consolidatedtrachyte, which must descend to the igneous reservoir. Any mass ofigneous matter, that might determine the further rupture of acollateral fissure, would result in the conduction of any changes ofpressure or vibrations, along the column of highly elastic trachyte;whilst the same earth-waves would be annulled or absorbed by theinelastic tufas surrounding it, so that the blow would be struckperpendicularly to the surface, and in a small area with well definedlimits. The undulatory sensations, after the principal local shock, were those that arrived from the great centre of impulse beneathCasamenella. " [30] The above paragraph is a summary of the reasoning stated withadmirable clearness by Dr. Johnston-Lavis. It should be mentioned thatthe late Professor Palmieri, relying on the extremely limiteddisturbed area, dissented from this view; but his difficulty is met bysupposing the focus to be small as well as shallow, a supposition thatis supported by the shortness of the meizoseismal band, as well as bythe elongation of the isoseismal lines in the direction perpendicularto this band. CHAPTER IV. THE ANDALUSIAN EARTHQUAKE OF DECEMBER 25TH, 1884. In most countries the principal seismic districts are of limitedextent. Thus, in central Japan, the east coast is frequently visitedby earthquakes, while the west coast is relatively undisturbed. Of theearthquakes felt in the kingdom of Greece during the years 1893-98, 63per cent. Were observed in Zante, and were for the most part confinedto that island. In the interior of the Iberian peninsula--in Leon andin New and Old Castile--destructive earthquakes are practicallyunknown; while the littoral regions of central and southern Portugal, Andalusia, and Catalonia are noted for their disastrous shocks. During the eighteenth century seismic activity was chieflyconcentrated in Portugal, and culminated in the great Lisbonearthquake of 1755. In the following century the seat of disturbancewas transferred from the west to the south of the peninsula; Portugalremained throughout in comparative repose, while Almeria experienceddestructive shocks in 1804, 1860, and 1863, and Murcia in 1828-29 and1864, leading up to the Andalusian earthquakes of 1884-85, describedin the present chapter. The preparation for the principal earthquake of December 25th, 1884, was unusually indistinct. For a day or two before, shocks were felthere and there in Andalusia, but so weak were they that they passedalmost unperceived. During the night of December 24-25, one slightshock was noticed at Colmeñar (Fig. 19) and another at Zafarraya. Onthe 25th, a faint movement of the ground was noticed at Malaga, and afew weak tremors at Periana; and shortly after came the great shock atabout 8. 50 P. M. Mean time of Malaga, or about 9. 8 P. M. Greenwich meantime. This earthquake was investigated by no fewer than three officialcommittees. The first in the field was nominated by the SpanishGovernment on January 7th, 1885, and consisted of four members, thePresident being Señor M. F. De Castro, the director of the GeologicalSurvey of Spain. The report of this commission was presented to theMinister of Agriculture, etc. , on March 12th. Early in February aFrench Commission, appointed by the Academy of Sciences, proceeded tothe scene of the disaster. With Professor F. Fouqué as chief, and MM. Lévy, Bertrand, Barrois, Offret, Kilian, Bergeron, and Bréon asmembers, this committee resolved itself after a time into one forstudying the geology of the central area; and, of their voluminousreport of more than 700 quarto pages (published in 1889), only 55 areimmediately concerned with the earthquake. At the beginning of April, Professors Taramelli and Mercalli, sent by the Italian Government, arrived in Andalusia; and their memoir, read a few months later beforethe Reale Accademia dei Lincei, forms by far the most valuablecontribution to our knowledge of the earthquake. DAMAGE CAUSED BY THE EARTHQUAKE. The meizoseismal area (see Figs. 19 and 20) lies in a mountainousdistrict, almost equidistant from the cities of Malaga and Granada. Inthis area, which contains nearly 900 square miles, the shock wasdisastrous to all but well-built houses. Whole villages wereoverthrown. In the surrounding zone many buildings escaped seriousdamage, and only a few were completely destroyed. It is estimated bythe Spanish Commission that, in the province of Granada, 3, 342 houseswere totally, and 2, 138 partially, ruined; in the province of Malaga, 1, 057 houses were totally, and 4, 178 partially, ruined; while in thetwo provinces together 6, 463 houses were damaged; making a total of17, 178 buildings more or less seriously injured. As usual in the South of Europe, bad construction and narrow streetswere largely responsible for the loss of property, houses that wereregularly built and made of good materials being only slightlyinjured. But, in this case, the great slope of the ground, the badquality of the foundations, and the nature of the underlying rockswere contributing factors. Many buildings also had been damaged byprevious shocks, and their ruin was only completed by the earthquakeof 1884. The total loss of life is variously estimated. According to theSpanish Commission, 690 persons were killed and 1, 426 wounded in theprovince of Granada, while 55 were killed and 59 wounded in that ofMalaga, making a total of 745 persons killed and 1, 485 wounded. TheItalian seismologists, having additional materials at their disposal, raise the total figures to 750 persons killed and 1, 554 severelywounded. Careful inquiries were also made on this subject by theconductors of the newspaper _El Defensor de Granada_. In Granadaalone, they reckon that 828 persons were killed and 1, 164 wounded. From the table given in the Italian report, it appears that 330persons were killed at Alhama, 118 at Arenas del Rey, 102 atAlbuñuelas, 77 at Ventas de Zafarraya, and 40 at Periana; thepercentage of mortality being 9 at Arenas del Rey, about the same atVentas de Zafarraya, and 3 or 4 at Alhama, Albuñuelas and Periana. Comparing these latter figures with the death rates of 71 per cent. AtMontemurro, caused by the Neapolitan earthquake, and of about 45 percent. At Casamicciola, by the Ischian earthquake of 1883, it will beseen that the loss of life during the Andalusian earthquake wascomparatively small--an exemption which is attributed by the Italiancommissioners to the absence of inhabited places from the immediateneighbourhood of the epicentre, and to the fact that the destructivevibrations occurred towards the end of the shock, thus allowingopportunity for escape. ISOSEISMAL LINES AND DISTURBED AREA. Fig. 19 shows the principal isoseismal lines as drawn by the Italiancommissioners. The meizoseismal area, which included all places atwhich the shock was disastrous, is bounded by an ellipse (marked 1 onthe map) 40 miles long from east to west, 28 miles wide, and about 886square miles in area. The next isoseismal (2) includes the places inwhich some buildings were ruined, but not as a rule completely, and inwhich there was no loss of life. Its bounding line is also elliptical, the longer axis being about 71 miles long and running nearly east andwest. Towards the south this zone is interrupted by the sea. It willbe noticed that these isoseismals are not concentric, the secondextending much farther to the west and south-west than in theopposite direction. A third isoseismal (not shown in the map) enclosesthe district in which the shock was "very strong, " or just capable ofproducing cracks in the walls of houses. It is similar in form to thesecond isoseismal, reaching as far as Estepone to the south-west, Osuna, Cordova, and Seville to the west, Jaen to the north, whiletowards the east it stops short of Almeria. [Illustration: FIG. 19. --Isoseismal lines of Andalusian earthquake. (_Taramelli and Mercalli. _)] The French Commission have also published a map of the earthquake, and, though the work of an experienced seismologist like ProfessorMercalli is probably more trustworthy, it is interesting to comparehis isoseismal lines with those obtained by his French colleagues, which are reproduced in Fig. 20. The curves in this figure are drawnso as to include the places that were, respectively, ruined, seriouslydamaged, and slightly damaged, by the shock. They should thereforecorrespond with the lines in Fig. 19. It will be seen that they differconsiderably in form, but at the same time they present certain pointsof agreement, such as the east and west elongation of the meizoseismalarea, and the great extension of the two outer isoseismals towards thewest and south-west The greatest difference is to be found in theeastern portion of the third isoseismal, which, according to theItalians, extends beyond the limits included in Fig. 20, and, according to the French, is bayed back by the great masses of theSierra Nevada. Outside Andalusia the earthquake was sensibly felt to the north as faras Madrid and Segovia, to the west at Huelva, Cárceres and Lisbon, andto the east at Valencia and Murcia. Towards the south, the greaterpart of the disturbed area was cut off by the Mediterranean, and thereare no records forthcoming from the opposite coast of Africa. Thetotal area disturbed by the earthquake is roughly estimated by theFrench Commission at about 154, 000 square miles, and by the ItalianCommission at about 174, 000 square miles; but, as the shock wasstrong enough to stop clocks and ring bells at Madrid, it is evidentthat even the greater of these values is too small. [Illustration: FIG. 20. --Isoseismal lines of Andalusian earthquake. (_Fouqué, etc. _)] THE UNFELT EARTHQUAKE. Far beyond the limits of the disturbed area, however, the long slowwaves sped over the surface, disturbing magnetographs and otherdelicate instruments. More than a century before, the great Lisbonearthquake of 1755 had caused oscillations in Scottish lakes, and onother occasions the effects of remote earthquakes had been witnessedat isolated places. But, in 1884, the concurrent registration of theAndalusian earth-waves at distant observatories attracted generalattention, and in part suggested the world-wide network ofseismological stations, the foundation of which was laid beforeanother decade had passed. In Italy, probable records of the earthquake were obtained at twoobservatories, but, owing to the approximate times given, theirconnection with it is not established. At Velletri, near Rome, Professor Galli's seismodynamograph registered a very slight movementat 10 P. M. , and at Rome itself Professor de Rossi found a tromometermaking unusual oscillations at 10. 15 P. M. [31] The most interesting records, however, are those furnished by themagnetographs at Lisbon, Parc Saint-Maur (near Paris), Greenwich, andWilhelmshaven. At Lisbon, the records are extremely clear. The curvesof the declination, horizontal force and vertical force magnets, asseen in Fig. 21, are abruptly broken at 8. 33 P. M. (Lisbon time, or 9h. 9m. 45s. , G. M. T. ). The disturbances, which are greatest on thedeclination curve and least on the vertical force curve, lasted in allthree for about 12 minutes, and are quite distinct from the ordinarymagnetic perturbations. At Parc Saint-Maur, the magnetographs seem tobe ill-adapted to act as seismographs, for only a slight mark wasdiscovered on a re-examination of the curves, beginning at 9. 24 P. M. (Paris time, or 9h. 14m. 39s. , G. M. T. ) At Greenwich, Mr. W. Elliswrites, there was "a small simultaneous disturbance of the declinationand horizontal force magnets, occurring at 9h. 15m.... Both magnetswere at this time set into slight vibration, the extent of vibrationin the case of declination being about 2' of arc, and in horizontalforce equivalent to . 001 of the whole horizontal force nearly. " Of thethree instruments at Wilhelmshaven, only one showed any movement atthe time of the earthquake. The declination magnet was undisturbed, the horizontal force curve was accidentally interrupted, but thevertical force curve indicated a very perceptible shock. Beginning at9. 52 P. M. (Wilhelmshaven mean time, or 9h. 29m. 29s. , G. M. T. ), thecurve was broken for four minutes, for the rapid swinging of theneedle could not be registered until the motion became fainter. Further disturbances also occurred at 9. 59, 10, 10. 2, and 10. 5P. M. [32] [Illustration: FIG. 21. --Magnetograph records of Andalusian earthquake at Lisbon. (_Fouqué, etc. _)] POSITION OF THE EPICENTRE. The innermost isoseismal being too large, and the time-records tooinaccurate, to give the position of the epicentre, both Commissionsresorted to observations of the direction, Professor Fouqué and hiscolleagues depending chiefly on the oscillation of hanging lamps, andProfessors Taramelli and Mercalli on the fall or displacement ofstatues and other objects, and all avoiding as far as possible theevidence of fissures in buildings. The Italian observers point out that, among the divergent directionsvisible at any place, there is generally one more distinctly markedthan the others, and this, they consider, corresponds to the movementcoming almost directly from the centre of disturbance. Plotting thesedirections (36 in number), they find that they converge as a rulewithin the triangle formed by joining Ventas de Zafarraya, Alhama, andJatar, while a large number of them traverse the elliptical area, whose boundary is represented by the dotted line in Fig. 19. This areais about 9 miles long and 2-1/2 miles wide, its longer axis runsnearly east and west, and its centre coincides with the western focusof the ellipse which forms the boundary of the meizoseismal area. Itlies, moreover, close to Ventas de Zafarraya and Arenas del Rey, thetwo places where the seismic death-rate was highest, while its majoraxis almost coincides with the line joining them. The evidence of hanging lamps collected by the French Commission wasmore consistent than that of the fallen objects. At every place, theplane in which the lamps oscillated was nearly constant, thedeviations being generally attributable to irregularities in the modeof suspension. The azimuths again intersect within an elliptical area, which, according to the Commission, differs little from the centralregion of the earthquake (Fig. 20). It Is clear, however, from the mapaccompanying the French report, that the majority converge towards anarrow band extending east and west from near Arenas del Rey to nearVentas de Zafarraya, and therefore agreeing closely with theepicentral area as determined by Professors Taramelli andMercalli. [33] DEPTH OF THE SEISMIC FOCUS. If the depth of the seismic focus amounts to several miles, one of themost serious objections to Mallet's method lies in the varyingrefractive power of the different strata traversed by the earth-waves(p. 28). At present we have no way of meeting this objection, and allcalculations of the depth of the focus are therefore more or lessdoubtful. A difficulty in practice has also been urged, depending onthe widely differing inclinations of the fractures at any place; butthe Italian observers found that the errors from this source weregreatly reduced by avoiding all fissures in poorly-built houses, orwhich start from windows or other apertures, and selecting only thosewhich occur in homogeneous walls directed towards the epicentre. Thebest angles of emergence thus measured by them are thirteen in number, all made at places lying within 5 and 23 miles from the centre of theepicentral area, and, with two exceptions, inside the meizoseismalzone (Fig. 19). The depths corresponding to the different wave-pathsvary from 5. 3 to 23. 0 miles, the mean depth of the focus given by allthirteen observations being 7. 6 miles. The only estimate made by the French Commission--and it is one thatthey rightly regarded with considerable doubt--was based on a methoddevised by Falb. As the sound generally precedes the shock, Falbassumes that it travels with a greater velocity. If the velocities ofboth series of waves are known, and if they start at the same instantand from the same region, the interval that elapses between thearrivals of the sound and shock should give the distance traversed bythem and consequently the depth of the focus. It is unnecessary tomention more than two of the serious objections to this method. Theduration of the preliminary sound should increase rapidly with thedistance from the focus, and of this there is not the slightestevidence. Moreover, the sound-vibrations that are first heard do notnecessarily come from the same part of the focus as those which causethe shock, but, as will be seen in Chapter VIII. , probably from itsnearer lateral margin. The French Commission, finding the averageduration of the fore-sound near the epicentre to be 5 seconds, estimate the depth of the focus at about 7 miles--a result whichagrees remarkably with that obtained from the angles of emergence, butwhich is not, on that account, entitled to credit. NATURE OF THE SHOCK. In the nature of the shock, there was a singular uniformity throughoutthe whole disturbed area, the chief variation noticed being evidentlydependent on the observer's distance from the epicentre. For instance, in the meizoseismal area (Fig. 19), at Ventas deZafarraya, a loud sound like thunder was first heard, and before itceased there came a violent subsultory movement preceded by a verybrief oscillation, then a pause of one or two seconds, and lastly amore intense and longer series of undulations, the whole movementlasting 12 seconds. At Cacin, three phases were distinguished, thefirst a slight undulatory movement coincident with the sound, followedimmediately by the subsultory motion, a pause, and strongerundulations, the total duration being 15 seconds. The variationsnoticeable in this zone seem to have been apparent only, sensitiveobservers perceiving a tremulous motion before the verticalvibrations, and in the pause between them and the concludingundulations. In both phases, the intensity increased to a maximum andthen gradually decreased. The movement at Ventas de Zafarraya andCacin is represented by Professors Taramelli and Mercalli by thecurves _a_ and _b_ in Fig. 22. In the second zone (Fig. 19), the same two phases were universallyobserved, but the subsultory movement was less pronounced or themovement was partly subsultory and partly undulatory, and occasionallyboth phases are described as undulatory. The motion near Malaga isrepresented by the curve _c_ in Fig. 22. [Illustration: FIG. 22. --Nature of shock of Andalusian earthquake. (_Taramelli and Mercalli. _)] Outside the ruinous zone, the first phase rapidly lost what remainedof its subsultory form, and the pause between the two parts wasnoticeably longer than near the epicentre. Thus, at Seville andCordova, two shocks were felt, separated by an interval of someseconds; the second according to some observers at Seville, terminating with vertical tremors. At Madrid, also, the two parts wereperceived, the interval between them being 3 or 4 seconds in length;but, as a rule, outside Andalusia, only a single undulatory shock wasfelt, without any preliminary sound. That the changes observed in the shock were merely an effect of lessor greater distance, will be obvious from Fig. 23, in which theintensity at any moment is that represented by the distance of thecorresponding point on the curve from the different base-lines, thebase-line _a_ corresponding to a place near the epicentre, and _b_, _c_, _d_, etc. , to places at gradually increasing distances. Thus, ata place corresponding to the base-line _b_, the intensity of thetremors during the intervening pause (represented by the short linePN) was so slight that they frequently escaped notice, while thepreliminary tremors observed by some near the epicentre werealtogether imperceptible. At the places corresponding to thebase-lines _c_, _d_, _e_, _f_, the duration of the whole shock and ofeach part gradually diminished, while the interval between the twoparts increased owing to the gradual extinction of the finalvibrations of the first part and of the initial vibrations of thesecond. At the farthest of these places (_f_) the first part was soweak that it sometimes passed unobserved. Lastly, at a placecorresponding to the base-line _g_, the first part was imperceptibleto all observers, and the shock consisted of a single series ofhorizontal undulations. [Illustration: FIG. 23. --Diagram to illustrate variation in nature of shock of Andalusian earthquake. ] _Origin of the Double Shock. _--If the double shock were observed atonly a few places, we should naturally look for some local explanationof the peculiarity. The second shock, for instance, might be asubterranean echo, the earth-waves being reflected at the boundingsurface of two different kinds of rock. In the case of the Andalusianearthquake, such an explanation is precluded by the almost universalobservation of the double shock, the greater intensity of the secondpart, and the longer period of its vibrations. The Italian observers, who paid considerable attention to the doubleshock, give a more general explanation. They regard the two parts ofthe shock as corresponding in the main to longitudinal and transversalwaves starting simultaneously from the same focus (see p. 13). Theformer vibrations would be vertical at the epicentre and wouldgradually become horizontal in spreading outwards; the latter would behorizontal at the epicentre and at a distance from it (_e. G. _ atSeville) nearly vertical. Also, as the longitudinal waves travel morerapidly than others, the interval between the two parts of the shockwould increase with the distance from the origin. Owing again, to thelarge size of the focus, the first part of the shock would at no placebe instantaneous, and its later vibrations might coalesce with theearlier transverse vibrations, so that, within and near themeizoseismal area, the second part of the shock might be stronger thanthe first. A similar result might be produced in the same district ifthe transverse vibrations coincided with reflected longitudinalvibrations, and Professors Taramelli and Mercalli think that suchreflection would occur from the old crystalline rocks of the Sierra deAlmijara and possibly also from the calcareous and crystalline rocksto the south-west of Cartama. Satisfactory as it seems to be in some respects, this explanation isopen to serious objections, of which I will mention only two. Thefirst is that, though the pause between the two parts of the shockdoes increase with the distance, it does not increase rapidly enough;at Seville, it should be two or three minutes, instead of "someseconds" in length. A more fatal objection, however, is that, if theexplanation were correct, every earthquake-shock should consist of twoparts, and this is only the case with a small minority. On the other hand, if the velocities of the waves composing each partwere the same, the slight increase in the length of the interval isreadily accounted for, as we have seen, by the gradual extinction ofits weak terminal vibrations. But in any case, the long interval thatelapsed between the beginnings of the two parts at a place so near theepicentre as Ventas de Zafarraya, shows that each part was due to adistinct impulse; and, judging from the directions of the respectivemovements, it would seem that the focus of the first impulse wassituated at a greater depth than the focus of the second. Whether theepicentres corresponding to the two foci were coincident or more orless separate is not clear from the nature of the shock; but it isprobable that they were nearly or quite detached, and that a secondepicentre was situated near the eastern focus of the ellipse boundingthe meizoseismal area. SOUND-PHENOMENA. In the Neapolitan earthquake, the sound was only heard in a districtof about 3, 300 square miles immediately surrounding the epicentres, while the whole area disturbed by the shock was not less than 39, 000square miles. A similar limitation was noticed in the Andalusianearthquake. According to the Spanish Commission, the sound was heardat only one place (Cordova) outside the provinces of Granada andMalaga; and its audibility was a rule confined to the area withinwhich buildings were damaged by the shock. It was compared atdifferent places to the noise of a passing train or a carriage heavilyladen running on a paved road, of distant thunder, a great storm, orthe discharge of heavy guns. At every place where the sound was heard, it distinctly preceded theshock, frequently allowing time for escape from houses that wereafterwards ruined. Its duration within the meizoseismal area was on anaverage about five or six seconds, rarely perhaps did it exceed tenseconds. At some places in the same area, it overlapped the beginningof the shock, but generally it was separated from the latter by a veryshort interval, estimated at a second. From this precedence of thesound, the Italian Commission conclude that the sound-waves travelledmore rapidly than those which formed the shock, an inference thatdepends on the assumption that both waves started simultaneously fromwithin precisely the same focal limits. A different explanation, notbased on these assumptions, will be considered more fully in ChapterVIII, dealing with the recent earthquakes of Hereford and Inverness. VELOCITY OF THE EARTH-WAVES. If, in a highly-civilised country, the time-records of an earthquakevary within wide limits, it is not surprising that those given for theAndalusian earthquake should be wholly untrustworthy. Even the clocksin public buildings and railway stations differed by as much as 25minutes in their indications. An interesting observation is, however, described in the French report and is worth repeating, though it doesnot lead to any accurate result. At the time of the principal shock, two telegraph-clerks were in communication, one at Malaga and theother at Velez-Malaga. The latter, surprised by the shock, suddenlystopped his message; and, about six seconds later, the arrival of theearth-waves at Malaga explained the interruption to his colleague. As, according to the French report, Velez-Malaga is 9 kms. (or about 5-1/2miles) nearer than Malaga to the mean epicentral point, it followsthat the velocity of the earth-waves must have been about 1. 5 kms. , ornearly a mile, per second. [34] The only observations of any real value in determining the velocityare those given by the stopped clock at the observatory of SanFernando (Cadiz) and by the magnetographs at Lisbon, Parc Saint-Maur, Greenwich, and Wilhelmshaven. Taking the times at Cadiz, Lisbon, Greenwich, and Wilhelmshaven at 9. 18, 9. 19, 9. 25, and 9. 29 P. M. Respectively (Paris mean time) and the mean epicentral point ascoinciding with Alhama, the French Commission estimates roughly themean surface-velocity between Cadiz and Lisbon at 3. 6 kms. Per second, between Cadiz and Greenwich at 4. 5 kms. Per second, between Cadiz andWilhelmshaven at 3. 1 kms. Per second, and between Greenwich andWilhelmshaven at 1. 6 kms. Per second. Dr. Agamennone, however, notices that the distances from Alhama are not correctly measured, andsubstitutes for the above figures 4. 83, 3. 43, 2. 82, and 1. 75 kms. Persecond respectively. These results apparently show a decrease in the velocity with theoutward spread of the earth-waves, but, as Dr. Agamennone again pointsout, a comparatively small error in the time at Cadiz would neutralisethe apparent decrease. It is not to be supposed that the astronomicalclock at this observatory was wrong by more than a second or two, butthe behaviour of clocks during an earthquake is so irregular--somestopping at once, others staggering on for some seconds beforearrest--that the Cadiz time may differ from the true time by severalseconds. Besides this possible error, there is also considerable uncertainty inthe records from the magnetic observatories, owing to the slow rate atwhich the photographic paper travels. At Parc Saint-Maur this rate isonly 10 mm. Per hour, and at the other observatories about 15 mm. Perhour. Allowing, therefore, for an error of half-a-minute in thetime-record at Cadiz, of one minute in those of Lisbon, Greenwich, andWilhelmshaven, and of two minutes in that at Parc Saint-Maur, andtaking the mean epicentral point as determined by the Italianobservers, Dr. Agamennone, applying the method of least squares, findsthe probable value of the velocity of propagation to be 3. 15 kms. (ornearly 2 miles) per second, with a possible error of . 19 kms. Persecond. This result agrees closely with the value found for the longslow undulations of more recent earthquakes. MISCELLANEOUS PHENOMENA. _Connection between Geological Structure and the Intensity of theShock. _--While a great part of the injury to buildings must beattributed to their faulty construction, the connection between thenature of the underlying rock and the amount of damage was veryclearly marked. Other conditions being the same, houses built onalluvial ground suffered most of all; and the destruction was alsogreat in those standing on soft sedimentary rocks such as clays andfriable limestones. On the other hand, when compact limestones orancient schists formed the foundation-rock, the amount of damage wasconspicuously less than in other cases. The members of both the French and the Italian Commissions agree inascribing the peculiar form and relative positions of the isoseismallines to geological conditions. To the east of the epicentre, theschists and crystalline limestones form a deep, uniform, and compactmass; while, to the west, the old crystalline rocks are covered byjurassic, cretaceous, and eocene formations, constituting a lesshomogeneous and less elastic mass, in which the intensity of the shockwould fade off much more rapidly, with the result that the epicentreoccupies the western focus of the elliptical boundary of themeizoseismal area (Fig. 19). [35] That mountain-ranges have an important influence on the form ofisoseismal lines is evident from both maps (Figs. 19 and 20), butespecially from that published by the French Commission (Fig. 20). The resistance offered by the Sierra Nevada to the propagation of theearth-waves is shown in the former map by the approximation of thefirst and second isoseismals at the east end, and in the latter by thegreat bay in the third isoseismal line. Whichever interpretation ofthe evidence is the more accurate, the action of the mountainous massis clearly to lessen rapidly the intensity of the shock--an effectwhich is probably due to the abrupt changes in the direction andnature of the strata encountered normally by the earth-waves. On theopposite side of the epicentre, the waves meet the Sierra de Rondaobliquely. In traversing this range, the shock lost a great part ofits strength, while it continued to be felt severely along its easternfoot, thus giving rise to the south-westerly extension of the thirdisoseismal in Fig. 20, and, though to a less extent, that of thesecond in Fig. 19. _Fissures, Landslips, etc. _--The earthquake resulted in manysuperficial changes, such as fissures, landslips, and derangement ofthe underground water-system--all changes of the same order as thedestruction of buildings--but, so far as known, in no fault-scarps orother external evidence of deep-seated movements. Some of the fissures were of great length. One of the most remarkableoccurred at Guevejar, a village built on the south-west slope of theSierra de Cogollos. It was in the form of a horse-shoe, and was abouttwo miles long, from ten to fifty feet wide, and of great depth. Inits neighbourhood, innumerable small cracks appeared, someperpendicular and others parallel to the great fissure. The groundwithin, a bed of clay resting on limestone, also slid down towards theriver. Houses near the centre of the fissured tract were shifted asmuch as thirty yards within the first month, and others near itsextremity about ten feet; while the accumulation of the material atthe south end of the fissure resulted in the formation of a smalllake, of about 250 to 350 square yards in area and about 30 feet deep. All streams within the fissured zone disappeared, and the spring, which provided the drinking-water of the village, ceased to flow. The underground water-system was generally affected throughout thecentral area. In some places, mineral springs disappeared; in others, new springs broke out or old ones flowed more abundantly. At Alhama, the increased flow was accompanied by a permanent rise in temperaturefrom 47° to 50° C. , and by a marked change in character. AFTER-SHOCKS. Frequent after-shocks are a characteristic of the earthquakes ofSouthern Spain. After the Cordova earthquake of 1170, they continuedfor at least three years. The Murcian earthquake of 1828 was followedby 300 minor shocks during the next twenty-four hours, and for morethan a year slight tremors were often felt. For some time after thegreat earthquake of 1884, the movements of the ground were extremelynumerous in the immediate neighbourhood of the epicentre, farther awaythey were rarer and of less intensity, and outside the area of damagedbuildings they were nearly absent. Thus, during the night of December 25-26, 110 after-shocks werecounted at Jatar, from 14 to 17 at Alcaucin, Ventas de Huelma, Motril, Cacin, Durcal, Malaga, etc. ; about 11 at La Mala and Albuñuelas; 9 atVelez-Malaga and Lenteje; and from 5 to 7 at Frigiliana, Riogordo, andCartama. The strongest of these shocks occurred at 2. 20 A. M. , and, though none was violent, several helped to complete the ruin of manyhouses that had been damaged by the principal shock. From this time, after-shocks occurred almost daily until the end ofMay, after which they became much less frequent. According to the listgiven in the Italian report, which closes at the end of January 1886, 237 shocks were felt, 23 up to the end of December, 30 in January1885, 25 in February, 27 in March, 46 in April, and 43 in May. In June1885, only three are recorded, and the average number during each ofthe following seven months lies between five and six. This list, however, does not include the very weak shocks, [36] for nearly allthose contained in it were felt as far as Malaga or its neighbourhood. The shocks varied considerably in intensity as well as in frequency, five of them being much more violent than the rest. One that occurredon December 30th was felt strongly in all the damaged area, two otherson January 3rd and 5th caused fresh injury to buildings, a fourth, onFebruary 27th, disturbed an area bounded roughly by the secondisoseismal of the principal earthquake (Fig. 19), while the fifth andstrongest, that of April 11th, was felt over a large part of the zonebeyond. At places within and near the meizoseismal area, earth-sounds weresometimes heard without any sensible shock; occasionally, also, tremors were felt with no attendant sound; but, as a rule, the shockswere accompanied by sound, and in every such case, as in the principalearthquake, the sound preceded the shock, or at most was partlycontemporaneous with it. Several of the after-shocks resembled the principal earthquake intheir division into two parts separated by an interval of rest orweaker movement from half a second to a second in length, though thewhole duration of the shock itself in no case exceeded five or sixseconds. Occasionally, the likeness was still closer, in thesuccession of sound, subsultory motion and concluding horizontalundulations. GEOLOGY OF THE MEIZOSEISMAL AREA AND ORIGIN OF THE EARTHQUAKES. The meizoseismal area and surrounding zones lie in the midst of themountainous region that separates the basin of the Guadalquiver fromthat of the Mediterranean, the essential structure of which, accordingto the geologists of the French Commission, is outlined in Fig. 24. Inthis sketch-map, the lightly-shaded bands correspond to an upperseries of crystalline schists, and the cross-shaded bands to the lowerseries of mica-schists and dolomites that form the anticlinal folds ofthe Sierra de Ronda, the Sierra de Mijas, and the Sierra Tejeda. In addition to the faulting and intense folding in the direction oftheir strikes, these rocks are also intersected by three nearlyparallel transverse faults of post-Triassic age, which, aided bysubsequent denudation, have cut up the whole range into a number ofdistinct sierras. They are represented by the broken lines in Fig. 24. [Illustration: FIG. 24. --Structure of meizoseismal area of Andalusian earthquake. (_Fouqué, etc. _)] One of these faults, that which passes near Motril, traverses themeizoseismal area, whose boundary, as laid down by the FrenchCommission, is indicated by the dotted line on the sketch-map. [37] Inthe neighbourhood of Zafarraya, the fault intersects the brokenanticlinal fold of the Sierra Tejeda, and the epicentre is thussituated in one of the most disturbed tracts of the whole region. Theevidence, both seismic and geological, is insufficient to support anyprecise view as to the origin of the earthquake, but there can belittle doubt that it was closely connected with movements along one ormore of the system of faults that intersect not far from Zafarraya. REFERENCES. 1. AGAMENNONE, G. --"Alcune considerazioni sui different metodi fino ad oggi adoperati nel calcolare la velocità di propagazione del terremoto andaluso del 25 dicembre 1884. " Roma, _R. Accad. Lincei, Rend. _, vol. Iii. , 1894, pp. 303-310. 2. ---- "Velocità superficiale di propagazione delle onde sismiche in occasione della grande scossa di terremoto dell' Andalusia del 25 dicembre 1884. " _Ibid. _, vol. Iii. , 1894, pp. 317-325. 3. CASTRO, M. F. De. --_Terremotos de Andalucía: Informe de la comision nombrada para su estudio dando cuenta del estado de los trabajos en 7 de marzo de 1885. _ (Madrid, 1885; 107 pp. ) 4. FOUQUÉ, F. , etc. --"Mission d'Andalousie: Études relatives au tremblement de terre du 25 décembre 1884, et à la constitution géologique du sol ébranlé par les secousses. " Paris, _Acad. Sci. Mém. _, vol. Xxx. , pp. 1-772. 5. MACPHERSON, J. --"Tremblements de terre en Espagne. " Paris, _Acad. Sci. , Compt. Rend. _, vol. C. , 1885, pp. 397-399. 6. NOGUÉS, A. F. --"Phénomènes géologiques produits par les tremblements de terre de l'Andalousie, du 25 décembre 1884 au 16 janvier 1885. " _Ibid. _, pp. 253-256. 7. ROSSI, M. S. De. --"Gli odierni terremoti di Spagna ed il loro eco in Italia. " _Bull. Vulc. Ital. _, anno xii. , 1885, pp. 17-31. 8. TARAMELLI, T. , and G. MERCALLI. --"I terremoti Andalusi cominciati il 25 dicembre 1884. " Roma, _R. Accad. Lincei, Mem. _, vol: iii. , 1885, pp. 116-222. 9. Paris, _Acad. Sci. , Compt. Rend. _, vol. C. , 1885, pp. 24-27, 136-138, 196-197, 256-257, 598-601, 1113-1120, 1436 (the last three by F. Fouqué). FOOTNOTES: [31] These times correspond to about 9. 10 and 9. 25 P. M. , Greenwichmean time. The earthquake stopped a clock at the Royal Observatory ofSan Fernando (Cadiz), at 8h. 43m. 54. 5s. Mean local time, corresponding to 9h. 8m. 44s. , G. M. T. [32] The earthquake is also said to have been registered at theobservatory of Moncalieri, near Turin, but I have not been able toascertain the time of occurrence. A movement felt at about 10. 20 P. M. At Ramsbury, in Wiltshire, was attributed to the earthquake, thoughthe time is about an hour too late. On December 26th, an astronomicalclock was stopped at Brussels and its pillar displaced; and, on theevening of the same day, the large telescope at the observatory wasalso found to have been shifted. These effects, it is suggested, werecaused by the Andalusian earthquake, but the connection between themseems to me very doubtful. [33] The French observers have also applied a method depending on thetime of occurrence of the shock. Joining places where the recordedtimes were the same, they notice that the perpendicular bisectors ofthese lines intersect within an area which agrees practically withthat determined by the azimuths. The inaccuracy of the time-recordsmust, however, lessen the significance of this result. [34] Dr. Agamennone points out that, according to the Italian report, the difference in distance is 22 kms. (or 13-3/4 miles), leading to avelocity of about 3. 6 kms. , or 2. 3 miles per second. [35] It should be remembered that it is not improbable that there weretwo detached epicentres, coinciding roughly with the two foci of thiscurve. [36] Only eight are recorded during the night of December 25-26. Onseveral occasions during April and May 1885, groups of slight shockswere felt; but as their individual times are not given, they areregarded as equivalent to one shock each in the above totals. [37] The boundary, as drawn in this figure, differs slightly from thatgiven in Fig. 20. CHAPTER V. THE CHARLESTON EARTHQUAKE OF AUGUST 31ST, 1886. The Charleston earthquake stands alone among the great earthquakesdescribed in this volume, and indeed among nearly all greatearthquakes, in visiting a region where seismic disturbances werealmost unknown. Calabria and Ischia, the Riviera and Andalusia, Assamand the provinces of Mino and Owari in Japan, are all regions whereearthquake-shocks are more or less frequent and occasionally ofdestructive violence. But, from the foundation of Charleston in 1680until 1886, that is, for more than two centuries, it is probably nottoo much to say that few counties in Great Britain were so free fromearthquakes as the State of South Carolina. [38] The practical isolation of the earthquake of 1886 left its trace onthe character of the investigation. Not only were the observersuntrained, but the investigators themselves were unprepared. Forinstance, the scale of intensity used in drawing the isoseismal lineswas not adopted until after the first letters of inquiry were issued. On the other hand, nothing could exceed the energy and ability withwhich the epicentral tracts were examined by Mr. Earle Sloan and thecollection of time-records made by Mr. Everett Hayden. To them, and toMajor C. E. Dutton, whose valuable monograph supersedes all otheraccounts, we are indebted for the two chief additions to our knowledgeresulting from the study of the Charleston earthquake. These are thedetermination of the double epicentre, and the measurement of thevelocity with which the earth-waves travelled. DAMAGE CAUSED BY THE EARTHQUAKE. The land-area disturbed by the earthquake and the isoseismal lines areshown in Fig. 25, the small black oval area (which IncludesCharleston) being that within which the greatest damage to buildingsoccurred. The chief part of the epicentre, however, lies from 12 to 15miles to the west and north-west of Charleston, in a forest-claddistrict, containing only two villages and various scattered houses. The city of Charleston, whose population between 1880 and 1891increased from fifty to fifty-five thousand, is built on a peninsulabetween the Cooper River on the east and the Ashley River on thesouth-west. Originally, this was an irregular tract of comparativelyhigh and dry land, intersected by numerous creeks, which, as the citygrew, were filled up to the general level of the higher ground. It ison this "made land" as a rule that the more serious damage tobuildings occurred. At 9. 51 P. M. (standard time of the 75th meridian), the greatearthquake occurred, and, one minute later, there was left hardly abuilding in the city that was not injured more or less seriously. "Thedestruction, " as Major Dutton remarks, "was not of that sweeping andunmitigated order which has befallen other cities, and in which everystructure built of material other than wood has been levelledcompletely to the earth in a chaos of broken rubble, beams, tiles, andplanking, or left in a condition practically no better. " The number ofhouses entirely demolished was not great, but several hundred lost alarge part of their walls, and many were condemned as unsafe andafterwards pulled down. A board of inspectors, appointed toinvestigate the condition of the houses, reported that not one hundredout of fourteen thousand chimneys examined by them escaped damage, andthat 95 per cent. Of those injured were broken off at the roof. Thetotal cost of the necessary repairs, it was estimated, would amount toabout one million pounds. According to the official records, 27 persons were killed inCharleston during the earthquake, but, by cold, exposure, etc. , thisnumber was brought up to not less than 83. The number of personswounded was never ascertained. ISOSEISMAL LINES AND DISTURBED AREA. In drawing the isoseismal lines (represented by the continuous curvesin Fig. 25), Major Dutton made use of the well-known Rossi-Forel scaleof seismic intensity, a translation of which is given below. [39] Thecurves range from the highest degree, 10, corresponding to disastrouseffects on buildings, down to the lowest but one, 2, which would beapplied to a shock felt only by a small number of persons at rest. Itis evident, I think, that these lines cannot be regarded as drawn withgreat accuracy. The number of records (nearly 4000, from about 1, 600places), great as it is, is hardly sufficient for the purpose; andmany were collected from newspapers. The circulars of inquiry alsocontained no distinct questions corresponding to the different degreesof the scale employed, and therefore it is not always certain that theintensity recorded was the maximum observed. But, if the curves mighthave varied in detail with a larger and more accurate series ofobservations, they must represent in their main features thedistribution of seismic intensity throughout the disturbed area. Onepoint of importance is the partial earthquake-shadow in the region ofthe Appalachian Mountains shown by the southward incurving of theisoseismals 4, 5, and 6, and especially by the first two of theselines. Another is the close grouping of the isoseismals in the Stateof Mississippi, illustrating a rapid fading of intensity as theearth-waves crossed the unconsolidated materials of the Mississippidelta. [Illustration: FIG. 25. --Isoseismal lines of Charleston earthquake. (_Dutton, etc. _)] Owing to the short distance between the epicentre and the sea-coast, it is impossible to make more than a rough estimate of the extent ofthe disturbed area. Even when the boundary lies on land, it traversessome districts which are thinly populated and others where theinhabitants are unobservant, and unlikely to notice the slowoscillations which were alone perceptible at great distances. Theshock was, however, felt at Boston (800 miles from the epicentre), LaCrosse on the upper Mississippi (950 miles to the north-west), atseveral places in Cuba (between 700 and 710 miles), and in Bermuda(950 miles). To the south, the limits are unknown, there being noreport from Yucatan, the nearest point of which is distant about 930miles. If we assume the disturbed area to have a mean radius of 950miles, then it must have covered no less than 2, 800, 000 square miles. And, that this estimate is not excessive, will be evident from thefact that the land-area disturbed (including parts of the great lakesand inlets in the sea-coast) amounted to about 920, 000 square miles. PREPARATION FOR THE EARTHQUAKE. The preparation for the earthquake seems to have begun about threemonths before. During June, and even earlier, slight but undoubtedtremors are said to have been felt in Charleston, but no record ofthem was kept until about 8 A. M. On August 27th, when a decidedearthquake occurred at Summerville, a village twenty-two miles to thenorth-west. The shock and sound were simultaneous, the shock a singlejolt or heavy jar, the sound loud and sudden; they were such as mighthave been caused by the firing of a heavy cannon or the explosion of aboiler or blast of gunpowder. At 4. 45 A. M. On August 28th, the shockand sound were repeated, only more strongly, the former beingdistinctly felt as far as Charleston. During that day and the next, there were several other shocks at Summerville, and then rest andquiet succeeded until the evening of August 31st. NATURE OF THE SHOCK. At 9. 51 P. M. (to take one of the best descriptions), the attention ofan observer in Charleston was "vaguely attracted by a sound thatseemed to come from the office below, and was supposed for a moment tobe caused by the rapid rolling of a heavy body, as an iron safe or aheavily-laden truck, over the floor. Accompanying the sound there wasa perceptible tremor of the building, not more marked, however, thanwould be caused by the passage of a car or dray along the street. Forperhaps two or three seconds the occurrence excited no surprise orcomment. Then by swift degrees, or all at once--it is difficult to saywhich--the sound deepened in volume, the tremor became more decided, the ear caught the rattle of window-sashes, gas-fixtures, and othermovable objects; the men in the office ... Glanced hurriedly at eachother and sprang to their feet.... And then all was bewilderment andconfusion. "The long roll deepened and spread into an awful roar, that seemed topervade at once the troubled earth and the still air above and around. The tremor was now a rude, rapid quiver, that agitated the wholelofty, strong-walled building as though it were being shaken--shakenby the hand of an immeasurable power, with intent to tear its jointsasunder and scatter its stones and bricks abroad.... "There was no intermission in the vibration.... From the first to thelast it was a continuous jar, adding force with every moment, and, asit approached and reached the climax of its manifestation, it seemedfor a few terrible seconds that no work of human hands could possiblysurvive the shocks. The floors were heaving under-foot, thesurrounding walls and partitions visibly swayed to and fro, the crashof falling masses of stone and brick and mortar was heard overhead andwithout.... "For a second or two it seemed that the worst had passed, and that theviolent motion was subsiding. It increased again and became as severeas before. None expected to escape. A sudden rush was simultaneouslymade to endeavor to attain the open-air and fly to a place of safety;but, before the door was reached all stopped short, as by a commonimpulse, feeling that hope was vain--that it was only a question ofdeath within the building or without, of being buried beneath thesinking roof or crushed by the falling walls. The uproar slowly diedaway in seeming distance. The earth was still, and oh! the blessedrelief of that stillness. " If somewhat sensational in form, this report gives an extremely vividand generally accurate account of the great shock. Other observers inCharleston concur in dividing the movement into five phases. Thepreliminary tremors and murmuring sound lasted about twelve seconds, and, although they increased in strength, they were succeeded somewhatsuddenly by the violent oscillations of the second phase, followed bya third phase of much less intensity and a fourth of strongeroscillations, these three phases lasting altogether about fiftyseconds. The fifth phase, in which the tremors died out ratherrapidly, continued about eight seconds; so that the total duration ofthe earthquake was not less than seventy seconds. The variation of theintensity with the time is represented roughly by the curve in Fig. 26. [Illustration: FIG. 26. --Curve of intensity at Charleston. (_Dutton. _)] At Charleston, there were thus two decided maxima of intensity, nearlyequal in strength, though the first seems to have been slightly morepowerful than the second. As in the Andalusian earthquake, theintervening tremors were imperceptible at a distance from theepicentre, and the earthquake appeared in the form of two distinctshocks, separated by an interval the average duration of which wasestimated at slightly less than half a minute. At most places, thefirst shock is described as the stronger, but the difference inintensity of the two parts could not have been great, for both werenoticed at several places more than 600 miles from the epicentre. _Visible Earth-Waves. _--Many persons in the meizoseismal area assertthat they saw waves moving along the surface of the ground. AtCharleston, according to an observer who was facing a street-lamp atthe time, "the progress of the waves as they passed the house, goingtowards the south-east, was plainly observed, although they travelledwith incomparable swiftness. The shadow of each moving ridge cast fromthe gas-light was distinctly seen. The waves were not in long rollers, but had rather the appearance of 'ground-swells' in deep water, " theheight of which from crest to trough he estimated at not less than twofeet. In the words of another observer, "The vibrations increasedrapidly and the ground began to undulate like the sea. The street waswell lighted, having three gas-lamps within a distance of 200 feet, and I could see the earth waves as they passed as distinctly as I havea thousand times seen the waves roll along Sullivan's Island beach. The first wave came from the south-west, and as I attempted to make myway ... I was borne irresistibly across from the south side to thenorth side of the street. The waves seemed then to come from both thesouth-west and north-west, and crossed the street diagonally, intersecting each other, and lifting me up and letting me down as if Iwere standing on a chop sea. I could see perfectly, and made carefulobservations, and I estimate that the waves were at least two feet inheight. " THE DOUBLE EPICENTRE. For seismological purposes, it is unfortunate that the epicentraldistrict should be one containing so few buildings and other objectsthat could preserve the effects of the shock. It is for the most parta barren, forest-clad region, in places swampy, with occasionalscattered houses. But it is crossed by three lines of railwaydiverging from Charleston, and the damage which they sufferedsupplements to some extent the defects arising from the scarcity ofbuildings. These railway lines are the South Carolina, theNorth-Eastern, and the Charleston and Savannah, denoted by the lettersA, B, and C, respectively, in Figs. 28 and 29. [40] It will beconvenient to follow Major Dutton, and trace the variation ofintensity exhibited along each line. For six miles along the South Carolina Railway (A) the damage to theline, though indicative of a strong shock, was of little consequence. In the first half of this distance no repairs were required, but at3-2/3 miles the rails were bent and the joints between them opened; at5 miles, the fish-plates were torn from their fastenings and thejoints between the rails opened seven inches; and at nearly 6 milesthe joints were again opened, and the road-bed depressed six inches. After this point, deflections of the line and elevations anddepressions of the road-bed were no longer rare. Near the 9-milepoint, the intensity of the shock seemed to increase most rapidly;lateral displacements of the line became more frequent as well asgreater in amount. The distortions of the lines were probably greatestbetween 10 and 11 miles; here they were often displaced laterally, sometimes depressed or elevated, and occasionally twisted intoS-shaped curves, while many hundred yards of the track were shovedbodily towards the south-east. "The buckling always took place whenthis lateral shoving encountered a rigid obstacle, usually a longrigid trestle. At the north-western end of the trestle theaccumulation of rails resulted in a sharp kink. Correspondingextensions of the track by the opening of the joints and shearing ofthe fish-plate bolts occurred some distance to the north-westward. " At11-1/2 miles, the lines were again stretched and the joints opened byabout seven inches; but, from this point for more than four miles, thesharp kinks revealing a sliding of the track were entirely absent, though there were still long slight flexures in the lines and changesof level in the road-bed. The railway in this section traverses adistrict which is partly a swamp and partly a rice-field; and thus itmay be, as Major Dutton suggests, that the ground was less fitted topreserve the effects of the shock. [41] At about 18 miles, the linereaches higher and firmer ground; and, from here to Summerville(21-2/3 miles), there were many sinuous flexures. For six milesfarther, violent distortions of the rails ceased to occur, the rate ofdecrease in intensity being most marked near the 23-mile point. Thelast flexure occurred at Jedburgh (27-1/2 miles) at the south end of along, heavy trestle (Fig. 27). [Illustration: FIG. 27. --Flexure of rails at Jedburgh. (_Dutton. _)] There is thus a certain symmetry in the damage to this line withrespect to a point about 15 or 16 miles from the Charleston terminus. The changes of intensity are most rapid at distances of about 9 and 23miles from the terminus. Also, on the south-east side of the 16-milepoint, the longitudinal displacements of the line are always to thesouth-east; on the other side, always to the north-west. Major Duttontherefore infers that the epicentre must be on a line drawn nearlythrough the 16-mile point at right angles to the railway. Somewhat similar changes were noted along the North-Eastern Railway(B), the Charleston terminus of which is about three-quarters of amile to the south-east of that of the South Carolina Railway. Slightflexures in the line occurred at distances of 1-1/2 and 4 miles fromthe terminus, and at about 6 miles the road-bed was depressed, in onepart by as much as 22 inches. At about 6-1/3 miles, the joints betweenthe rails were opened 14 inches, and there were slight sinuousflexures in the line near the 7-mile and 8-mile points. Theindications of great intensity then rapidly increased, the rate ofchange being greatest near the 9-mile point. Here, there was a longlateral flexure with a shift of 4 inches eastward. Half-a-milefarther, the fish-plates were broken and the rails parted 8-1/2inches. A little beyond the 10-mile point, an embankment 15 feet highwas pushed 4-1/2 feet eastward along a chord of 150 feet. At the12-mile point and beyond, fish-plates were broken, lines were bent andthe joints opened; the road-bed was cut by a series of cracks, one ofwhich was 21 inches wide, while the beginning of a long trestle wasshifted 8-1/3 feet to the west. From 12-1/2 to 14-1/2 miles, severalbuildings were damaged or destroyed by a movement which was clearlymore vertical than horizontal. Near the 16-mile point, the ground wasfissured and thrown into ridges, the rails being similarly bent in avertical plane. Soon after this, the line reaches a broad, sandytract, and, though the thickness of the sand is probably not much morethan 40 feet in any place, the disturbances diminish almost at once, and, for a distance of more than two miles, there was little damagedone to the line. At Mount Holly Station (18 miles), the intensity wasso slight that the houses suffered no injury more serious than theloss of chimneys. Half-a-mile farther, the ground becomes less sandy, and the effects of the shock more distinct. The lines were bent inplaces for about a quarter of a mile, after which they again pass intothe sandy area with a decrease of damage, the last flexure being nearthe 21-mile point. The rate of change of intensity in this part of theline appears to have been greatest at a distance of about 19-1/2 milesfrom the terminus, but the exact distance is obviously somewhatuncertain. There is again a rough symmetry in the damage to the line, the centralpoint being about 14 miles from the Charleston terminus. A line drawnthrough this point at right angles to the North-Eastern Railway (orrather to that part of it between the 9-mile and 19-1/2-mile points)should pass through the epicentre. It meets the corresponding line forthe South Carolina Railway in a point which is indicated in Figs. 27and 28 by a small circle (W). Houses and other buildings are rare inthe surrounding district; but, as the intensity of the shockdiminished outwards in all directions, this point must markapproximately the position of the epicentre. As it is close to theWoodstock Station on the South Carolina Railway, it is called by MajorDutton the Woodstock epicentre. The Charleston and Savannah Railway (C) uses the same lines as theNorth-Eastern for the first seven miles from Charleston, and thenturns off in a south-westerly direction. For 4-1/2 miles from thejunction the signs of disturbance were few and unimportant. Therailway then crosses the Ashley River, the banks of which slid towardsone another and jammed the drawbridge; but for four miles fartherthere was no serious damage done to the lines. At about 16-1/2 milesthe effects of the shock became rapidly more apparent. For nearly1-1/2 mile the entire railroad was deflected into an irregular curve, the displacement being greatest at the bridge, where it crosses theStono River. Here, it was as much as 37 inches to the south. AfterRantowles Station (18 miles), there were many displacements, bothlateral and vertical. At 18-1/2 miles, a long southward deflectionbegan, the amount of which reached 25 inches at the 19-mile point, 50inches half-a-mile farther on, and was still greater at 20-2/3 miles. For two miles more, sinuous flexures were continuous, but, at the22-2/3-mile point, they rapidly disappeared, the railroad passing onto higher and firmer ground. Between 25 and 27 miles, there wereoccasional slight flexures in the line or depressions of the railroad;but, after the 27-1/4-mile point, they seldom occur, and, when theydo, are of little consequence. Some of the effects described in the last paragraph may, as MajorDutton suggests, be due to the varying nature of the surface-rocks. Itis important to notice, however, that disturbances of the lines wereexceedingly rare in the section that lies nearest to the Woodstockepicentre, and that they increase in violence for some distance fromthat region, the maximum intensity being reached a mile or two to thewest of Rantowles Station. This points clearly to the existence of asecond focus. Unfortunately, there are very few houses or otherobjects in the neighbourhood, and the position of the correspondingepicentre cannot be determined accurately. Major Dutton places it inthe position indicated by a small circle (R), and calls it theRantowles epicentre from its vicinity to the station of that name. If the meizoseismal area had been a thickly populated one, theevidence of ruined and damaged houses would have provided materialsfor the construction of isoseismal lines surrounding the twoepicentres. It is difficult, as it is, to gauge the equality of theeffects on objects so different as railway-lines and buildings; andthe isoseismals shown in Figs. 28 and 29 can therefore lay no claim toaccuracy. Fig. 28 shows the epicentral isoseismals as they are drawn by Mr. Earle Sloan. They do not correspond to the degrees of any definitescale of seismic intensity; but they may be taken as representing theimpressions of a very careful observer, who traversed the districtimmediately after the occurrence of the earthquake, and who, whendrawing these lines, was biassed by no preconceived theory. Major Dutton, relying chiefly on Mr. Sloan's written notes, interpretsthe evidence differently, and obtains the series of curves shown inFig. 29. In this case, also, the isoseismals correspond to noexpressed standard of intensity. They are intended merely to representthe forms of the curves, and, by their less or greater distance apart, the more or less rapid rate at which the intensity varied. The chief difference between the two maps concerns the form of theWoodstock isoseismals. Major Dutton draws them approximatelycircular, leaving the map blank towards the north, where hardly anyevidence was forthcoming. Mr. Sloan attributes the scantiness ofeffects here to a diminution of intensity, and makes the lines curvein towards the epicentre. They certainly must do so in crossing theNorth-Eastern Railway; and the somewhat southerly trend of Mr. Sloan'scurves to the east of this railway seems to me to furnish the betterrepresentation of the distinctly greater intensity in that region. [Illustration: FIG. 28. --Epicentral isoseismal lines of Charleston earthquake according to Mr. Sloan. (_Dutton. _)] [Illustration: FIG. 29. --Epicentral isoseismal lines of Charleston earthquake according to Major Dutton. (_Dutton. _)] More important, however, than this divergence of opinion is theagreement in one respect between the two sets of curves. Both show amarked expansion around the points known as the Woodstock andRantowles epicentres, especially about the former, and a contractionin the intermediate region. The evidence of these isoseismalstherefore confirms that of the damaged railway lines, and establishesMajor Dutton's inference that there were two distinct foci, theepicentres of which were about thirteen miles apart. ORIGIN OF THE DOUBLE SHOCK. In the last chapter, it was shown that the double shock of theAndalusian earthquake could be due only to two distinct impulsestaking place either within the same focus or, more probably, in twodetached foci. Similar reasoning applies to the Charleston earthquake. The double maximum or double shock was observed in no less thanfourteen States. Moreover, the interval between the two maxima atCharleston appears from Fig. 26 to have been about 34 seconds inlength. Thus, the duplication of the shock cannot have been a merelylocal phenomenon, nor can it have resulted from the separation intotwo parts of the earth-waves proceeding from a single disturbance. Each maximum must therefore be connected with a distinct impulse. Combining this inference with Major Dutton's discovery of the doublefocus, no doubt can remain as to the origin of the repeated shock. Itis clear, also, that the impulse at the Woodstock focus was thestronger of the two; for the isoseismals spread out more widely roundthe corresponding epicentre, and there was no rapid decline ofintensity from that point, such as might be associated with a weakerdisturbance within a shallow focus. [Illustration: FIG. 30. --Planes of oscillation of stopped pendulum clocks at Charleston. ] Again, since the earlier part of the shock is almost uniformlydescribed as the stronger, it follows that the Woodstock focus was thefirst in action. A curious fact recorded by Major Dutton supports thisinference. In Charleston, four clocks were stopped by the shock, theerrors of which at the time were certainly less than eight or nineseconds. The planes in which their pendulums oscillated are shown bythe lines lettered A, B, C, and D in Fig. 30, the broken lines W and Rrepresenting respectively the directions from Charleston of theWoodstock and Rantowles epicentres. Clock A stopped at 9h. 51m. 0s. , Bat 9h. 51m. 15s. , C at 9h. 51m. 16s. , and D (which had been reset tothe second earlier in the day) at 9h. 51m. 48s. Now, if the plane ofoscillation coincided nearly with the direction of the shock, the onlyeffect would be a temporary change in the period of oscillation; butif it was at right angles to the direction of the shock, the clockmight be stopped by the point of the pendulum catching behind thegraduated arc in front of which it oscillated. The planes of the firstthree clocks, it will be seen, were approximately at right angles tothe direction of the Woodstock epicentre, and B and C were indeedstopped in the manner just described; while the plane of shock D wasnearly perpendicular to the direction of the Rantowles epicentre. Asthe pendulums of B and C might make a few staggering oscillationsbefore their final arrest, Major Dutton assigns 9h. 51m. 12s. As theepoch of the first maximum at Charleston; and, as the interval betweenthe two maxima was about 34 seconds, this would give about 9h. 51m. 46s. For the epoch of the second maximum--a time which agrees veryclosely with that given by clock D. Thus, clocks A, B, and C must havebeen stopped by the Woodstock vibrations, and clock D abouthalf-a-minute later by those coming from the Rantowles focus. DEPTH OF THE SEISMIC FOCI. Two methods of estimating the depth of the seismic focus have beendescribed in the preceding pages--namely, Mallet's, depending on theangle of emergence, and Falb's, based on the interval between theinitial epochs of the sound and shock. To these, Major Dutton adds athird method, in which he relies on the rate at which the intensity ofthe shock varies with the distance from the epicentre. _Dutton's Method of determining the Depth of the Focus. _--If theseismic focus is either a point or a sphere, and the initial impulseequal in all directions, and if the intensity of the shock diminishesinversely as the square of the distance from the focus, then thecontinuous curve in Fig. 31 will represent the variation of intensityalong a line passing through the epicentre E. The form of the curve onthese assumptions does not depend in any way on the initial intensityof the impulse; it is governed solely by the depth of the focus. Thedeeper the focus, the flatter becomes the curve, as we have seen indiscussing the Ischian earthquakes (p. 68). In all directions from theepicentre, the intensity at first diminishes slowly; but the rate ofchange of intensity with the distance soon becomes more rapid, untilit is a maximum at the points C, C; after which it again diminishesand dies out very slowly when the distance becomes great. It will beevident from Fig. 18 that the deeper the focus the greater also is thedistance EC of the points where the intensity of the shock changesmost rapidly. It may be easily shown, indeed, that this distancealways bears to the depth of the focus the constant ratio of 1 tosqrt(3), or about 1 to 1. 73. [42] Now, if a series of isoseismals could be drawn corresponding tointensities which differ by constant amounts, we should have a seriesof circles like those surrounding the Woodstock epicentre in Fig. 29, the distance between successive lines at first decreasing graduallyuntil it is a minimum at the dotted circle and afterwards graduallyincreasing. This dotted circle is obviously that which passes throughall points where the intensity of the shock changes most rapidly. Major Dutton calls it the _index-circle_ and, when its radius isknown, the depth of the focus is at once obtained by multiplying theradius by 1. 73. In 1858, Mallet proposed a method which bears some resemblance to theabove, [43] but depending only on the intensity of the longitudinalwaves. Major Dutton claims for his method that the effects of thelongitudinal and transverse waves are not separated, that it takesaccount of the "total energy irrespective of direction or kind ofvibration. " [Illustration: FIG. 31. --Diagram to illustrate Dutton's method of determining depth of seismic focus. ] _Objections to Dutton's Method. _--I have described this methodsomewhat fully, though it seems to me open to more serious objectionsthan Mallet's first method which it is intended to replace. We have, in the first place, no reason for supposing that the focus iseither a point or a sphere, or that the initial impulse is uniform inall directions. If the earthquake were caused by fault-slipping, bothassumptions would be untrue, and it is for those who employ the methodto prove their validity. But of greater consequence is the fact that, if the method werecorrect, all earthquakes originating at the same depth must haveindex-circles of equal radii. If the depth of the focus were, say, tenmiles, then the index-circle must have a radius of about six miles, whether the initial disturbance be of extreme violence or so weak thatit is not felt at the surface at all, much less so far as six milesfrom the epicentre. The law of the inverse square is of course onlytrue for a perfectly elastic and continuous medium, and the real curveof intensity is not that of the continuous line in Fig. 31, butsomething of the form represented by the dotted line. In this case, the rate of change of intensity is greatest at some point C', nearerthan C to the epicentre, and the application of Major Dutton's rulewould give a point F', nearer the surface than F, for the focus. Thus, assuming that the method can be applied in practice--and the testinvolved is one so delicate that it would be difficult to apply exceptwith refined measurements--then all that we can assert is that thecalculated depth is certainly less than the true depth. _Dutton's Estimate of the Depth of the Seismic Foci. _--In applying themethod, the chief difficulty is to obtain a series of isoseismal linescorresponding to equidistant degrees of intensity. As already pointedout, those given in Fig. 29 are merely diagrammatic; but theindex-circle of the Woodstock focus, represented by the dotted line, is made to pass through the places where the rate of change ofintensity was found to be greatest. The radius of this circle beingvery nearly seven miles, it follows that the resulting depth of theWoodstock focal point would be about twelve miles. Major Duttonregards this estimate as probably correct within two miles. In the neighbourhood of the Rantowles epicentre, the isoseismals inboth Figs. 28 and 29 are elongated in form. The _index-circuit_, as itwould be called in such a case, cannot be drawn completely, but itsradius parallel to the shorter axis of the curves is about 4-1/2miles, and the resulting depth of the Rantowles focal point would benearly eight miles. VELOCITY OF THE EARTH-WAVES. The recognition of the double epicentre is, from a geological point ofview, the most important fact established by the investigation of theCharleston earthquake. But of equal interest, from a physical point ofview, is the estimate of the velocity of the earth-waves, which isprobably more accurate than that determined for any previous shock. Owing to the existence of the standard time system in the UnitedStates, the exact time is transmitted once a day to every town andvillage within reach of a telegraph line; and the effect of smallerrors in the observations is considerably lessened by the greatdistance traversed by the earth-waves, sixty good reports coming fromplaces more than 500 miles from the epicentre, and ten from placesmore than 800 miles distant. The total number of time-records collected is 316, but of these 130had to be rejected, either because they were obviously too early ortoo late, or because they were only given to the nearestfive-minutes' interval. There remain 186 observations which aredivided by Major Dutton into four classes according to their probablevalue. In an earthquake of such great duration (about 70 seconds atCharleston), it is necessary in the first place to select some specialphase of the movement as that to which the records mainly refer, andthen to determine as accurately as possible the time of occurrence ofthis phase at the origin. There can be little doubt as to which phase should be chosen. Theshock began with a series of tremors, which passed somewhat abruptlyinto the oscillations that formed the first and stronger maximum. These were clearly felt all over the disturbed area, and, as thebeginning of the first maximum at places near the epicentre and thebeginning of the shock at distant stations were probably due to thesame vibrations, this particular phase may be fairly selected as thatto which the time-measurements refer. The time of this phase at the origin can only be ascertained from thetime at which it reached Charleston, and our knowledge of this dependschiefly on the evidence of stopped clocks. How unreliable this may beis well known. Clocks may indeed be stopped at almost any phase of themovement; and, whenever stopped clocks can be compared with reallygood personal observations, they almost invariably show a later time. At Charleston three good clocks were stopped by the vibrations fromthe Woodstock focus, two of them being in close agreement (p. 121);and, allowing for a few oscillations before their final arrest, MajorDutton places the time of arrival of the selected phase at Charlestonat 9h. 51m. 12s. P. M. The evidence of these clocks is also supportedby that of other observations, which show that 9. 51 was certainly thenearest minute to the time of arrival, and favour a somewhat laterinstant much more strongly than one a little earlier. Now, the distance of Charleston from the Woodstock epicentre issixteen miles, and from the corresponding focus (with the calculatedvalue of its depth) twenty miles. A first estimate of the velocitygives a value of a little more than three miles a second, and the timeat the Woodstock focus may therefore be taken as 9h. 51m. 6s. With aprobable error of a few seconds. [44] Proceeding to the observations at a distance, we find them, even afterall rejections, to be very different in value. They were thereforedivided into groups consisting of observations which are as nearly aspossible homogeneous. The first group contains five records from places between 452 and 645miles from the Woodstock epicentre. They give the time to within 15seconds, obtained from an accurately kept clock, or from a clock orwatch that was compared with such within a few hours of theearthquake. The resulting velocity is 3. 236 plus or minus . 105 miles(or 5205 plus or minus 168 meters) per second. [45] In the second group there are eleven observations (between distancesof 438 and 770 miles) which satisfy the same conditions as those inthe first group, except that the time is only given to the nearestminute or half-minute. The velocity obtained from them is 3. 226 plusor minus . 147 miles (or 5192 plus or minus 236 metres) per second. The third group included all but the above records and those obtainedfrom stopped clocks. They are 125 in number (between distances of 80and 924 miles), but it is uncertain whether they correspond to theselected phase of the movement, and the errors of the clocks andwatches used were unknown. They give a mean velocity of 3. 013 plus orminus . 027 miles (or 4848 plus or minus 43 metres) per second. In the fourth group, we have the evidence of 45 stopped clocks (atplaces between 20 and 855 miles), which apparently give a velocity of2. 638 plus or minus . 105 miles (or 4245 plus or minus . 168 metres) persecond. At six places, however, the times indicated by stopped clockscan be compared with good personal observations; and these show thatthe time of traverse from the origin obtained from the former is on anaverage 1. 28 times the time of traverse obtained from the latter. If asimilar correction be made for all the stopped clocks, the correctedvelocity of the earth-waves would be from 3. 17 to 3. 23 miles (or 5100to 5200 metres) per second. In obtaining the mean value of the velocity from all the observations, those of the fourth group are omitted, and the weights of the firstthree groups are taken inversely as the squares of the probableerrors--that is, as 2: 1: 4. The resulting mean velocity is 3. 221 plusor minus . 050 miles (or 5184 plus or minus 80 metres) per second; and, though it does not follow that all other estimates are erroneous (forthe velocity may vary with the strength of the earthquake and withother conditions), it is probable that this result is more nearlyaccurate than any other previously obtained. MISCELLANEOUS PHENOMENA. _Fissures and Sand-Craters. _--In point of size, there was nothingremarkable about the fissures in the ground produced by the Charlestonearthquake. The largest were only a few hundred yards long, and, except near the river-banks, they rarely exceeded an inch in width. They seem, however, to have been unusually abundant; for, within anarea of nearly 600 square miles surrounding the two epicentres, theywere almost universal, and over a much wider area they still occurredin great numbers, though with somewhat less continuity. From many of these fissures water was ejected, carrying with it largequantities of sand and silt, and so abundantly that every stream-bed, even though generally dry in summer, was flooded. By the passage ofthe water, some part of the fissures was often enlarged into a roundhole of considerable size, ending in a craterlet at the surface. Certain belts within the fissured area contained large numbers ofthese craterlets, of all sizes up to twenty feet or more in diameter. One near Ten-Mile Hill was twenty-one feet across. In this district, they were apparently larger and more numerous than elsewhere; manyacres of ground being covered with sand, which, close to the orifices, was two feet or more in depth. Here and there, the water was ejected with considerable violence, aswas manifest from the heights to which it spurted. The testimony ofwitnesses on this point is of course doubtful, for the earthquakeoccurred after nightfall, but in a few places the branches and leavesof trees overhanging the orifices were smirched with sand and mud upto a height of fifteen or twenty feet. _Effects on Human Beings. _--It is interesting to notice the behaviourof different races under the influence of a violent earthquake, andperhaps no greater contrast could be observed than between thecalmness exhibited by the Japanese in the presence of disaster and thewild fear merging into helpless panic that characterised theresidents, and especially the negroes, of Charleston. "As we dasheddown the stairway, " says a writer already quoted (p. 108), "and outinto the street, from every quarter arose the shrieks, the cries ofpain and fear, the prayers and wailings of terrified women andchildren, commingled with the hoarse shouts of excited men.... Onevery side were hurrying forms of men and women, bareheaded, partiallydressed, some almost nude, and all nearly crazed with fear andexcitement.... A few steps away, under the gas-lamp, a woman liesprone and motionless on the pavement, with upturned face andoutstretched limbs, and the crowd which has now gathered in the streetpasses her by, none pausing to see whether she is alive or dead ... Noone knows which way to turn, or where to offer aid; many voices arespeaking at once, but few heed what is said. " Between the selfish rush for safety here described and the calminterest of the most distant observers, Major Dutton records nearlyevery possible variety of mental effects, the actions resulting fromwhich may be roughly classified as follows: A. No persons leave their rooms. B. Some persons leave their houses. C. Most persons run into the streets, which are full of excitedpeople. D. People rush wildly for open spaces and remain all nightout-of-doors. In the map of the isoseismal lines (Fig. 25), the dotted curves boundthe areas in which the effects corresponding to the three highestdegrees of the above scale were observed. The curve for the firstdegree (A) coincides of course with the isoseismal line of intensity2. It will be seen that there is a certain rough correspondence betweenthese curves and the isoseismal lines. The curve D and the isoseismal8 are close together; in other words, people thought it wiser to campout-of-doors for the night if the shock was strong enough to damagebuildings slightly. The curve C and the isoseismal 6 are similarlyconnected; that is, if the movement made pictures swing, etc. , peoplerushed into the streets. On the whole, the curve B and the isoseismal3 roughly coincide, or, if the shock was not strong enough to makedoors and windows rattle, some persons left their houses and publicmeetings were dispersed. _Feeling of Nausea. _--A feeling of nausea was experienced by manypersons at the time of the earthquake, somewhat rarely it appears inthe neighbourhood of the epicentre and even outside the isoseismal 7, but more frequently beyond these limits, and perceptible as far as thebroken line in Fig. 25. The most distant places at which it wasnoticed are Blue Mountain Creek (New York) and Dubuque (Iowa), whichare respectively 823 and 886 miles from Charleston. AFTER-SHOCKS. As Summerville lies six miles to the north-west of the Woodstockepicentre and Charleston 16 miles to the south-east, it is probablethat many of the after-shocks were unfelt and a still greater numberunrecorded. In Charleston, seven shocks, all much slighter than theprincipal shock, were felt during the night of August 31--September 1, and thirty before the end of September. Of these, the shock ofSeptember 3rd, at 11 P. M. , was the strongest, but those which occurredon September 1st, 2nd, 21st, and 27th were also described as severe, and the remainder as moderate or slight. For weeks after the greatshock, curious sensations were distinctly perceptible during the stillhours of the night "as though the crust of the earth were resting on agelatinous mass in constant motion. " The last shock felt in Charlestonseems to have been one recorded on March 18th, 1887. At Summerville, many shocks occurred that were scarcely perceptible inCharleston, and those noticed in both places were usually stronger, and the motion more nearly vertical, at Summerville. "The peculiarcharacteristic of all of them was the deep, solemn, powerful boom, like the report of a heavy cannon, usually accompanied by a quick, short jar. Sometimes it was prolonged into a heavy roar or rumble, asif many reports were delivered in a volley. The number of them wasnever recorded. " On September 3rd, Mr. W. J. McGee, of the UnitedStates Geological Survey, arrived at Summerville. During the eveningof that day, detonations were heard at intervals, averaging perhapshalf-an-hour, accompanied occasionally by very slight spasmodictremors of an instant's duration. They were much like peals of thunderat a distance of half-a-mile or more, though rather more muffled. "Itwas my impression, " Mr. McGee remarks, "that the sound was sometimesabout as grave as the ear can perceive, resembling somewhat thetremulous roar sometimes accompanying combustion in locomotives. "These sounds continued, but with diminishing frequency, throughout theremainder of the year and as late as July 1st, 1887. ORIGIN OF THE EARTHQUAKE. Major Dutton's valuable monograph is a record of theearthquake-phenomena. He offers no theory as to the cause of theshock, and is therefore in no way responsible for the account given inthe remaining part of this chapter. That there were two seismic foci he has shown, I think, conclusively;and my object is now to trace out briefly the probable nature of themovements that produced the double shock. Referring to Figs. 28 and 29, it will be seen that, according to bothMr. Sloan and Major Dutton, the isoseismals surrounding the Rantowlesepicentre are distorted along a line which runs from a few degreeseast of north to a few degrees west of south. Their oval form is inall probability connected with a focus elongated in about the samedirection. Near the Woodstock epicentre, the isoseismals are drawndifferently in the two maps, and in neither case do they offer anysure guide as to the form of the seismic focus. If, however, we followMr. Sloan's interpretation of the evidence, and suppose the earthquaketo have been fault-formed, then it is probable that in this region thefault bends round slightly towards the east. The only other evidence on this point is that afforded by the regionsof defective intensity, real or apparent, along the threerailway-lines diverging from Charleston. One of these occurred nearMount Holly Station on the North-Eastern Railway (B, Figs. 28 and 29), another for four miles starting from the 11-1/2-mile point on theSouth Carolina Railway (A), and a third along the Charleston andSavannah Railway (C) over a distance of four miles from the AshleyRiver. In the first two cases, Major Dutton suggests that the lessamount of damage was due to the nature of the soil traversed by therailway; but it is on the softer ground that the effects of anearthquake-shock are generally the more disastrous. On the whole, itseems to me probable that the three tracts referred to are reallyregions of less intensity, and it is worthy of notice that they liealong a nearly straight line. To show the bearing of these remarks, let CD (Fig. 32) represent thesection of a fault and EF that of the surface of the earth, andsuppose the rock-mass A to slip slightly and suddenly downwards. Thenthe particles of A at the surface of the fault will, by impulsivefriction, be drawn sharply upwards, and those of B correspondinglydownwards; so that the earth-waves in the two rock-masses will startin opposite phases of vibration. Along the line of fault, everyparticle of rock, being urged upwards and downwards almost equally, will remain practically at rest. Thus, regions of defective intensitymay arise from partial interference by the spreading of eitherearth-wave in the adjoining rock-mass. [Illustration: FIG. 32. --Diagram to explain origin of regions of defective intensity. ] If this be the correct explanation, the path of the originating faultmay be taken as that indicated by the broken line in Fig. 28, a linewhich is nearly parallel to the chief branches of the isoseismalcurves. [46] As both epicentres lie on the west side of this line, thefault must hade or slope in this direction. The distortion of theWoodstock isoseismals towards the north-west confirms the latterinference, for the intensity of the shock is greater on the sidetowards which the fault hades. From the comparative absence of earthquakes in South Carolina, we mayinfer that the fault is one subject to displacements at wide intervalsof time. The gradually increasing stress along its surface wasrelieved at one or two points in or near the Woodstock focus on August27th and 28th, and perhaps during the preceding months. But the firstgreat slip took place suddenly in that focus, and spread graduallysouthwards--for there was no interruption in the movement--until abouthalf-a-minute later it reached the Rantowles focus, where the secondgreat slip occurred. Eight or ten minutes afterwards there was anotherslip--in what part of the fault is uncertain--and this was followed atirregular intervals by many small movements gradually diminishing infrequency and in focal area. Within a year from the first disturbance, the fault-system attained once more its usual condition of rest. REFERENCES. 1. DUTTON, C. E. --"The Charleston Earthquake of August 31st, 1886. " _Amer. Geol. Survey, Ninth Annual Report_, pp. 209-528. 2. _Nature_, vol. Xxxv. , 1887, pp. 31-33; vol. Lxiii. , 1901, pp. 165-166. FOOTNOTES: [38] The authorities for this statement are Mallet's Catalogue ofRecorded Earthquakes (_Brit. Assoc. Rep. _, 1852, pp. 1-176; 1853, pp. 117-212; 1854, pp. 1-326), which closes with the year 1842, and Fuchs'_Statistik der Erdbeben von 1865-1885_. According to Mallet, there wasan earthquake in S. Carolina in November 1776, and the New Madridearthquake of December 16th, 1811, was felt at Charleston. Fuchsrecords two earthquakes at Charleston on May 12th, 1870, and December12th, 1876; and two in S. Carolina on December 12th and 13th, 1879. [39] 1. Recorded by a single seismograph, or by some seismographs ofthe same pattern, but not by several seismographs of different kinds, the shock felt by an experienced observer. 2. Recorded by seismographs of different kinds; felt by a small numberof persons at rest. 3. Felt by several persons at rest; strong enough for the duration ordirection to be appreciable. 4. Felt by several persons in motion; disturbance of movable objects, doors, windows; creaking of floors. 5. Felt generally by every one; disturbance of furniture and beds;ringing of some bells. 6. General awaking of those asleep; general ringing of bells;oscillation of chandeliers, stopping of clocks; visible disturbance oftrees and shrubs; some startled persons leave their dwellings. 7. Overthrow of movable objects, fall of plaster, ringing of churchbells, general panic, without damage to buildings. 8. Fall of chimneys, cracks in the walls of buildings. 9. Partial or total destruction of some buildings. 10. Great disasters, ruins, disturbance of strata, fissures in theearth's crust, rock-falls from mountains. [40] In order to simplify these figures, the rivers, most of theinlets, and other details are omitted. Small figures are added alongthe railway lines to denote the distance in miles from the stations inCharleston. [41] If this were so, the decrease in intensity would be onlyapparent; but it may have been real, and a possible explanation onthis supposition is given later on (p. 135). [42] If _c_ be the depth of the focus, _a_ the intensity at unitdistance from the focus, and _y_ the intensity on the surface atdistance _x_ from the epicentre, then y=a/(c^2+x^2) The inclination of the curve at any point is given by dy/dx=-2*a*x/(c^2+x^2)^2, and this is a maximum when d^2y/dx^2 or (3*x^2-c^2)/(c^2+x^2)^3 is zero, which is satisfied when c=x*sqrt(3) [43] _British Association Report_, 1858, pp. 101-103. [44] The above time would have to be increased by one second if thedepth of the focus were very small, and diminished by one second if itwere as great as 23 miles; the difference in either case being lessthan the probable error. [45] The method employed is as follows: Let t_0 be the computed time(9h. 51m. 6s. ) at the focus, _x_ seconds the error in this estimate, _t_ the reported time at a given place, _D_ its distance from thefocus in miles, and _y_ the number of seconds required to travel onemile; then, assuming that _y_ does not vary with the distance, we havex+Dy=t+t_0. An equation of this form is obtained from eachobservation, and the method of least squares is then applied todetermine the most probable values of _x_ and _y_. [46] This seems to me the more probable course. It is possible, however, that the fault-line may pass from Mount Holly Station to theeast of the Woodstock epicentre as shown in Fig. 28, and then to thewest of the Rantowles epicentre, the fault changing its direction ofhade in the intermediate district. CHAPTER VI. THE RIVIERA EARTHQUAKE OF FEBRUARY 23RD, 1887. Few earthquakes have aroused a more widespread interest than thosewhich struck the thronged cities of the Riviera on February 23rd, 1887. The first and greatest of the shocks occurred at about 6. 20A. M. , the second nine minutes later, and the third, intermediate instrength, at about 8. 51 A. M. [47] All three shocks were of destructiveviolence, the damage wrought by them extending along the coast and fora short distance inland from Nice to beyond Savona. Most of the injuryto property and nearly all the loss of life were, however, concentrated on the eastern side of the frontier; and it thereforefell to the lot of the Italian Government to provide for thescientific investigation of the earthquakes, as well as to meet thewants of those deprived of home and support. Professors Taramelli andMercalli, who two years before had studied the earthquakes inAndalusia, were again nominated, the former to examine the geology ofthe central regions, and the latter to report on the seismicphenomena. Their joint memoir forms one of the most complete accountsthat we possess of any earthquake, and is the chief authority for thedescription given in this chapter. Another valuable monograph is thatprepared by Professor A. Issel, of Genoa, who received an independentappointment from the same Ministry. A third official commission wasalso sent to estimate the amount of damage caused by the earthquakesin the Italian towns and villages. In France, the destruction ofproperty was much less serious, and attention was confined chiefly tothe records of the shock provided by magnetographs and otherinstruments in distant observatories. In Switzerland, the effectsremarked were merely those due to the evanescent vibrations of aremote earthquake; but many interesting records were collected by thepermanent seismological commission established in that country. DAMAGE CAUSED BY THE EARTHQUAKES. Owing to variations in the nature, foundation, and site of buildings, there is always great diversity in the destructive effects of anearthquake. In one and the same town, most of the houses may be razedto the ground, while in their midst may be found some that areshattered but still standing, and others perhaps that are practicallyunharmed. The stronger after-shocks often complete the ruin of thepartially damaged houses; though in such cases the real loss is as arule comparatively small. The close succession of the two strong after-shocks of February 23rdmade it impossible as a rule to separate their effects from those dueto the first shock; but it has been roughly estimated that aboutone-quarter of the total damage was caused by the two after-shockstogether. To them also must be referred in part the comparativelysmall number of wounded, many persons buried beneath the ruins havingno doubt perished from subsequent falls before they could beextricated. Taking all three shocks together, the total loss to property, according to Professor Mercalli, must be valued at about 22 millionfrancs in Italy alone. For the province of the Alpes Maritimes inFrance, full details are wanting, but the loss there cannot fall farshort of three million francs. The total amount of damage musttherefore be placed at about a million pounds. From the figures givenby the official commissions, it appears that the earthquakes were mostdisastrous at Diano Marina and Diano Castello; while other places, such as Oneglia, Bussana, Baiardo, Pompeiana, and Vallecrosia, suffered only a little less severely. At Mentone about 155 houses, andat Nice about 61 houses, were rendered uninhabitable, and many otherswere badly injured. In Italy, 633 persons were killed, 432 seriously wounded, and 104slightly wounded; in France, 7 persons were killed and 30 seriouslywounded, the number of persons slightly wounded being unknown. Themajority of the deaths occurred in two or three places. Thus, at DianoMarina, 190 persons were killed and 102 wounded; at Baiardo, 220 werekilled and 60 wounded; at Bussana, there were 53 killed and 27wounded. The death-rates were, however, comparatively small, amountingfor the above places to not more than 8-1/2, 14, and 6-1/2 per cent. , respectively; figures which only slightly exceed those obtained forplaces in the meizoseismal area of the Andalusian earthquake. Though the damage can hardly be regarded as excessive, it wasnevertheless largely due to the peculiar architecture prevalent in theRiviera. Arches in the walls are common even in the upper storeys, and, in Oneglia and Diano Marina, if not also in other places, thefloors are nearly always brick arches abutting against the walls andwithout other lateral support. Professor Mercalli believes that, inprivate houses, more than 90 per cent. Of the dead bodies were crushedbeneath these fallen arches. The height of the buildings is also greatin proportion to the foundation and to the thickness of the walls; andthe main walls are interrupted by numerous apertures, from the cornersof which nearly all the fissures sprang. In some of the coast towns, the houses are built of rounded stones gathered from the beach, or ofrubble with stones of all shapes and sizes, bound by cement of thepoorest quality. Lastly, as much of the damage due to previousearthquakes had been badly repaired, it is evident that thedestructiveness of the Riviera earthquakes must to a great extent bereferred to preventable causes. The occurrence of the principal shock shortly after six on the morningof Ash Wednesday must also have increased the death-rate; for manypersons, after a night of amusement, had lain down for a short timeand were sleeping heavily; while others had already risen and werecollected in the churches; the circumstances in either case renderingescape more difficult. Taking account, however, of this accidental increase in the number ofvictims, Professor Mercalli considers that the earthquake of 1887 wasthe most disastrous of all those which have visited either theRiviera or northern Italy in the last three centuries; though, duringthe nineteenth century, there were three Italian earthquakes of fargreater destructive power, but all confined to the southern part ofthe peninsula--namely, the Neapolitan earthquakes of 1805 and 1857, and the Ischian earthquake of 1883. PREPARATION FOR THE EARTHQUAKES. It is difficult, as usual, to specify the exact moment when the firstearthquake of the 1887 series took place; but it is evident that thepreparation for the great shock was very brief. At Oneglia, it isalleged that faint shocks and sounds were observed many times, chieflyat night, during the month preceding February 23rd; though they werenot at the time supposed to be of seismic origin. A slight shock isalso reported from Diano at about midnight on February 21-22. The first undoubted shock occurred on February 22nd, at about 8. 30P. M. , or ten hours before the principal earthquake. Though veryslight, it was felt throughout the Riviera and in part of Piedmont. Another shock, also weak, took place at about 11 P. M. ; and a third, sensible only in the eastern part of the Ligurian Apennines, onFebruary 23rd, at about 1 A. M. ; at which time the tide-gauge at Genoarecorded some abnormal oscillations. An hour later, a more important, though by no means a strong, shock occurred; this was perceptible allover the Riviera, in Piedmont, and in Corsica; in other words, itdisturbed a region agreeing closely with the central area of thedisastrous shock. At about 5 A. M. , a fifth shock, somewhat weakerthan the preceding, was felt over the same area, concurrently, ornearly so, with another abnormal oscillation of the tide-gauge atGenoa; while a sixth shock was noticed at several places a few minutesbefore the great shock. During the night of February 22-23, nervous persons in many towns andvillages were agitated without apparent reason. Birds and animals, more sensitive than human beings to faint tremors, were moredistinctly affected, especially for some minutes before theearthquake. Horses refused food, were restless or tried to escape fromtheir stables, dogs howled, birds flew about and uttered cries ofalarm. As these symptoms were noticed at more than one hundred andthirty places within the Italian part of the central area, there canbe little doubt that they were caused by microseismic movements forthe most part insensible to man. ISOSEISMAL LINES AND DISTURBED AREA. The only complete map of the isoseismal lines is that drawn byProfessor Mercalli. [48] In this map, reproduced in Fig. 33, thecontinuous curves represent the principal isoseismal lines; the dottedcurves define the disturbed areas of two of the stronger after-shocks. The meizoseismal area, bounded by the curve marked 1 in Fig. 33, isalso shown on a larger scale in Fig. 34. At the places denoted bysmall circles in the latter figure, the principal shock was"disastrous, " some of the houses in each being either totally orpartially ruined. At those marked by a small cross, the shock was"almost ruinous"; in other words, numerous houses were damaged, but inno case was the injury of a serious character. The meizoseismal areais thus a narrow band, skirting the Riviera coast from Mentone toAlbissola, a distance of 106 miles, and extending inland for not morethan from nine to twelve miles. The greatest intensity, correspondingto the ruin of many houses with considerable loss of life, wasreached at only a few places between Bussano and Diano Marina, alllying within a littoral band about twenty miles in length and three tothree and a half miles in width. If, however, the epicentre had lainon land, the area would have been much greater, Professor Mercalliestimates about four times greater, than its actual amount. [Illustration: FIG. 33. --Isoseismal lines of the Riviera earthquake. (_Mercalli. _)] The curve marked 2 (Fig. 33) bounds the "almost ruinous" zone; itsexpansion towards the north and contraction towards the west, north-west, and east, being its most noteworthy features. The nextzone, that of slight damage, is contained between the isoseismals 2and 3, the latter curve probably grazing the north end of Corsica. Beyond this lies the "strong" zone, in which the shock was generallyfelt without causing any damage to buildings. Its boundary (marked 4)passes near Marseilles, Como, and Parma, and includes nearly the wholeof Corsica; towards the north-west, in the valley of Aosta, it curvesin towards the isoseismal 3. In the outermost zone of all the shock was "slight, " and towards themargin was only just perceptible. The boundary, which of coursedefines that of the disturbed area, reaches as far north as Basle andDijon, to Perpignan on the west, Trento, Venice, and Pordenone on theeast, and to the south as far as Tivoli (near Rome) and the northernend of Sardinia. In eastern Switzerland, it shows a marked curveinwards; possibly, as Professor Mercalli suggests, from the vibrationshaving to cross the northern Apennines in a direction nearly at rightangles to their axis. Except for this bay, however, the curve differslittle from a circle, the centre of which lies in the sea, a little tothe south of Oneglia, close to the position assigned by other evidenceto the epicentre. The radius of this circle being about 264 miles, itfollows that the disturbed area must have contained about 219, 000square miles--by no means a large amount for so strong an earthquake. POSITION OF THE EPICENTRES. It is evident, from the form of the meizoseismal area shown in Fig. 33, that a mere fringe of it lies upon land, and that the epicentremust be situated some distance out at sea. Other facts may bementioned which point to the same conclusion. There were, forinstance, no purely vertical movements observed, even in the districtswhere the damage done by the shock was greatest. Nor were any largelandslips to be seen in those areas; there were no lasting changes inthe underground water-system; and in general, as Professor Mercalliremarks, all the superficial distortions of the ground which are socharacteristic of the epicentral area of a great earthquake wereconspicuous by their absence. There is evidence, again, of somedisturbance of the sea-bed in the death and flight of fishes fromgreat depths and in the seismic sea-waves recorded by the tide-gaugesat Genoa and Nice. These phenomena will be described in a latersection, but reference should be made here to an interestingobservation at Oneglia on the occurrence of some of the strongerafter-shocks. Persons on the coast, it is said, saw the sea curlingand moving, and immediately afterwards the shock was felt. In determining the position of the epicentre, Professor Mercalli hadrecourse as usual to observations on the direction of the shock, especially those derived from the oscillation of lamps or othersuspended objects, the projection or fall of bodies free to move, fractures, etc. , in damaged houses, and the stopping of pendulumclocks. Such observations were made at 120 places--72 in the westernRiviera and the Alpes Maritimes, and 48 at Piedmont, Lombardy, andTuscany. At many of these places the movement was extremely complicated. Innearly all parts of the area most strongly shaken, for instance, thedirection of the shock changed more than once; and it was thereforenecessary to select whenever possible the principal direction of theshock at each place. In some towns, such as Oneglia, Mentone, Antibes, Cuneo, etc. , the shock had two dominant directions, and these appearedto be sensibly at right angles to one another; an inclination which, as Professor Mercalli suggests, may be due in part to theapproximation of the real directions to those of the principal wallsof the houses in which the observations were made. Most of the lines of direction, when plotted on the map, convergetowards an area lying between the meridians of Oneglia and San Remo, and between nine and fifteen miles from the coast. For places near theepicentre, the most trustworthy, in Mercalli's opinion, are those madeat Oneglia, Mentone, Taggia, Bordighera, Castel Vittorio, Nice, andGenoa; and the points in which these lines Intersect one another areIndicated by small crosses on the map of the meizoseismal area (Fig. 34). All of them lie at sea at distances between six and fifteen milesto the south of Oneglia. The most probable position of the principalepicentre is that marked by the small circle A, which is situatedabout fifteen miles south of Oneglia. [Illustration: FIG. 34. --Meizoseismal area of the Riviera earthquake. (_Taramelli and Mercalli. _)] There are, however, several lines of direction which can have noconnection with this epicentre. Besides the east and west lines atNice, Mentone, and Antibes, there are others at the same places whichrun north and south or nearly so. Professor Mercalli believes thatthey were due to vibrations coming from a second focus lying to thesouth of Nice, and there are also several lines of direction at moredistant places which converge towards the neighbourhood of thecorresponding epicentre. This conclusion receives unexpected support from some of the besttime-records. At the railway-stations of Loano and Pietra Ligure, thetimes of occurrence were given as 6h. 20m. 5s. And 6h. 20m. Respectively--estimates which are probably accurate to within a fewseconds; for, at the moment of the shock, the officer who brought theexact time along the railway-line from Genoa was at Loana, and hadjust passed through Pietra Ligure. On the other hand, the estimatesfor Mentone and Nice--namely, 6h. 18m. 35s. And 6h. 19m. 43s. , if notequally exact, cannot err by many seconds, certainly not by so much asone minute. Since the distances of Loana and Pietra Ligure from theprincipal epicentre are 31 and 32 miles, and those of Mentone and Nice28 and 37 miles, it is therefore clear that the vibrations whicharrived first at Nice and Mentone must have come from a local focus, where the impulse preceded that at the principal focus by severalseconds. DEPTH OF THE PRINCIPAL FOCUS. Inaccurate as are all the methods of determining the depth of focus, it seems probable, as Professor Issel argues, that the principalRiviera focus was situated at a considerable distance from thesurface. In no part of the meizoseismal area was the shock a reallyviolent one; yet its intensity must have faded very slowly outwards, for it was strong enough to stop clocks at places in Switzerland andelsewhere not less than 250 miles from the origin. Professor Mercalli regards Mallet's method with greater favour thanmost seismologists. He points to the gradual increase in the angle ofemergence from the outer zones disturbed by the Riviera earthquaketowards the meizoseismal area, where several good observations weremade from fissures in walls parallel to the dominant direction of theshock. The angles of emergence which he considers as most trustworthyare those of 35° at Taggia, 40° at Oneglia, and about 30° atBordighera. The corresponding depths for the focus are 10. 4, 10. 4, and11. 6 miles, giving an average of about 10-3/4 miles. There are no similar observations forthcoming for the depth of thesecondary focus near Nice and Mentone; but Professor Mercalli observesthat it must have been shallower than the other, for the verticalcomponent of the vibrations from this focus was much less sensiblethan that of the motion coming from the principal focus. NATURE OF THE SHOCK. _The Double Shock. _--In the valuable collection of records made byProfessors Taramelli and Mercalli there appears at first sight to bethe utmost diversity in the evidence with regard to the nature of theshock. Thus, in the province of P. Maurizio alone, the shock wasdescribed as subsultory first and then undulatory or vorticose at 25places, undulatory and then subsultory at 22, undulatory and thensubsultory and again undulatory or vorticose at 13, and subsultoryfirst, then undulatory, and finally subsultory and vorticose at twoplaces. It is clear that the shock was of considerable duration, notless than half-a-minute as a rule, and that there were several phasesin the movement; and it would seem that one or more of these phasesmay have passed unnoticed owing to the alarm occasioned by the shock, and to the fact that most of the observers were asleep when theearthquake began. Defects of memory must also have an influence not tobe neglected, for, even with the simple shocks felt in the BritishIsles, persons in the same or neighbouring places differ greatly intheir testimony. But, if we confine ourselves to the accounts of careful persons alone, the discrepancies to a large extent disappear. Indeed, all over theruinous area (Fig. 33) the shock maintained a nearly uniformcharacter. At Oneglia, for instance, there were two well-markedphases, the first of which began with a brief subsultory movement, followed by more horizontal undulations of longer period; a pause, lasting but for an instant, was succeeded by vibrations which, thoughnot vertical, were highly inclined to the horizon; they continuedthroughout the second phase, but, towards the end, new undulationswere superposed, and these, coming from different directions, resultedin an apparently vorticose movement. Professor Mercalli represents themotion diagrammatically by the curve _a_ in Fig. 35. At Diano Marina, as will be seen from the curve _b_, the shock again consisted of twophases, each beginning with a few subsultory vibrations and endingwith horizontal undulations of much longer period. In the first phase, the undulations were marked by a dominant direction, but, towards theclose of the second phase, there was no determinate direction, and theimpression was again that of a vorticose shock. At Savona, themovement, which is represented by the curve _c_, must have lasted fromtwenty-five to thirty seconds. It also consisted of two phases, withsubsultory vibrations and undulations in the same order; and it wasnoticed that the second part of the shock was much stronger than thefirst. According to some observers, the concluding movements werevorticose. [Illustration: FIG. 35. --Nature of shock of Riviera earthquake. (_Taramelli and Mercalli. _)] In the zone surrounding the ruinous area, the vertical component ofthe motion was observed to diminish with the intensity; but, in otherrespects as well as in duration, the shock retained the same generalform. At Genoa, Turin, Acqui, Alessandria, Antibes, and other places, two distinct phases were perceived, occasionally separated by a briefpause, the first being invariably the weaker. At some places, theobservers speak of two shocks at about 6. 20 A. M. , separated by aninterval of a few seconds; and this division was noticeable as far asSalò on the shore of Lake Garda and Vicenza in Venetia. Only inSwitzerland and other districts near the boundary of the disturbedarea did the weaker part of the shock become insensible, the otherconsisting of horizontal oscillations, remarkable for their slownessand regularity, and lasting for as much as twenty or thirty seconds. We may thus conclude, with Professor Mercalli, that the earthquakeresulted from the almost immediate succession of two distinct shocks, in each of which the nearly vertical vibrations were more marked atthe beginning, while the slower undulations predominated towards theclose, those of the second phase generally becoming vorticose throughthe superposition of movements coming from different directions. Thesecond part of the shock in all of the more carefully written accountsis described as the stronger, especially as regards the subsultoryvibrations in the meizoseismal area; except in the immediateneighbourhood of Nice, where the second phase was generally regardedas the weaker, or at any rate as not stronger than the first. _Origin of the Double Shock. _--These observations show, not only thatthe principal earthquake consisted of two distinct shocks, but alsothat the shocks originated in different foci. For, if the vibrationsof both had started from one focus, the second shock would have beeneverywhere the stronger; instead of which there was a small area nearNice where the intensity of the first was the greater. This pointsclearly to the existence of another focus situated not far from Nice;and it is evident that the greater intensity of the first part in thatdistrict was due solely to the proximity of this focus, for, stillfarther to the west, at Antibes, the second part was again thestronger. There is thus a striking agreement in the inferences drawn fromobservations on the direction, time of occurrence, and nature of theshock. In the face of such concurring testimony, little doubt canremain as to the existence of two foci, one to the south of Onegliaand the other to the south of Nice, the initial impulse at the latterbeing decidedly the weaker, and preceding that at the eastern focus byan interval of some seconds, long enough at any rate for the resultingvibrations to reach the Oneglia focus and to spread beyond it beforethe vibrations from that focus started on their outward journey. _Seismographic Records. _--In 1887, the Riviera and the districtsadjoining it were unprovided with accurately constructed seismographs. The observatories at Alessandria, Milan, Monza, Parma, Florence, andother places in Italy contained seismoscopes and other pendulums, andthese all registered the fact that an earthquake had occurred, and inmany cases traced a series of elliptical or elongated curves. A recordof the shock was also given by a Cecchi seismograph at Perpignan inFrance, but the distance from the epicentre was too great to allowdetails to be shown. The most valuable record was that obtained from aCecchi seismograph at the observatory of Moncalieri, near Turin, aboutninety miles north of the principal epicentre. In this seismograph, the pendulums are provided with pointers, thetips of which touch vertical sheets of paper attached to the sides ofan upright rectangular box. When an earthquake occurs, this box ismade to descend slowly with a uniform velocity, while the movingpointers trace curves upon the smoked paper. The north-and-southcomponent of the horizontal motion is inscribed on the sheet of paperfacing west, and the east-and-west component on the paper facingsouth. [Illustration: FIG. 36. --Seismographic record of the Riviera earthquake at Moncalieri. (_Denza. _)] During the principal Riviera earthquake, the former pendulum furnishedan indistinct record, while the other traced the diagram reproduced inFig. 36. The movement, as here represented, began at about 6h. 21m. 50s. A. M. (mean time of Rome) with a series of small tremors, whichlasted for about twelve seconds. Then followed some largeoscillations, always in a nearly east-and-west direction, which at 6h. 22m. 21s. Gave place to a second series of tremors similar to those atthe beginning of the shock, but of greater amplitude. These continuedfor at least twelve seconds, at the end of which time the motion ofthe smoked paper ceased. The total duration of the movement atMoncalieri cannot therefore have been less than forty-three seconds. Interesting as this record is, it is doubtful how far it representsaccurately the movement of the ground. The Moncalieri instrument waserected before the modern type of seismograph was designed, in whichsome part remains steady, or very nearly steady, during thecomplicated movements of the ground that take place in an earthquake. It will be noticed that the curve in Fig. 36 shows no sign of thedivision of the shock into two distinct parts, and this may perhaps bedue to the swinging of the pendulum itself; in which case, the curvedescribed by the pointer would be the resultant of the oscillations ofthe ground and the proper motion of the pendulum. SOUND-PHENOMENA. The sounds that preceded and accompanied the Riviera earthquake haveattracted but little study, although they seem to have been widelyobserved. No attempt was made to define the limits of the area overwhich they were audible; but Professor Mercalli states that in the twoouter zones (Fig. 33) the sound generally passed unobserved. It was, however, heard near Piacenza in Lombardy and Reggio in Emilia, placeswhich are about 115 and 140 miles from the principal epicentre. In the area in which the shock was most violent, the sound resembledthat of trains and vehicles in motion; while, outside this area itgenerally appeared to be like the hissing of a violent wind. In only afew places was it compared to detonations, the crashes of artillery ordistant thunder. Some observers describe the sound as appearing atfirst as if a strong wind were rising, and then as the roaring of aheavy railway-train passing. Nearly all the observers, who were awake at the beginning of theearthquake, agree in asserting that the sound distinctly preceded anymovement of the ground. From this, as in the case of the Andalusianearthquake, Professor Mercalli infers that the sound-vibrationstravelled with the greater velocity; but, as will be shown in ChapterVIII. , the general precedence of the sound admits of another and moreprobable explanation. THE UNFELT EARTHQUAKE. If the Andalusian earthquake first drew general attention to thedistant spread of unfelt earth-waves, the Riviera earthquake showedthat this was no isolated phenomenon. We know now that the propagationof such waves is only limited by the surface of the earth, but in 1887some doubt was felt at first as to the nature of the disturbance, whether it was magnetic or mechanical in its origin. In 1884, the only observatories at which magnetographs were disturbedwere those of Lisbon, Parc Saint-Maur (near Paris), Greenwich, andWilhelmshaven. In 1887, the magnetographs registered the Rivieraearthquake at these and several other observatories, the distributionof which is shown in Fig. 37. In this sketch-map, the position of theprincipal epicentre is represented by the small cross, while thenearly circular line shows the boundary of the disturbed area. [Illustration: FIG. 37. --Distribution of observatories at which magnetographs were disturbed by the Riviera earthquake. ] Three of the observatories, those of Nice, Lyons, and Perpignan, lieinside this area. At Nice (which is thirty-seven miles from theprincipal epicentre), M. Perrotin states that the magnetograph curvesshow nothing of any interest, except a notable magnetic perturbationon the vertical force curve, the time of which, however, is notstated. [49] At Lyons (211 miles), the declination, horizontal forceand vertical force, magnets were all disturbed at 6h. 25m. 47s. A. M. , and Perpignan (264 miles), all three magnets, but especially those forthe declination and horizontal force, were set abruptly oscillatingat 6h. 25m. 20s. Elsewhere in France, the disturbances were noticed at theobservatories of Parc Saint-Maur and Montsouris, near Paris (about 447miles), and at Nantes (538 miles). At Parc Saint-Maur, all threecurves show a very clear trace of the earthquake at 6h. 25m. 35s. , theoscillations lasting several minutes, and at Montsouris they alsobegan at the same time. At Nantes, the perturbations were so slightthat they escaped notice on a first examination. In Austria, disturbances were observed at Pola (295 miles) and Vienna(506 miles), beginning at 6h. 28m. 35s. And 6h. 30m. 35s. , respectively. They reached Brussels (522 miles) at 6h. 29m. 27s. , andUtrecht (600 miles) at 6h. 28m. 38s. [50] At Wilhelmshaven (690 miles), only the vertical force magnet was affected, the oscillationsbeginning at 6h. 30m. 35s. , and lasting for fourteen minutes. At 6h. 27m. 55s. , the declination and horizontal force magnets of Greenwichobservatory (642 miles) were set vibrating, but no similardisturbances were revealed by the vertical force curve or by the twoearth-current registers. At Kew (652 miles), the horizontal forcemagnetograph was moved by the earthquake at about 6h. 29m. 55s. Thecurves at Stonyhurst and Falmouth show no sign of any disturbance, nordo those at Pawlovsk in Russia, or Seville. At Lisbon (951 miles), however, the three curves indicate disturbances at 6h. 32m. 35s. , butso feeble are they that they would have escaped discovery if theoccurrence of the earthquake had been unknown. The effects registered on the magnetograms are quite different fromthose which correspond to ordinary magnetic perturbations; but theyare not unlike those produced by the action of the momentary currentswhich are used for making the hour-marks, except that theearthquake-oscillations lasted several minutes (see Fig. 21). In eachcase, then, the magnetic bars must have received a succession ofseveral or many impulses. Now, the effect of these impulses on each magnet must depend on therelations which exist between the period of oscillation of the magnet, the rate of damping of such oscillations, and the interval between thesuccessive impulses. Also, the apparent commencement of the phenomenamay be delayed if two impulses of contrary sense should follow oneanother before the bar is perceptibly displaced. It is therefore to beexpected, as M. Mascart points out, that the disturbances of the threeinstruments need not be of the same order of magnitude, that withdifferent forms of apparatus the effects may be very variable, andthat the deflection of one instrument may precede that of another atone and the same place. In all the magnetographs, the record is made on photographic paper, which travels so slowly that the time of a movement can only beascertained to the nearest minute. As the disturbances on the Frenchcurves were apparently almost simultaneous, and as no two of theothers differed in time of occurrence by more than five minutes, thereis thus some colour for M. Mascart's contention that the magneticapparatus registered, not the movements of the ground, but the passageof electric currents produced in the ground at a certain epoch of theearthquake. [51] On the other hand, it is important to notice that, in the central partof the disturbed area, at Nice, two, if not all three, of themagnetographs were unaffected at the time of the earthquake. At first sight, this fact seems equally opposed to a mechanicalexplanation of the disturbance. But, when the vibrations are veryrapid, as they are in the neighbourhood of the epicentre, the magneticbars, owing to their mode of suspension, have not sufficient time tobe sensibly deflected in the brief interval between successive phasesof the impulse. The magnetograms of the Montsouris observatory show, for instance, hardly any perceptible trace of disturbance during thepassage of railway trains along two adjacent lines. The farther, however, the earth-waves travel from the origin, the longer becomesthe period of their vibrations. In Switzerland, they were remarkablefor their slowness, even to the unaided senses. Thus, at places moreor less remote from the Riviera, the magnets would receive impulses atintervals approximating to their own periods of vibration, and theywould then oscillate freely for some time. Again, notwithstanding some variations, it will be remarked that onthe whole the retardation of the initial epoch of the disturbancesincreases with the distance from the epicentre. It thus seems clear, Ithink, that the cause of the disturbances must be sought in the shockitself; although their initial epochs at different places are tooroughly defined for ascertaining the velocity with which theearth-waves travelled. EFFECTS OF THE EARTHQUAKE AT SEA. The Riviera earthquake, owing to its submarine origin, was marked bycertain phenomena that were absent from the other earthquakesdescribed in this volume. _Nature of the Earthquake at Sea. _--At the time of the earthquake, several vessels were close to the epicentral area. One, about threemiles off Diano Marina, was shaken twice at about 6. 20 A. M. , and soviolently that it seemed as if the masts would be broken off. Another, about ten miles south of P. Maurizio, also experienced two shocks, afew minutes apart, as if each time it had struck the bottom. Theseobservations are chiefly interesting in showing that the double shockwas felt at sea as well as on land. As transverse vibrations are notpropagated through water, it follows that the second part of the shockcannot, as some maintain, have been composed of transverse vibrations. _Destruction of Fishes. _--During the days immediately following theearthquake, a large number of deep-sea fishes were found dead orhalf-dead either in shallow water or stranded on the beach, especiallyin the neighbourhood of Nice. Among them were numerous specimens, mostly dead and floating, of _Alepocephalus rostratus_, a typicaldeep-sea form, several of _Pomatomus telescopium_, _Scopeluselongatus_, and _S. Humboldti_, and many of _Dentex macrophthalmus_and _Spinax niger_. The death and flight of these fishes must havebeen due to a sudden shock, almost like that caused by the explosionof dynamite, and communicated simultaneously to the whole surface oftheir bodies. _Seismic Sea-Waves. _--Immediately after the earthquake, the searetired a short distance, variously estimated at from ten to thirtymetres, laying bare some rocks that were usually immersed. At P. Maurizio, the surface was lowered by a little more than a metre; andafter a few minutes it rose to nearly a metre above its originallevel, returning to it after a series of continually-decreasingoscillations. At San Remo, a fall of about the same amount took place, the sea returning after five minutes, and a ship anchored in theharbour broke from her moorings. Again, at Antibes, the sea wassuddenly lowered by about a metre, so that ships afloat in the harbourwere aground for some instants, and then returned with someimpetuosity to its original level. [Illustration: FIG. 38. --Record of tide-gauge at Nice. (_Issel. _)] The evidence of eye-witnesses is confirmed by that of the tide-gaugesat Nice and Genoa, the curves of which are reproduced in Figs. 38 and39. At Nice, the first arrest of the curve in its usual courseoccurred at 6. 30 A. M. ;[52] the sea-level sank somewhat abruptly, andafter a few marked oscillations gradually returned to its normalposition at 7. 50 A. M. At Genoa, the shock caused the writing-pen ofthe tide-gauge to dent the paper on which the record is made, and soonafterwards the curve shows a series of irregular oscillations, abouteight taking place every hour, and gradually decreasing until theyceased to be perceptible about two hours after the principalearthquake. [Illustration: FIG. 39. --Record of tide-gauge at Genoa. (_Issel. _)] MISCELLANEOUS PHENOMENA. _Connection between Geological Structure and the Intensity of theShock. _--As with the Andalusian earthquake, faulty construction anddefective materials were responsible for much of the damage caused bythe Riviera earthquake. But, if we may judge from the sharp localvariations in its amount, the nature of the surface-rocks must haveexerted a still more potent influence. At Cervo, for example, theinjury to property amounted to less than £3 per head of thepopulation; at Diano Marina, only two or three miles to the west, itrose to £22 per head. The death-rate at Cervo was about one-tenth, andat Diano Marina about 8-1/2 per cent. Again, at Mentone, the damagemust have been considerable, for about 155 houses were rendereduninhabitable; while Monte Carlo, only a few miles farther west, escaped almost unharmed. Now, Mentone and Diano Marina are for themost part built on clay or alluvial deposits, and Monte Carlo on afoundation of limestone. Even within the limits of a single town, variations no less strikingwere perceptible. In Mentone, the greatest damage occurred to housesof two storeys built on alluvial soil in the low-lying parts near thesea and in the valleys. The effect of the foundation in this part waswell shown in the case of two equally well-built houses not more than300 yards apart. One in the valley, with doubtful foundations, wasvery much shattered; the other, built on rock, was uninjured. Thelarge hotels, especially those on high ground, suffered least, few ofthem having their main walls seriously damaged. These buildings riseto heights of from four to six storeys, and of necessity have a firmand solid foundation. Professors Taramelli and Mercalli have made a careful study of thesubject of this section. The general conclusions at which they arriveare that the intensity of the shock was greatest at places built onpliocene conglomerates, beds of clay superposed on compact old rocks, patches of alluvium, miocene formations of some thickness formed ofrepeated alternations of strata of incoherent marls and limestones orcompact sandstones, beds of chalk, or somewhat rotten dolomite. The shock was also more destructive on the summits of isolated hillsand ridges and on the steep slopes of mountains. The influence of theform of the ground was, however, subordinate to that exerted by thenature of the subsoil. Thus, at Mentone, as we have seen, and also atNice and Genoa, houses built on rock in elevated positions sufferedmuch less than those situated on the plains below that are composed ofsand and recent alluvium. _Observations of the Earthquake in Railway-Tunnels. _--Observationsmade in mines at various times and places have proved that anearthquake is felt less strongly in deep workings, if felt at all, than on the surface of the ground. In the railway-tunnels of theRiviera, as Professor Issel has shown, the same result was establishedduring the earthquake of 1887. On the line which runs northward from Genoa to Piedmont, a tunnel morethan five miles in length pierces the hilly ground between Ponterossoand Ronco, the greatest thickness of rock above being about a thousandfeet. At the time of the earthquake, the tunnel was not everywhereopened out to its full width, and men were at work in differentsections. Outside, the shock was strong enough to damage buildings. Inside, at about 200 yards from the south end, only a feeble shock wasfelt; at 1, 350 and 1, 625 yards, some bricks were seen to fall from thefacing, but the shock was not otherwise perceived, and only a fewyards farther nothing unusual was noticed by the men at work. Again, in an unfinished tunnel, about three-quarters of a mile long, between the harbour of Genoa and the eastern railway-station, thevibrations were very slightly felt. Even in the tunnels traversed bythe coast railway from Genoa to Nice--that is, in those situatedwithin the meizoseismal area--the shock was either very weak or notfelt at all, and not one of the tunnels suffered the slightest injury. To men at work inside a long tunnel, the conditions for observingearthquakes are somewhat imperfect, but these facts, nevertheless, bring out very clearly the inferior intensity of the shock at somedepth below the surface. AFTER-SHOCKS. While the unfelt earth-waves of the great earthquake were stillwending their way over the zone that surrounds the disturbed area, thecentral regions were again shaken, at 6. 29 A. M. , by a shock strongenough to produce fresh ruins in the stricken towns along the coast. Nearly two and a half hours of quiet followed, broken only by a fewsubterranean rumblings in the central part of the meizoseismal area. Then, at 8. 51 A. M. , occurred another shock, short and sharp, andinferior in strength only to the principal earthquake. Both of theseafter-shocks were felt in Western Switzerland; indeed, they wereperceptible nearly as far as the great shock; the second, however, alittle farther than the first, for it alone was noticed at such placesas Vicenza, Forlì, and Florence. The shock at 6. 29 was usuallydescribed as long and its vibrations as undulatory only; that at 8. 51as rather subsultory than undulatory and of very brief duration. Thelatter, however, was followed after an interval of a few seconds byanother shock so weak that it generally passed unobserved. Both shockswere preceded by a rumbling sound. During the next two days, tremors and earth-sounds were frequent inthe Riviera; once an hour, on an average, the greater part of themeizoseismal area was shaken by vibrations more or less slight. But, between one shock and another, at Diano Marina and Alassio, and evenas far as Nice, it only required attention from a careful observer toperceive an almost continual throbbing of the ground. Only one of these shocks, that of February 24th, at 2. 10 A. M. , wasstrong enough to cause slight damage to buildings. It disturbed anarea, not exceeded by any of the later shocks, the boundary of which, shown by the dotted line A in Fig. 33, extends to the north and eastas far as Piacenza and Spezia, while to the west it includes Cannes. The centre of the curve so drawn lies on land, but, as the shock wasnot felt in Corsica, there is no evidence as to the southerlyextension of the disturbed area; and it is probable, as ProfessorMercalli suggests, that the shock originated in the eastern or Onegliafocus of the great earthquake. After February 25th, slight shocks were felt during the nextfortnight, at the rate of three or four a day, until March 11th, whenthe last after-shock resulting in slight damage occurred at about 3. 12P. M. The boundary of its disturbed area, represented in Fig. 33 by thedotted line B, passes a little to the east of Savona, and then throughAlessandria, Moncalieri, and Marseilles. The shock, however, was notobserved in Corsica, so that the exact position of the epicentre isunknown; but Professor Mercalli believes it to coincide with thewestern or Nice epicentre of the principal earthquake. At the momentof the shock, the sea was observed from Alassio to curl and to riseslightly, while the tide-gauge at Nice, which had traced a continuouscurve earlier in the day, showed a characteristic notch about 3. 7 P. M. Of the remaining after-shocks, only two attained any notable degree ofstrength. One, on May 20th at about 8. 15 A. M. , disturbed an areanearly concentric with that of the great earthquake, and with aboundary coinciding nearly with the isoseismal 2 in Fig. 33. Again, onJuly 17th at 11. 30 P. M. , occurred a shock felt over an area nearly aslarge as that disturbed on February 24th at 2. 10 A. M. , and situated inthe same part of the country. Altogether, during the year following the Riviera earthquake, Professor Mercalli records 190 after-shocks, most of them slight oronly just felt. With the exception of the first two (on February23rd), none was observed outside the isoseismal 4 of the principalearthquake (Fig. 33); and, of the rest, only the four whose dates aregiven above disturbed an area of more than one-eighth of that of thegreat shock. Some of them, like the shock of March 11th, were strongerin the western part of the meizoseismal area; but the majorityaffected most the eastern portion and seem to be closely associatedwith the Oneglia focus. From February 26th to April 20th, Professor Rumi made observations onthe after-shocks by means of the Foucault pendulum erected at Genoafor demonstrating the rotation of the earth. In nearly every case, theoscillations took place along a north-east and south-west line, or inthe same direction as the first great shock--a resemblance whichsupports the inference that many of the after-shocks originatedwithin the Oneglia focus. ORIGIN OF THE EARTHQUAKES. _Recent Movements in the Riviera. _--The earliest movements thatresulted in the great range of the Maritime Alps and the LigurianApennines date from pre-Carboniferous times, when the centralcrystalline massifs in part emerged. At the end of the Liassic epoch, the secondary formations of the district were uplifted, and it was atthis time that the range assumed its characteristic curved form. Laterstill, at the close of the Eocene period, an elevation of more than9000 feet took place, for upper Eocene beds are found at this heightin the Maritime Alps. Since that time, other important movements have occurred. Pliocenedeposits have been found in the Riviera at an altitude of 1, 800 feet. Recent soundings in the Gulf of Genoa have also shown that all thevalleys of the Riviera between Nice and Genoa are continued far belowthe level of the sea to depths of not less than 3000 feet. Thus, atthe end of the Pliocene or beginning of the Quaternary period, therewas an elevation of nearly 5000 feet, accompanied or followed by theerosion of the valleys which, later on, during the Quaternary period, were submerged about 3000 feet. Even in still more recent times, probably in the Palæolithic age, minor movements continued. Traces ofrecent elevation, varying in amount from a few feet to sixty feet ormore, occur at the Balzi Rossi in the Alpes Maritimes, near Bergeggi, and in Genoa; while evidences of submergence are to be found nearMonaco, at Beaulieu and at Diano Marina. It is important to noticethat the great movements dating from the end of the Eocene period arealmost confined to the Maritime Alps and the western portion of theRiviera. In the parts of Piedmont lying to the north of Cuneo and inthe eastern Riviera, they produced hardly any sensible effect. _Seismic History of the Riviera. _--The movements just referred to arethose which, in course of time, have become sensible to the eye. Theyrepresent the sum of a long-continued series of displacements that mayonce have been on a large scale, but are now comparatively small. Theearthquakes that occur in the Riviera show, however, that the finalstage has not yet been reached. Their epicentres indicate the regionsin which slips are still taking place, and the magnitude of theseslips is roughly measured by the intensity of the resulting shocks. The map in Fig. 40 is one of a series drawn by Professor Mercalli torepresent the distribution of seismic activity in Piedmont and theRiviera. It corresponds to the period from 1801 to 1895. The wholearea is divided into a number of seismic districts, each of which isdistinguished by a particular degree of activity. In estimating thisquantity, Professor Mercalli takes intensity as well as frequency intoaccount. Thus, the lowest degree, represented by the lightest tint ofshading, corresponds to one or two strong earthquakes with a fewmoderate or slight shocks; the eighth and highest to four or fiveruinous or disastrous earthquakes followed by trains of after-shocks. The map shows very clearly that, during the last century, the seismicactivity was greatest in the Maritime Alps and the westernRiviera--that is, in the very districts in which the recentmountain-making movements have been most conspicuous. [53] [Illustration: FIG. 40. --Distribution of seismic activity in the Riviera. (_Mercalli. _)] In all these districts, Professor Mercalli distinguishes severalwell-marked seismic centres, to each of which he traces the origin oftwo or more earthquakes. In the districts with which we are at presentconcerned, those of the Alpes Maritimes and the western Riviera, themost important centres are situated near Oneglia (in the sea), nearTaggia, in the valleys of the Vesubia and Tinea (near Nice), and inthe sea to the south of Nice. To the first of these centres belongsthe disastrous earthquake of February 23rd, 1887, as well as itsafter-shocks on February 24th, May 20th, July 17th, and September 30thof the same year, also the ruinous earthquakes of 1612 and 1854, andseveral others of a lesser degree of intensity. All of these werelongitudinal earthquakes, the axes of their meizoseismal areas beingparallel to the neighbouring mountain-ranges. A few miles to the westof Oneglia lies the Taggia centre, with which were connected thedisastrous earthquake of 1831, the violent earthquake of 1874, andother strong or very strong shocks. These were for the most parttransversal earthquakes, their axes being perpendicular to those ofthe Oneglia centre. Some of the strongest earthquakes in this region originated in acentre lying to the north of Nice in the valleys of the Vesubia andTinea. Among them may be mentioned the ruinous earthquakes of 1494, 1556, 1564, and 1644, and probably also the disastrous earthquake of1227. A fourth centre, and one of considerable interest, is that whichlies at sea, a short distance to the south of Nice, and nearly alongthe continuation of the valleys above-mentioned. This is the secondarycentre of the earthquake of 1887, and probably also of that ofDecember 29th, 1554. It is occasionally in action apart from theOneglia centre, as on November 27th, 1771, June 19th, 1806, andDecember 21st, 1861; but such shocks, though rather strong, neverreach a high degree of intensity. _Origin of the Earthquakes of 1887. _--The most important feature inthe principal earthquake of 1887 is its origination in two distinctfoci, which are sometimes in action almost simultaneously, but moreoften separately. The earthquakes belonging to the two foci differgreatly in intensity and number, and the stronger part of the shock in1887 originated in the focus associated with the more disastrous andmore frequent earthquakes. The existence of two foci would of course give rise to a meizoseismalarea elongated in the direction of the line joining them. It is clear, however, that the Oneglia focus was also extended in the samedirection; for, in the after-shock of February 24th, the isoseismalsdrawn by Professor Mercalli are parallel to this line; and this wasalso the case in the shock of March 11th. As both foci were under thesea, it is difficult to locate them with precision; but it seems veryprobable that they occupy portions of a submarine fault that runsparallel or nearly so to the Apennine axis between the meridians ofOneglia and Nice. A brief period of preparation is a characteristic of the Rivieraearthquakes. In 1887, two at least of the preliminary shocks onFebruary 23rd (those of about 2 and 5 A. M. ) originated in the Onegliafocus. At 6. 20 A. M. The first and weaker movement took place in thewestern focus; and, a few seconds after the resulting vibrationsreached the eastern focus, the second and greater slip took placethere. The occurrence of seismic sea-waves is probably evidence of theformation of a small, though sensible, fault-scarp in the same region. To relieve the additional stresses thus brought into action along thefault-surface, numerous small slips took place in different parts, some as far to the west as the Nice focus, but the greater numberprobably within or close to the focus in the neighbourhood of Oneglia. REFERENCES. 1. BERTELLI, T. --"Osservazioni fatte in occasione di una escursione sulle Riviera Ligure di ponente dopo i terremoti ivi seguiti nell' anno 1887. " _Boll. Mens. Dell' Oss. Di Moncalieri_, vol. Viii. , 1888, Nos. 6, 7, 8. 2. CHARLON, E. --"Note sur le tremblement de terre du 23 février 1887. " _Bull. Del Vulc. Ital. _, anno xiv. , 1887, pp. 18-23. 3. DENZA, F. --_Alcune notizie sul terremoto del 23 febbraio 1887_ (Turin). 4. ISSEL, A. --"Il terremoto del 1887 in Liguria. " _Boll. Del R. Com. Geol. D'Italia_, anno 1887, supplemento, pp. 1-207. 5. MERCALLI, G. --_I terremoti della Liguria e del Piemonte_. (Naples, 1897, 146 pp. ) 6. ODDONE, E. --"I dati sismici della Liguria in rapporto alla frequenza ed alla periodicità. " _Boll. Della Soc. Sismol. Ital. _, vol. Ii. , 1896, pp. 140-151. 7. OFFRET, A. --"Sur le tremblement de terre du 23 février 1887. Discussion des heures observés dans la zone épicentrale. " Paris, _Acad. Sci. , Compt. Rend. _, vol. Civ. , 1887, pp. 1150-1153. 8. ----. "Tremblements de terre du 23 février 1887. Heures de l'arrivée des secousses en dehors de l'épicentre. " _Ibid. _, pp. 1238-1242. 9. ROSSI, M. S. DE. --"Relazione sui terremoti del febbraio 1887. " _Bull. Del Vulc. Ital. _, anno xiv. , 1887, pp. 5-17. 10. ----. "Bibliografia: Sul terremoto ligure del 23 febbraio 1887. " _Ibid. _, pp. 60-62, 107-112, 115-128. 11. TARAMELLI, T. , and G. MERCALLI. --"Il terremoto ligure del 23 febbraio 1887. " _Annali dell' Uff. Centr. Di Meteor. E di Geodin. _, vol. Viii. , parte iv. , 1888. (Roma, 298 pp. ) 12. UZIELLI, G. --_Le commozioni telluriche e il terremoto del 23 febbraio 1887_ (Turin). 13. _Nature_, vol. Xxxv. , 1887, pp. 438, 462, 534-535; vol. Xxxvi. , 1887, pp. 4, 151-152. 14. Paris, _Acad. Sci. Compt. Rend. _, vol. Civ. , 1887, pp. 556-557, 606-612, 634-635, 659-667, 744-745, 757-758, 759-760, 764-766, 822-823, 830-835, 884-890, 950-951, 1088-1089, 1243-1245, 1350-1352, 1416-1419; vol. Cv. , 1887, pp. 202-203; vol. Cviii. , 1889, p. 1189; vol. Cix. ; 1889, pp. 164-166, 272-274, 660. FOOTNOTES: [47] The above times and all others in this chapter are given in Romemean time, which is 50m. Earlier than Greenwich mean time. [48] Professor Uzielli has also published a map of the isoseismallines for the Italian part of the disturbed area. [49] It seems doubtful whether this movement was connected with theearthquake. M. Offret does not include Nice in his list ofobservatories at which magnetographs were disturbed. [50] This is the time given by M. Offret. According to M. Mascart, itshould be 6h. 25m. 40s. [51] In order to test the truth of this explanation, M. Moureauxsuspended a bar of copper at the Parc Saint-Maur observatory by twothreads in the same way as the horizontal force-magnet. The directionof this bar was also registered photographically, and it remainedunmoved during the Verny earthquake of July 12th, 1889, and theDardanelles earthquake of October 25th, 1889, while one or more of themagnets were disturbed. The experiment, however, was ineffective; for, in order that the magnet may rest in a horizontal position, its centreof gravity must be at unequal distances from the two points ofsupport. [52] The hour-marks in Fig. 38 refer to Paris mean time, and those inFig. 39 to Genoa mean time. [53] In the seventeenth century, the maximum seismic activity wasmanifested in the neighbourhood of Nice, and in the eighteenth centuryin Piedmont. CHAPTER VII. THE JAPANESE EARTHQUAKE OF OCTOBER 28TH, 1891. Although several years have elapsed since the occurrence of thegreatest of Japanese earthquakes, the final report that will embodythe labours of all its investigators is yet to be written. Severalimportant contributions to it, however, have already been made. Professor Koto, in an admirable memoir, has traced the course of thegreat fault-scarp and discussed the origin of the earthquake;Professor Omori, with equal care and thoroughness, has investigatedthe unrivalled series of after-shocks; Mr. Conder studied the damagedbuildings from an architect's point of view; Professor Tanakadate andDr. Nagaoka devoted themselves to a re-determination of the magneticelements of the central district, [54] while, by the compilation of hisgreat catalogue of Japanese earthquakes during the years 1885-92, Professor Milne has provided the materials for a further analysis ofthe minor shocks that preceded and followed the principal earthquake. The part of Japan over which the earthquake was sensibly felt isshown in Fig. 41. The small black area in the centre is that in whichthe shock was most severe and the principal damage to life andproperty occurred. The other bands, more or less darkly shadedaccording to the greater or less intensity of the shock, will bereferred to afterwards. Fig. 45 represents the meizoseismal area on alarger scale; and, as the greater part of it lies within the twoprovinces of Mino and Owari, the earthquake is generally known amongthe Japanese themselves as the Mino-Owari earthquake of 1891. [Illustration: FIG. 41. --Sketch-Map of Disturbed Area and Isoseismal Lines. (_Masato. _)] THE MEIZOSEISMAL AREA. More than half of the meizoseismal area occupies a low flat plain ofnot less than 400 square miles in extent. On all sides but the south, the plain, which is a continuation of the depression forming the Seaof Isé, is surrounded by mountain ranges, those to the west, north, and north-east being built up mainly of Palæozoic rocks, and those onthe east side of granite. A network of rivers and canals converts whatmight otherwise have been unproductive ground into one of the mostfertile districts in Japan. A great garden, as it has been aptlytermed, the whole plain is covered with rice-fields, and supports apopulation of about 787 to the square mile--a density which isexceeded in only six counties of England. As a rule, the soil is aloose, incoherent, fine sand, with but little clayey matter; and itis, no doubt, to its sandy nature that the disastrous effects of theearthquake were largely due. In the northern half of the district, themeizoseismal area is much narrower, and here it crosses a greatmountain-range running from south-west to north-east and separatingthe river-systems of the Japan sea from those of the Pacific. To thenorth, the meizoseismal area terminates in another plain, in thecentre of which lies the city of Fukui, where the destructiveness ofthe earthquake was only inferior to that experienced in the provincesof Mino and Owari. There is also a detached portion of the area lyingto the east of Lake Biwa, but it is uncertain whether the exceptionalintensity there was due to the nature of the ground or to theoccurrence of a secondary or sympathetic earthquake in its immediateneighbourhood. [Illustration: FIG. 42. --General Plan of Geological Structure of Meizoseismal Area. (_Koto. _)] The general plan of the geological structure of the central districtis represented in Fig. 42. The thick line, partly continuous andpartly broken, shows the course of the great fault, to the growth ofwhich the earthquake chiefly owed its origin; while the thincontinuous lines represent the changing direction of strike of thePalæozoic rocks which surround the Mino-Owari plain, and thearrowheads the direction of the dip. It will be seen that thedirection of the strike forms an S-shaped curve, and it is clear thatthe present torsion-structure of the district could not have beenproduced without the formation of many fractures at right angles andparallel to the lines of strike. Professor Koto points out that theregular and parallel valleys of the rivers Tokuno-yama, Neo, Mugi, andItatori, indicated by broken lines in Fig. 42, have probably beenexcavated along a series of transverse fractures running fromnorth-west to south-east; while fractures which are parallel to theline of strike may be responsible for the zigzag course of thevalleys. DAMAGE CAUSED BY THE EARTHQUAKE. The great earthquake occurred at 6. 37 A. M. , practically withoutwarning, and in a few seconds thousands of houses were levelled withthe ground. Within the whole meizoseismal area there was hardly abuilding left undamaged. The road from Nagoya to Gifu, more thantwenty miles in length, and formerly bordered by an almost continuoussuccession of villages, was converted into a narrow lane between twolong drawn-out banks of _débris_. "In some streets, " says ProfessorMilne, "it appeared as if the houses had been pushed down from theend, and they had fallen like a row of cards. " Or, again, a mass ofheaped-up rubbish might be passed, "where sticks and earth and tileswere so thoroughly mixed that traces of streets or indications ofbuilding had been entirely lost. " At Gifu, Ogaki, Kasamatsu, and othertowns, fires broke out after the earthquake. In Kasamatsu thedestruction was absolutely complete; nothing was left but a heap ofplaster, mud, tiles, and charred timbers. At Ogaki, not more thanthirty out of 8000 houses remained standing, and these were all muchdamaged. Within the whole district, according to the official returns, 197, 530 buildings were entirely destroyed, 78, 296 half destroyed, and5, 934 shattered and burnt; while 7, 279 persons were killed, and 17, 393were wounded. Next to buildings, the embankments which border the rivers and canalssuffered the most serious damage, no less than 317 miles of such workshaving to be repaired. Railway-lines were twisted or bent in manyplaces, the total length demolished being more than ten miles. Incuttings, twenty feet or more in depth, both rails and sleepers wereunmoved; it was on the plains that the effects of the earthquake weremost marked. The ground appeared as if piled up into bolster-likeridges between the sleepers, and in many places the sleepers had movedend-ways. When the line crossed a small depression in the generallevel of the plain, the whole of the track was bowed, as if the groundwere permanently compressed at such places. "Effects of compression, "says Professor Milne, "were most marked on some of the embankments, which gradually raise the line to the level of the bridges. On some ofthese, the track was bent in and out until it resembled a serpentwriggling up a slope.... Close to the bridges the embankments hadgenerally disappeared, and the rails and sleepers were hanging in theair in huge catenaries. " ISOSEISMAL LINES AND DISTURBED AREA. The land area disturbed by the earthquake and the different isoseismallines are shown in Fig. 41. The "most severely shaken" district, thatin which the destruction of buildings and engineering works wasnearly complete, contains an area of 4, 286 square miles, or abouttwo-thirds that of Yorkshire. This is indicated on the map by theblack portion. Outside this lies the "very severely shaken" district, 17, 325 square miles in area, extending from Kobe on the west toShizuoka on the east, in which ordinary buildings were destroyed, walls fractured, embankments and roads damaged, and bridges brokendown. The third or "severely shaken" district contains 20, 183 squaremiles; and in this some walls were cracked, pendulum clocks stopped, and furniture, crockery, etc. , overthrown. Tokio and Yokohama lie justwithin this area. In the fourth region the shock was "weak, " themotion being distinctly felt, but not causing people to runout-of-doors; and in the fifth it was "slight, " or just sufficient tobe felt. These two regions together include an area of 51, 976 squaremiles. Thus, the land area disturbed amounts altogether to 93, 770 squaremiles--_i. E. _, to a little more than the area of Great Britain. According to Professor Omori, the mean radius of propagation was about323 miles, and the total disturbed area must therefore have been about330, 000 square miles, or nearly four times the area of Great Britain. Considering the extraordinary intensity of the shock in the centraldistrict, this can hardly be regarded as an over-estimate. The isoseismal lines shown in Fig. 41 are not to be regarded as drawnwith great accuracy; for there is no marked separation between thetests corresponding to the different degrees of the scale ofintensity. The seismographs at Gifu and Nagoya were thrown downwithin the first few seconds, and failed to record the principalmotion. But a great number of well-formed stone lanterns andtombstones were overturned, and, from the dimensions of these, Professor Omori calculated the maximum horizontal accelerationnecessary for overturning them at fifty-nine places within themeizoseismal area. [55] At five of these it exceeded 4000 millimetresper second per second, an acceleration equal to about five-twelfths ofthat due to gravity. Making use of these observations, Professor Omorihas drawn two isoseismal lines within the central district, which areshown in Fig. 44. At every point of the curve marked 2, the maximumacceleration was 2000 millimetres per second per second, and of thatmarked 1, 800 millimetres per second per second. The dotted linewithin the curve marked 2 represents the boundary of the meizoseismalarea, which, it will be observed, differs slightly from that given byProfessor Koto (see Fig. 45). The difference, however, is apparentlydue to the standard of intensity adopted, Professor Koto's boundaryagreeing rather closely with the curve marked 2 in Fig. 44. NATURE OF THE SHOCK. Little has yet been made known with regard to the nature of the shock, and the published records of the accompanying sound are so rare thatit seems as a rule to have passed unheard. The seismographs at Gifuand Nagoya registered the first half-dozen vibrations, and were thenburied beneath the fallen buildings. In the following table, the datafrom these two stations are therefore incomplete:-- PRINCIPAL MEASUREMENTS OBTAINED FROM SEISMOGRAPHIC RECORDS. -------------------+------------+----------+----------+--------------- | | | | Tokio | Gifu. | Nagoya. | Osaka. | (Imp. Univ. ). -------------------+------------+----------+----------+---------------Maximum horizontal | | | |motion | > 18 mm. | > 26 mm. | 30 mm. | > 35 mm. | | | |Period of ditto | 2. 0 secs. | 1. 3 sec. | 1. 0 sec. | 2. 0 secs. | | | |Maximum vertical | | | |motion | > 11. 3 mm. | 6. 2 mm. | 8 mm. | 9. 5 mm. | | | |Period of ditto | 0. 9 sec. | 1. 5 sec. | 1. 0 sec. | 2. 4 secs. -------------------+------------+----------+----------+--------------- If the period of the principal vibrations were known, the observationsof Professor Omori on the overturning of bodies would enable us todetermine the range of motion at different places. For instance, themaximum acceleration at Nagoya was found by these observations to be2, 600 millimetres per second per second, and if we take the period ofthe greatest horizontal motion to be the same as that of the initialvibrations--namely, 1. 3 second, the total range (or double amplitude)would be 223 millimetres, or 8. 8 inches. With the same period, and themaximum acceleration observed (at Iwakura and Konaki) of more than4, 300 millimetres per second per second, the total range would begreater than 14. 5 inches. [56] In the meizoseismal area, many persons saw waves crossing the surfaceof the ground. At Akasaka, according to one witness, the waves camedown the streets in lines, their height being perhaps one foot, andtheir length between ten and thirty feet. To the north of the samearea, we are told that "the shoreline rose and fell, and with thisrising and falling the waters receded and advanced. " Even at Tokio, which is about 175 miles from the epicentre, the tilting of the groundwas very noticeable. After watching his seismographs for about twominutes, Professor Milne next observed the water in an adjoining tank, 80 feet long and 28 feet wide, with nearly vertical sides. "At thetime it was holding about 17 feet of water, which was running acrossits breadth, rising first on one side and then on the other to aheight of about two feet. " Still clearer is the evidence of theseismographs in the same city. Instead of a number of irregular waves, all the records show a series of clean-cut curves. The heavy masses inthe horizontal pendulums were tilted instead of remaining as steadypoints. They were not simply swinging, for the period of theundulations differed from that of the seismograph when set swinging, and also varied in successive undulations. It was ascertainedafterwards, by measurement with a level, that to produce thesedeflections, the seismograph must have been tilted through an angle ofabout one-third of a degree. _Direction of the Shock. _--Shortly after the earthquake, ProfessorOmori travelled over the meizoseismal area and made a large number ofobservations on the directions in which bodies were overturned, takingcare to include only those in which the direction of falling would notbe influenced by the form of the base, such as the cylindrical stonelanterns so frequently found in Japanese gardens. At some placesthese bodies fell in various directions, at others with considerableuniformity in one direction. For instance, at Nagoya, out of 200 stonelanterns with cylindrical stems, 119 fell between west and south, and36 between east and north; the numbers falling within successiveangles of 15° being represented in Fig. 43. The mean direction of fallis W. 30° S. , coinciding with that in which the majority of thelanterns were overturned. Similar observations were made at forty-twoother places within and near the meizoseismal area, and the resultingmean direction for each such place in the Mino-Owari district is shownby short lines in Fig. 44, the arrow indicating the direction towardswhich the majority of bodies at a given place were overturned. It willbe seen from this map that the direction of the earthquake motion wasgenerally at right angles, or nearly so, to that of the neighbouringpart of the meizoseismal zone, and that on both sides of it, themajority of overturned bodies at each place fell towards this zone. [Illustration: FIG. 43. --Plan of Directions of Fall of Overturned Bodies at Nagoya. ] [Illustration: FIG. 44. --Map of Mean Directions of Shock and Isoseismal Lines in Central District. (_Omori. _)] VELOCITY OF THE EARTH-WAVES. The times of the great earthquake and of sixteen minor shocks onOctober 28th and 29th and November 6th were determined at the CentralMeteorological Observatory at Tokio, and at either two or three of theobservatories of Gifu, Nagoya, and Osaka, each of which is providedwith a seismograph and chronometer. The after-shocks referred tooriginated near a point about 6 miles west of Gifu, and the differencebetween the distances of Tokio and Osaka from this point is 89-1/2miles, of Tokio and Nagoya 147 miles, and of Tokio and Gifu 165 miles. The mean time-intervals between these three pairs of places were 67, 111, and 128 seconds respectively; and these give for the meanvelocity for each interval 2. 1 kilometres (or 1. 3 mile) per second. Thus there appears in these cases to be no sensible variation in thevelocity with the distance from the origin. As might be expected, an earthquake of such severity was recorded bymagnetometers at several distant observatories. Disturbances on theregisters of Zikawei (China), Mauritius, Utrecht, and Greenwich havebeen attributed to the Japanese earthquake, but the times at whichthey commenced are too indefinite to allow of any determination of thesurface-velocity of the earth-waves to great distances from theorigin. THE GREAT FAULT-SCARP. As in all disastrous earthquakes, the surface of the ground wasscarred and rent by the shock. From the hillsides great landslipsdescended, filling the valleys with _débris_; and slopes which wereformerly green with forest, after the earthquake looked as if they hadbeen painted yellowish-white. Innumerable fissures cut up the plains, the general appearance of the ground, according to Professor Milne, being "as if gigantic ploughs, each cutting a trench from 3 to 12 feetdeep, had been dragged up and down the river-banks. " But by far themost remarkable feature of the earthquake was a great rent or fault, which, unlike the fissures just referred to, pursued its courseregardless of valley, plain, or mountain. Although at first sightquite insignificant in many places, and some time hardly visible tothe untrained eye, Professor Koto has succeeded in tracing this faultalong the surface for a distance of forty miles, and he gives goodreasons for believing that its total length must be not less thanseventy miles. [Illustration: FIG. 45. --Map of Meizoseismal Area. (_Koto. _)] [Illustration: FIG. 46. --Ploughshare Appearance of the Fault near Fujitani. (_Koto. _)] [Illustration: FIG. 47. --The Fault-scarp at Midori. (_Koto. _)] The general character of the fault-scarp changes with the surfacefeatures. On flat ground, where the throw is small, it cuts up thesoft earth into enormous clods, or makes a rounded ridge from one totwo feet high, so that it resembles, more than anything else, thepathway of a gigantic mole (Fig. 46). When the throw isconsiderable--and in one place it reaches from 18 to 20 feet--thefault-scarp forms a terrace, which from a distance has the appearanceof a railway embankment (Fig. 47). Or, again, where the rent traversesa mountain ridge or a spur of hills, "it caused extensive landslips, one side of it descending considerably in level, carrying the forestwith it, but with the trees complicatedly interlocked or prostrate onthe ground. " [Illustration: FIG. 48. --Displacement of Field Divisions by the Fault near Nishi-Katabira. (_Koto. _)] At its southern end, the fault was seen for the first time crossing afield near the village of Katabira. The field was broken into clods ofearth, and swollen up to a height of 5-1/2 yards, while a greatlandslip had descended into it from an adjoining hill. A littlefarther to the north-west, the ground was sharply cut by the fault, the north-east side having slightly subsided and at the same time beenshifted horizontally through a distance of 3-1/4 to 4 feet to thenorth-west Adjoining fields were formerly separated by straight moundsor ridges running north and south and east and west, and these moundswere cut through by the fault and displaced, as shown in Fig. 48. Fromthis point the fault runs in a general north-westerly direction, thenorth-east side being always slightly lowered with respect to theother and shifted to the north-west. Near Seki it takes a morewesterly direction, and continues so to a short distance east ofTakatomi, where the north side is lowered by five feet, and movedabout 1-1/4 feet to the west. At the north end of Takatomi, a villagein which every house was levelled with the ground, the fault isdouble, and the continuous lowering towards the north has converted aonce level field into sloping ground. At this point, the small riverToba, flowing south, is partially blocked by the fault-scarp, and anarea of about three-quarters of a square mile, on which two villagesstand, was converted into a deep swamp (Fig. 49), so that, as theearthquake occurred at the time of the rice-harvest, the farmers wereobliged to cut the grain from boats. After passing Takatomi, the faultagain turns to the west-north-west, but, the throw being small, itresembles here the track of an enormous mole. At Uméhara it crosses agarden between two persimmon trees, appearing on the hard face of theground as a mere line; but the trees, which were before in aneast-and-west line, now stand in one running north and south, withoutbeing in the least affected by the movement (Fig. 50). From here toKimbara, where the fault enters the Neo valley, the north side isalways depressed and shifted westwards by about 6-1/2 feet. [Illustration: FIG. 49. --Map of Swamp formed by stoppage of River Toba by Fault-scarp. (_Koto. _)] [Illustration: FIG. 50. --Shifting of Trees by fault at Uméhara. (_Koto. _)] It was in the Neo valley that the supreme efforts of the earthquakewere manifested. Landslips were so numerous that the greater part ofthe mountain slopes had descended into the valley, the wholeappearance of which had changed. "Unfamiliar obstacles, " remarksProfessor Koto, "made themselves apparent, and small hills coveredwith forest had come into sight which had not been seen before. " Butthe ground was not only lowered and shifted by the fault; it waspermanently compressed, plots originally 48 feet in length afterwardsmeasuring only 30 feet. In fact, "it appears, " in the words ofProfessor Milne, "as if the whole Neo valley had become narrower. " A few miles after entering the Neo valley, the throw of the faultreaches its maximum at Midori. But instead of the relative depressionof the east side, which prevails throughout the rest of the line, thatside is here about 20 feet higher than the other. It is, however, shifted as usual towards the north, by about 13 feet; and thisdisplacement is rendered especially evident by the abrupt break in theline of a new road to Gifu (Fig. 47). That the east side has reallyrisen is clear, for, a little higher up, the river has changed from ashallow rapid stream 30 yards wide into a small lake of more thantwice the width, and so deep that a boatman's pole could not reach thebottom. At Itasho, about a mile north of Midori, both sides are nearlyon the same level, the fault appearing like a mole's track; and sevenmiles farther, at Nagoshima, the east side is relatively depressed bymore than a yard, and at the same time shifted about 6-1/2 feet to thenorth. [Illustration: FIG. 51. --Daily frequency of after-shocks at Gifu and Nagoya. ] At Nogo, the main Neo valley turns off at right angles to the east, and the fault continues its course up a side valley, the east side, with respect to the other, being continually depressed and shiftedtowards the north. It was traced by Professor Koto through Fujitani(Fig. 46), where there were many unmistakable evidences of theviolence of the shock, as far as the eastern shoulder of Haku-san; andhere, after following the fault for 40 miles, the lateness of theseason compelled him to return. There can be no doubt, however, thatit runs as far as Minomata; and it is probable, from the linearextension of the meizoseismal area, that it does not entirely die outbefore reaching the city of Fukui, 70 miles from its starting-point atKatabira. MINOR SHOCKS. For some hours after the earthquake, shocks were so frequent in themeizoseismal area that the ground in places hardly ever ceased fromtrembling. Without instrumental aid, detailed record was of courseimpossible; but fortunately the buried seismographs at Gifu and Nagoyawere uninjured, and in about seven hours both were once more inworking order. To the energy by which this result was accomplished, weowe our most valuable registers of the after-shocks of a greatearthquake. [Illustration: FIG. 52. --Monthly frequency of after-shocks at Gifu. (_Omori. _)] Until the end of 1893--that is, in little more than two years--thetotal number of shocks recorded at Gifu was 3, 365, and at Nagoya1, 298. None of these approached the principal earthquake in severity. Nevertheless, of the Gifu series, 10 were described as violent and 97strong; while of the remainder, 1, 808 were weak, 1, 041 feeble, and 409were sounds alone without any accompanying shock. The slight intensityof most of the shocks is also evident from the inequality in thenumbers recorded at Gifu and Nagoya, from which it appears that nearlytwo-thirds were imperceptible more than about 25 miles from the chieforigin of the shocks. Only 70 of the after-shocks during the first twoyears were registered at Osaka, and not more than 30 at Tokio. _Distribution of After-shocks in Time. _--The decline in frequency ofthe after-shocks was at first extremely rapid, the numbers recorded atGifu during the six days after the earthquake being 303, 147, 116, 99, 92, and 81, and at Nagoya 185, 93, 79, 56, 30, and 31; in fact, halfof the shocks up to the end of 1893 occurred by November 23rd at Gifu, and by November 6th at Nagoya. The daily numbers at these two placesare represented in Fig. 51, in which the crosses correspond to thenumbers at Gifu, and the dots to those at Nagoya; and the curves drawnthrough or near the marks represent the average daily number of shocksfrom October 29th to November 20th. It will be seen that these curvesare hyperbolic in form, the change from very rapid to very gradualdecline in frequency taking place from five to ten days after thegreat earthquake. Fig. 52 illustrates the distribution in time of theafter-shocks at Gifu to the end of 1893, the ordinates in these casesrepresenting the number of shocks during successive months. [57] A similar rapid and then gradual decline in frequency characterisesthe strong and weak shocks recorded at Gifu. Of the ten violentshocks, only one occurred after the beginning of January 1892; and ofthe 97 strong shocks, only three after April 1892. But at thecommencement of the series, feeble shocks (_i. E. _, shocks that couldjust be felt) and earth-sounds without any accompanying movement werecomparatively rare, and did not become really prominent until twomonths had elapsed. Of the 308 after-shocks recorded in 1893, nonecould be described as strong, only 10 were weak, while 263 were feebleshocks and 35 merely earth-sounds. The last two diagrams show at a glance that the decline in frequencyof after-shocks is very far from being uniform. Some of thefluctuations are due to the occurrence of exceptionally strong shocks, each of which is followed by its own minor train of after-shocks. [58]Others seem to be periodic, and possibly owe their origin to externalcauses unconnected with the earthquake. [59] _Method of representing the Distribution of After-shocks inSpace. _--The maps in Figs. 54-57 show the distribution of theafter-shocks in space during four successive intervals of two monthseach. They are founded on Professor Milne's great catalogue ofJapanese earthquakes, which give, among other data, the time ofoccurrence and the position of the epicentre for every shock until theend of 1892. For the latter purpose, the whole country is divided bynorth-south and east-west lines into numbered rectangles, eachone-sixth of a degree in length and breadth; and the position of anepicentre is denoted by the number of the rectangle in which itoccurs. The area included within the maps is bounded by the parallels34° 40' and 36° 20' lat. N. , and by the meridians 2° 10' and 3° 50'long. W. Of Tokio, so that ten rectangles adjoin each side of the map. The number of epicentres lying within each rectangle having beencounted, curves are then drawn through the centres of all rectanglescontaining the same number of epicentres, or through points whichdivide the line joining the centres of two rectangles in the properproportion. Taking, for example, the curve marked 5, if the numbers intwo consecutive rectangles are 3 and 7, the curve bisects the linejoining their centres; if the numbers are 1 and 6, the line joiningtheir centres is divided into five equal parts, and the curve passesthrough the first point of division reckoned from the centre of therectangle in which six epicentres are found. Thus the meaning of thecurve marked, say, 5 may be stated as follows:--If any point in thecurve be imagined as the centre of a rectangle whose sides aredirected north-south and east-west, and are respectively one-sixth ofa degree of latitude and longitude in length; then the number ofepicentres within this rectangle is at the rate of 5 for the timeconsidered. _Preparation for the Great Earthquake. _--At first sight, there appearsto have been but little direct preparation for the great earthquake. Except for a rather strong shock on October 25th, at 9. 14 P. M. , itoccurred without the warning of any preliminary tremors. But a closerexamination of the evidence shows, as we should indeed expect, thatthere was a distinct increase in activity for many months beforehand. The region had become "seismically sensitive. " Of the hundredrectangles included in the maps in Figs. 53-57, there are thirteenlying along the meizoseismal area of the earthquake of 1891, in whichnearly all the after-shocks originated. During the five years 1885-89, 53 out of 125 earthquakes (or 42 per cent. ) had their epicentres lyingwithin the thirteen rectangles; or, in other words, the averagefrequency in one of the rectangles of the meizoseismal area was fivetimes as great as in one of those outside it. In 1890 and 1891 (untilOctober 27th), the percentage in the thirteen rectangles rose to 61, and the average frequency in one of them to ten times that of one ofthe exterior rectangles. The curves in Fig. 53 illustrate the distribution of epicentres duringthe latter interval. It will be seen that they follow roughly thecourse of the meizoseismal area southwards to the Sea of Isé, and thatto the south-east they continue for several miles the short branch ofthe meizoseismal area which surrounds the southern end of thefault-scarp. [Illustration: FIG. 53. --Distribution of preliminary Shocks in Space. (_Davison. _)] Thus, the preparation for the great earthquake is shown, first, by theincreased frequency of earthquakes originating within its meizoseismalarea; and, secondly, by the uniformity in the distribution ofepicentres throughout the same region, the marked concentration ofeffort which characterises the after-shocks being hardly perceptibleduring the years 1890-91. [Illustration: FIG. 54. --Distribution of After-shocks in Space (November-December 1891). (_Davison_). ] _Distribution of After-shocks in Space. _--We have seen that theafter-shocks were subject to a fluctuating decline in frequency, rapidat first, and more gradual afterwards. It is evident, from Figs. 54-57, that a similar law governs the area within which theafter-shocks originated. During the first two months, epicentres occurover nearly the whole of the meizoseismal area, but afterwards theyare confined to a smaller district, which slowly, though notcontinually, decreases in size. [Illustration: FIG. 55. --Distribution of After-shocks in Space (January-February, 1892). (_Davison. _)] [Illustration: FIG. 56. --Distribution of After-shocks in Space (March-April). (_Davison. _)] The most important feature in the distribution of the epicentres isthe central region of extraordinary activity; but there are alsodistricts of minor and more short-lived activity near the threeextremities of the meizoseismal band. The seat of chief seismic actionshifts slightly from one part to another of the epicentral region, especially about the end of 1891, as will be seen by comparing theinnermost curves of Figs. 54 and 55. Thus, with the decline infrequency of the after-shocks and the decrease in their sphere ofaction, there took place concurrently a gradual but oscillatingwithdrawal of that action to a more or less central region of thefault. [Illustration: FIG. 57. --Distribution of After-shocks in Space (May-June, 1892). (_Davison. _)] _Sound Phenomena of After-shocks. _--While comparatively few observersseem to have noticed any noise with the principal earthquake, many ofthe after-shocks were accompanied by sounds. Professor Omori describesthem as belonging to two types. They were either rushing feeble noiseslike that of wind, or loud rumbling noises like those of thunder, thedischarge of a gun, or the fall of a heavy body. In the Neo valley, sounds of the second type were most frequent and distinct, but theyeither occurred without any shock at all, or the attendant tremor wasvery feeble; while, on the other hand, severe sharp shocks weregenerally unaccompanied by distinctly audible sounds. It is remarkable, also, that sounds were less frequently heard withthe early than with the later after-shocks. In November 1891, thepercentage of audible shocks was 17, and from December to thefollowing April always lay between 10 and 12. In May the percentagesuddenly rose to 39, and until the end of 1892 was always greater than32, while in November 1892, it rose as high as 49. This, of course, agrees with Professor Omori's observation that sounds attended feebleshocks more often than strong ones. The distribution of the audible after-shocks in space is shown in Fig. 58. These curves are drawn in the same way as those in Figs. 53-57, but they represent the percentages, not the actual numbers, of shocksaccompanied by sound. It will be noticed that all three groups ofcurves lie along the meizoseismal area, or the continuation of thesouth-east branch; while the axis of the principal group of curveslies to the west of the central regions in which most after-shocksoriginated. [Illustration: FIG. 58. --Distribution of Audible After-shocks in Space (November 1891-December 1892). (_Davison_. )] The explanation of these peculiarities is no doubt connected with thecomparative inability of the Japanese people to perceive the deepsounds which in Europe are always heard with earthquake shocks. Thesounds are rarely heard by them more than a few miles from theepicentre. [60] We may therefore conclude that slight after-shocksoriginated nearer the surface than strong ones, that the mean depthof the foci decreased with the lapse of time, and that the axes of thesystems of curves in Fig. 58 mark out approximately the lines of thegrowing faults. The separation of the two westerly groups of curvesappears to show that the main branch of the meizoseismal area isconnected with a fault roughly parallel to that traced by ProfessorKoto, but of which no scarp (if it existed) could be readilydistinguished among the superficial fissures produced by the greatshock. EFFECT OF THE EARTHQUAKE ON THE SEISMIC ACTIVITY OF THE ADJOININGDISTRICTS. So great and sudden a displacement as occurred along the fault-scarpcould hardly take place without affecting the stability of adjoiningregions of the earth's crust, and we should naturally expect to find adistinct change in their seismic activity shortly after October 28th. In Fig. 59 two such regions are shown, bounded by the straight dottedlines. The district in which the principal earthquake and itsafter-shocks originated is enclosed within the undulating dottedlines. The continuous lines inside all three districts are the curvescorresponding to 10 and 5 epicentres for the years 1885-92. Not farfrom the axes of the outer groups of curves there are probablytransverse faults, approximately parallel to the great fault-scarp andthe main branch of the meizoseismal band, and distant from them about45 and 55 miles respectively. [Illustration: FIG. 59. --Map of Adjoining Regions in which Seismic Activity was affected by the Great Earthquake. (_Davison. _)] In the district represented in the north-east corner of Fig. 59, 29earthquakes originated between January 1st, 1885, and October 27th, 1891, and 30 between October 28th, 1891, and December 31st, 1892, 7 ofthe latter number occurring in November 1891. In the south-westdistrict, the corresponding figures before and after the earthquakeare 20 and 36, 8 of the latter occurring in November 1891. Thus, inthe north-east district, for every shock in the interval before theearthquake there were six in an equal time afterwards, and at the rateof 10 during November 1891; and in the south-west district, for everyshock before the earthquake there were 10 afterwards, and at the rateof 16 during November 1891. Now, it is unlikely that the gradual increase of stress should be sonearly proportioned everywhere to the prevailing conditions ofresistance as to give rise to a marked and practically simultaneouschange in seismic activity over a large area; whereas the paroxysmaloccurrence of a strong earthquake might alter the surroundingconditions with comparative rapidity, and so induce a state of seismicexcitement in the neighbourhood. It therefore seems very probablethat the increased activity in the two districts here described was adirect consequence of the occurrence of the great earthquake. ORIGIN OF THE EARTHQUAKE. The preponderance of preliminary earthquakes within the meizoseismalarea and the outlining of the fault-system by the frequency curves of1890-91 (Fig. 53) point to the previous existence of the originatingfault or faults, and to the earthquake being due, not to the formationof a new fracture, as has been suggested, but to the growth of an oldfault. The last severe earthquake in the Mino-Owari plain occurred in 1859, so that for more than thirty years there had been but little relief tothe gradually increasing stresses. Now, the distribution of stressmust have been far from uniform throughout the fault-system, and alsothe resistance to displacement far from proportional to the stressesat different places. At certain points, therefore, the effectivestress would be greater than elsewhere, and it would be at thesepoints that fault-slips would first occur. Such slips tend to removethe inequalities in effective stress. Thus, the function of the slightshocks of 1890 and 1891 was, briefly, to equalise the effective stressover the whole fault-system, and so to clear the way for one or moregreat slips throughout its entire length. As to which side of the fault moved during the great displacement, orwhether both sides moved at once, we have no direct evidence but asregards the neighbourhood of Midori, and there the conditions wereexceptional. Professor Koto thinks that it was probably the rock onthe north-east side that was generally depressed and always shifted tothe north-west But the disturbance in reality seems to have been morecomplicated. That this was the case, that displacement occurred alongmore than one fault, is probable from the branching of themeizoseismal area, the isolation of the audibility curves of theafter-shocks (Fig. 58), and the sudden increase in seismic activityboth to the north-east and south-west of the epicentre. The detachedportion of the meizoseismal area near Lake Biwa may also point to aseparate focus. The whole region, indeed, was evidently subjected tointense stresses, and the depression on the north-east side of thefault-scarp can hardly fail to have been accompanied by othermovements, especially along a fault running near the western margin ofthe main branch of the meizoseismal area. The later stages of the movements are somewhat clearer. From a studyof the after-shocks, we learn that the disturbed masses began at onceto settle back towards the position of equilibrium. At first the slipswere numerous and took place over the whole fault-system, but chieflyat a considerable depth, where no doubt the initial displacement wasgreatest. After a few months, stability was nearly restored along theextremities of the faults; slips were confined almost entirely to thecentral regions, while a much larger proportion of them took placewithin the superficial portions of the faults. The official records bring down the history to the end of 1893. Sincethat time more than one strong shock has been felt in the Mino-Owariplain; but the stage of recovery from the disturbances of 1891 isprobably near its end, and we seem rather to be entering on a periodin which the forces are once more silently gathering that sooner orlater will result in another great catastrophe. REFERENCES. 1. CONDER, J. --"An Architect's Notes on the Great Earthquake of October 1891. " _Japan Seismol. Journ. _, vol. Ii. , 1893, pp. 1-91. 2. DAVISON, C. --"On the Distribution in Space of the Accessory Shocks of the Great Japanese Earthquake of 1891. " _Quart. Journ. Geol. Soc. _, vol. Liii. , 1897, pp. 1-15. 3. ---- "On the Effect of the Great Japanese Earthquake of 1891 on the Seismic Activity of the Adjoining Districts. " _Geol. Mag. _, vol. Iv. , 1897, pp. 23-27. 4. ---- "On the Diurnal Periodicity of Earthquakes. " _Phil. Mag. _, vol. Xiii. , 1896, pp. 463-476, especially pp. 466-468. 5. ---- "On Earthquake-Sounds. " _Phil. Mag. _, vol. Xlix. , 1900, pp. 31-70--especially pp. 49-53, 60-61. 6. KOTO, B. --"The Cause of the Great Earthquake in Central Japan, 1891. " _Journ. Coll. Sci. Imp. Univ. Japan_, vol. V. , 1893, pp. 295-353. 7. MASATO, H. --"Report on Earthquake Observations in Japan. " _Cent. Meteor. Obs. Of Japan_ (Tokio, 1892), pp. 16-18, 41, and map 30. 8. MILNE, J. --"A Note on the Great Earthquake of October 28th, 1891. " _Japan Seismol. Journ. _, vol. I. , 1893, pp. 127-151; _Brit. Assoc. Rep. _, 1892, pp. 114-128. 9. ---- "A Catalogue of 8, 331 Earthquakes recorded in Japan between 1885 and 1892. " _Japan Seismol. Journ. _, vol. Iv. , 1895, pp. 1-367--especially pp. 134-234, 303-353. 10. ---- "On Certain Disturbances in the Records of Magnetometers and the Occurrence of Earthquakes. " _Brit. Assoc. Rep. _, 1898, pp. 226-251--especially pp. 227, 232, 234, 241, and 245. 11. MILNE, J. , and W. K. BURTON. --"The Great Earthquake in Japan. " _Journ. Coll. Sci. Imp. Univ. Japan_, vol. V. , 1893, pp. 295-352. 12. OMORI, F. --"On the After-shocks of Earthquakes. " _Journ. Coll. Sci. Imp. Univ. Japan_, vol. Vii. , 1894, pp. 111-200; abstract in _Japan Seismol. Journ. _, vol. Iii. , 1894, pp. 71-80. 13. ---- "A Note on the Great Mino-Owari Earthquake of October 28th, 1891. " _Pub. Earthquakes Inves. Com. In Foreign Languages_, No. 4, Tokio, 1900, pp. 13-24. 14. ---- "Sulla velocità di propagazione e sulla lunghezza delle onde sismiche. " _Ital. Soc. Sismol. Boll. _, vol. I. , 1895, pp. 52-60--especially pp. 52-57. 15. ---- "Sull' intensità e sull' ampiezza del movimento nel gran terremoto giapponese del 28 ottobre 1891. " _Ital. Soc. Sismol. Boll. _, vol. Ii. , 1896, pp. 189-200. 16. ---- "Note on the After-shocks of the Mino-Owari Earthquake of Oct. 28th, 1891. " _Pub. Earthquakes Inves. Com. In Foreign Languages_, No. 7, Tokio, 1902, pp. 27-32. 17. ---- "Note on the relation between Earthquake Frequency and the Atmospheric Pressure. " _Tokyo Phys. -Math. Soc. Reports_, vol. Ii. , 1904, No. 8. 18. TANAKADATE, A. , and H. NAGAOKA. "The Disturbance of Isomagnetics attending the Mino-Owari Earthquake of 1891. " _Journ. Coll. Sci. Imp. Univ. Japan_, vol. V. , 1893, pp. 149-192. FOOTNOTES: [54] I have not referred to the results of this survey, for, thoughchanges in all the magnetic elements (especially in horizontalintensity) have taken place between 1887 and 1891-92, these changescannot be ascribed with confidence to the earthquake in the absence ofa thorough knowledge of the secular variation. [55] From the formula a=x*g/y, where _a_ is the maximum horizontalacceleration, _g_ the acceleration due to gravity, _y_ the height ofthe centre of gravity, and _x_ its horizontal distance from the edgeabout which the body was overturned. [56] These estimates are made, on the supposition of simple harmonicmotion, from the formula 2*a=alpha*t^2/(2*pi^2), where 2_a_ is thetotal range or double amplitude, _a_ the maximum acceleration, and _t_the period of the vibration. [57] Professor Omori finds that the mean daily number of earthquakes_y_ during the month _x_ (reckoned from November 1891) may beapproximately represented by the equation-- y = 16. 9 / (x + 0. 397); or, taking the semi-daily earthquake numbers during the five daysbetween October 29th and November 2nd, 1891, by the equation-- y = 440. 7 / (x + 2. 314), where _y_ denotes the number of earthquakes observed during the twelvehours denoted by _x_, the time being measured from the first half ofOctober 29th. It is interesting to notice that, taking account of themean annual frequency of earthquakes in ordinary years, the number ofshocks observed at Gifu during the two years 1898-99 should, accordingto the latter formula, be 163; the actual number recorded was 160. [58] The last violent shock before the end of 1893 occurred onSeptember 7th, 1892, and its effects on the frequency of after-shocksis shown by the daily numbers recorded at Gifu during the firstfortnight in September. These are--2, 2, 2, 3, 5, 5, 28 (on September7th), 8, 8, 5, 4, 3, 2, 4, 3. [59] The periodicity of after-shocks is discussed in the papersnumbered 4, 12, 16, and 17 at the end of this chapter. In these, theexistence of diurnal and other periods is clearly established. Professor Omori also shows that the mean daily barometric pressure issubject to fluctuations with maxima occurring on an average every5-1/2 days, and that earthquakes are least frequent on the days of thebarometric maxima and minima, and more frequent in the daysimmediately preceding and following them. [60] Of the Japanese earthquakes of 1885-92 originating beneath theland, twenty-six per cent. Were accompanied by a recorded sound; butless than one per cent. Of those originating beneath the sea and notmore than ten miles from the coast. CHAPTER VIII. THE HEREFORD EARTHQUAKE OF DECEMBER 17TH, 1896, AND THE INVERNESSEARTHQUAKE OF SEPTEMBER 18TH, 1901. Among the earthquakes described in this volume, the Hereford andInverness earthquakes hold but a minor place. The damage to buildings, though unusual for this country, was slight when compared with thatcaused by the preceding shocks; there was no loss of life, not asingle person was injured by falling masonry. The interest of theearthquakes lies entirely in the detailed study rendered possible bynumerous observations of the shock and sound, [61] and in the bearingof this evidence on the general theory of the origin of earthquakes. THE HEREFORD EARTHQUAKE OF DECEMBER 17TH, 1896. The principal earthquake of this series occurred at 5. 32 A. M. OnDecember 17th, and was preceded by at least nine minor shocks (thefirst of which was felt at about 11 or 11. 30 P. M. On December 16th), and followed by two others on the same day, and by a third and last onJuly 19th, 1897. The accounts of these preliminary movements will befound on a later page, as their bearing will be more fully apparentafter the discussion of the principal shock. [Illustration: FIG. 60. --Isoseismal and Isacoustic lines of Hereford earthquake. (_Davison. _)] ISOSEISMAL LINES AND DISTURBED AREA. On the map in Fig. 60, the continuous curves represent isoseismallines corresponding to the degrees 8, 7, 6, 5, and 4 of theRossi-Forel scale. The isoseismal 8, which is the most accuratelydrawn of the series, is an elongated oval, 40 miles long, 23 milesbroad, and containing an area of 724 square miles. The longer axis isdirected W. 44° N. And E. 44° S. Within this curve, there are 73places where buildings are known to have been damaged, 55 places beingin Herefordshire, 17 in Gloucestershire, and one in Worcestershire. The most important damage occurred in the city of Hereford, which, in1901, contained 4, 565 inhabited houses. Here, no fewer than 218chimneys had to be repaired or rebuilt. The Cathedral was slightlyinjured. The finial of a pinnacle of the Lady Chapel was thrown down, a fragment of a stone fell from one of the arches in the southtransept, and the three pinnacles of the western front were fractured. Several churches suffered to a similar extent, while, at the MidlandRailway Station, all the seven chimney-stacks were shattered. AtDinedor, Fownhope, Dormington, Withington, and a few other villages, the damage was also relatively greater than elsewhere, these placesall lying within a small oval about 8-1/2 miles long, which surrounds, not the centre, but rather the north-west focus, of the isoseismal 8. The isoseismal 7, which includes places where the shock was strongenough to overthrow ornaments, vases, etc. , is also very nearly anellipse, whose axes are 80 and 56 miles in length, and whose area is3, 580 square miles. Its longer axis, running from W. 42° N. To E. 42°S. , is practically parallel to that of the inner curve. Next insuccession comes the isoseismal 6, surrounding those places where theshock was strong enough to make chandeliers, pictures, etc. , swing;but, as most of the observers seem to have slept in darkened rooms, the number of determining points for this curve is less than usual, and its course is therefore laid down with a somewhat inferior degreeof accuracy. The error, however, is probably small, and we maytherefore regard the isoseismal 6 as another ellipse, 141 miles long, 116 miles broad, and containing an area of 13, 000 square miles. Itslonger axis is again nearly parallel to those of the precedingisoseismals. The next two isoseismals are nearly circular in form. It will benoticed that large portions of them, and especially of the isoseismal4, traverse the sea. In these parts, the paths of the curves are tosome extent conjectural. In drawing them, the chief guides are theirtrend before leaving the land and the known intensity along theneighbouring coastlines. The isoseismal 5 bounds the area within whichthe shock was perceptible as a sensible displacement and not merely aquiver. Its dimensions are 233 miles from north-west to south-east, and 229 miles from south-west to north-east, and its area 41, 160square miles. The isoseismal 4, which includes places where the shockwas strong enough to make doors, windows, etc. , rattle, is 356 milesfrom north-west to south-east, and 357 miles from south-west tonorth-east, and 98, 000 square miles in area; its centre coincidesnearly with that of the small oval area in the neighbourhood ofHereford, where the damage to buildings was relatively greater thanelsewhere. Outside the isoseismal 4, the earthquake was observed at severalplaces. The shock was certainly felt at Middlesbrough, 12-1/2 milesfrom the curve, and probably at Killeshandra (in Ireland), 65 milesdistant. Thus, if we consider the boundary of the disturbed area tocoincide with the isoseismal 4, its area would be 98, 000 squaremiles, or 1-2/3 that of England and Wales; if it were a circleconcentric with the isoseismal 4, and passing through Middlesbrough, its area would be 115, 000 square miles, or nearly twice that ofEngland and Wales; while, if it passed through Killeshandra, its areawould be 185, 000 square miles, or more than three times the area ofEngland and Wales. [62] _Position of the Originating Fault. _--The form, directions, andrelative positions of the isoseismal lines furnish important evidencewith regard to the originating fault. We conclude in the first placethat its mean direction is parallel to the longer axes of the threeinnermost isoseismal lines--that is, north-west and south-east, or, more accurately, W. 43° N. And E. 43° S. [63] In this case, theelongated forms of the isoseismal lines cannot be attributed tovariations in the nature of the surface rocks. The district embracedcontains about 13, 000 square miles, and it is improbable that the axesof the three isoseismals should retain their parallelism over so largean area, if these variations had any considerable effect. Moreover, inthe same district, an earthquake occurred in 1863, whose meizoseismalarea was elongated from north-east to south-west, or almost exactlyperpendicular to the direction in 1896. Secondly, it will be noticed (Fig. 60) that the isoseismal lines arenot equidistant from one another. On the north-east side, they areseparated by distances of 20, 34, 55, and 51 miles; and on thesouth-west side by distances of 13-1/4, 25, 60, and 77 miles. Itfollows from this that the fault-surface must hade or slope towardsthe north-east; for, near the epicentre, the intensity is greatest anddies out more slowly on the side towards which the fault hades. If we could ascertain any one place through which the fault passed, its position would thus be completely determined. Unfortunately, thereis no decisive evidence on this point. There are, however, severalplaces to the south-west of Hereford where the intensity of the shockwas distinctly less than in the surrounding district, and it ispossible that this was due to their neighbourhood to the fault-line(see p. 135). If so, the originating fault must have extended from apoint about a mile and a half west of Hereford for a distance of about16 miles to the south-east; and a fault in this position wouldcertainly satisfy all the details of the seismic evidence. NATURE OF THE SHOCK. Throughout the disturbed area, considerable variations were observedin the nature of the shock. These changes were due to the mere size ofthe focus, to its elongated form and, as will be seen, to itsdiscontinuity, and also to the distance of the place of observationfrom the epicentre. At places near the epicentre, rapid changes in the direction of theshock were observed owing to the large angle subtended by the focus;while, at considerable distances, this angle being small, the changesof direction were imperceptible. A further variation with the distancewas an increase in the period of the vibrations. Close to theepicentre, the general impression was that of crossing the wake of asteamer in a very short rowing-boat, or of riding in a carriagewithout springs. At distances of a hundred miles or more, the movementis described as being of a pleasant, gentle, undulating character, like that felt during the rocking of a ship at anchor or in a carriagewith well-appointed springs. The most remarkable feature of the shock, however, was its divisioninto two distinct parts or series of vibrations, separated by aninterval, lasting two or three seconds, of absolute rest and quiet. And this was no mere local phenomenon. With the exception of a narrowband that will be referred to presently, records of the double shockcome from nearly all parts of the disturbed area, even from districtsso remote as the Isle of Man and the east of Ireland. The two partsdiffered in intensity, in duration, and in the period of theirconstituent vibrations. For instance, at Oaklands (near Chard), ashivering motion was first felt, and then, after about three or fourseconds, a distinct rocking from side to side. At Exeter, there was asudden tremor lasting about two seconds, followed, after two or threeseconds, by another and more severe shaking lasting four or fiveseconds. Again, at West Cross (near Swansea), an undulatory movementfor about four seconds was followed soon after by a tremulous shock. At Liverpool, the durations of the first part, interval, and secondpart were respectively estimated at about six, two, and four seconds. As a first result of the observations, then, it appears that in thesouth-east half of the disturbed area, the second part of the shockwas the stronger, of greater duration and consisted of longer-periodvibrations (as at _a_, Fig. 61); while, in the north-west half, thesame features characterised the first part of the shock (_b_, Fig. 61). A closer examination of the records shows, however, that theboundary between the two portions of the disturbed area was not astraight line, but slightly curved, the concavity facing thesouth-east. The broken line on the map (Fig. 60), which is hyperbolicin form, represents roughly the position of this curved boundary. [64] [Illustration: FIG. 61. --Nature of shock of Hereford earthquake. ] Along this hyperbolic boundary-line, or rather within a narrow band ofwhich it is the central line, the shock lost its double character, andwas manifested as a single series of vibrations gradually increasingin intensity and then dying away. Close to the edges of this band, careful observers were able to distinguish two maxima of intensityconnected by a continuous series of tremors (_c_, Fig. 61). Thus, within the band, the two series of vibrations, which elsewhere wereisolated, must have been superposed on one another; while, near theedges of the band, the concluding tremors of the first seriesoverlapped the initial tremors of the second. _Origin of the Double Series of Vibrations. _--The Hereford earthquakethus belongs to the same class as the Neapolitan, Andalusian, Charleston, and Riviera earthquakes. As in these cases, the hypothesisof a single focus is inadmissible. The division of the disturbed areainto two regions of opposite relative intensity, duration, etc. , issufficient proof that a single series of vibrations was not duplicatedby reflection or refraction, or by separation into longitudinal andtransverse waves. It is equally conclusive against a repetition of theimpulse within the same focus. We must therefore infer that the focusconsisted of two nearly or quite detached portions arranged along anorth-west and south-east line, and that the impulse at the north-westfocus was the stronger of the two. The only question that remains tobe decided is whether the impulses at the two foci were simultaneousor not. Now, if the impulses occurred at the same instant, the waves from thetwo foci would travel with the same velocity, and would thereforecoalesce along a straight band which would bisect at right angles theline joining the two epicentres. But we have already seen that thisband is curved, and it thus follows that the two impulses were notsimultaneous. Again, since the concavity of the hyperbolic band facesthe south-east, the waves from the north-west focus must havetravelled farther than those from the south-east focus before the twomet along the hyperbolic band; in other words, the impulse at thenorth-west focus must have occurred two or three seconds before theimpulse at the other. _Position and Dimensions of the Two Foci. _--There can be little doubtthat the impulse at the north-west focus was responsible for thegreater damage to buildings at Hereford, Dinedor, Fownhope, etc. Thecentre of its epicentral area must therefore lie about three milessouth-east of Hereford. It is probable, also, that the correspondingcentre of the other focus is similarly placed with respect to thesouth-east portion of the isoseismal 8--that is, about two or threemiles north-east of Ross. These two points are eight or nine milesapart. Now, since, as we shall see, the mean surface-velocity of theearth-waves was about 3000 feet per second, and the mean duration ofthe quiet interval between the two series was 3-1/2 seconds, thenearest ends of the two foci must have been separated by a distance ofnot less than two miles. Moreover, since the series of vibrations fromthe north-west or Hereford focus lasted a few seconds longer than thatfrom the south-east or Ross focus, the former must have been about twomiles longer than the latter, and we may therefore estimate theirlengths at about eight and six miles respectively. Including theundisturbed intermediate portion, this would give a total length offocus of about 16 miles, a result we have already inferred from thedimensions of the isoseismal 8. DIRECTION OF THE SHOCK. Although no question was asked with regard to the direction of theshock, no fewer than 469 observers made notes on this point. As ageneral rule, their determinations are extremely rough, few referringto more than the eight principal points of the compass. Moreover, inany one place, the directions assigned to the shock are very varied. For instance, in the city and suburbs of Birmingham, eight observersgive the direction along a north and south line, eight east and west, eleven north-west and south-east, and five north-east and south-west, while there are five other intermediate estimates. But, when thesedirections are plotted on a map of the district, it is seen at oncethat they are either nearly parallel or perpendicular to the roads inwhich the observers were living; that is, the apparent direction ofthe shock was at right angles to one of the principal walls of thehouse. This, of course, is a result to be anticipated, for, whateverbe the direction of the earthquake-motion, a house tends to oscillatein a plane perpendicular to one or other of its walls. It is extraordinary to how great a distance the direction of the shockis perceptible. Records come from Brighton (137 miles from theepicentre), Maldon in Essex (144 miles), Harrogate (147 miles), Douglas in the Isle of Man (167 miles), Dublin (176 miles), andBaltinglass in Co. Wicklow (180 miles). Nevertheless, whatever the distance may be, the sense of directionmust be most perceptible in those houses whose principal walls are atright angles to the true direction of the earthquake-motion, and weshould therefore expect to find the observations of direction mostfrequently made in such houses, or in others which approximate to thissituation. Thus, the average of all the observations within a fairlysmall area should give a result not very far from the true directionof the shock; and, the smaller the area and the farther from theepicentre, the more reliable should be the result. Now, in Birminghamthe mean direction of the shock is E. 39° N. , which differs only by 2°from the line joining the city to the epicentre; in London it is E. 21° S. , the difference being again 2°. In other cases, theobservations from different counties are grouped together, and themean direction is taken to correspond to the centre of the county. Yet, even then, there is often a close agreement between the meandirection of the shock and the direction of the county-centre from theepicentre; the difference being not more than two or three degrees inthe counties of Buckingham, Devon, Stafford, Warwick, and York. Inother cases, where the deviation exceeds this amount, either thenumber of observations is small or the county is near the epicentreand so subtends a large angle. Two results of some importance follow from this analysis: (1) thatwhile, with a few isolated observations, the "method of directions" isalmost sure to fail, with a large number of observations closelygrouped, the position of the epicentre may be determined with a fairapproach to accuracy; and (2) that, at any rate outside a radius offorty miles, the earth-waves travelled in approximately straight linesoutwards from the epicentre. COSEISMAL LINES AND VELOCITY OF EARTH-WAVES. Coseismal lines were defined by Mallet as long ago as 1849, but, owingto the difficulty of ascertaining the correct time, they have so farbeen of little service in the investigation of earthquakes. In thecase of the Hereford earthquake, the distances traversed by theearth-waves are small; but, on the other hand, the time-records arenumerous and frequently trustworthy to the nearest minute. Rejectingall estimates earlier than 5. 32 A. M. , and later than 5. 36, as well asa number at 5. 35, there remain fairly good observations from 381places, and exceptionally accurate ones from 33 places. The latterwere obtained from signalmen and other careful observers who were inpossession of Greenwich time, or who compared their watches shortlyafterwards with well-regulated watches. With evidence so abundant, a new method of drawing coseismal linesbecomes possible. According to this method, each place of observationis indicated on the map by a mark corresponding to the particularminute recorded. If the records were quite correct, there would be acentral area occupied by the marks corresponding to 5. 32 A. M. , surrounded by a series of zones in which the times were respectively5. 33, 5. 34, and 5. 35. The curves separating these zones would becoseismal lines corresponding to the times 5. 32-1/2, 5. 33-1/2, and5. 34-1/2. Owing, however, to the inevitable inaccuracy of all the time-records, these different zones intrude on one another, and the coseismal lineshave therefore to be drawn about half-way through the overlappingregions, special weight being attributed to the apparently moreaccurate observations. [Illustration: FIG. 62. --Coseismal lines of the Hereford earthquake. (_Davison. _)] The coseismal lines obtained in this manner are represented by thecontinuous curves in Fig. 62. The isoseismals, which are added for thesake of comparison, are indicated by the dotted lines. It will be seenthat the coseismal lines are elongated in the same direction as theisoseismals, but to a less extent, and this no doubt is due to thefact that the epoch selected by the majority of observers was one notfar from, and slightly preceding, that of the maximum intensity ofthe shock. Now, the average distance between the two inner coseismals is 32-3/4miles, between the two outer ones (so far as drawn) 35-1/6 miles, andbetween the first and third 67-1/6 miles. The mean surface-velocitybetween the two inner coseismals is therefore 2, 882 feet per second, and between the two outer ones 3, 095 feet per second. There is thus anapparent increase in the velocity with the distance, but the accuracyof the coseismal lines is unequal to establishing this as a fact. Themean surface-velocity of 2, 955 feet per second between the first andthird coseismals is probably, however, the most accurate estimate ofthe surface-velocity yet made in a slight earthquake. SOUND-PHENOMENA. _Nature of the Sound. _--The sound which accompanied the shock was ofthe same character as that heard during all great earthquakes. It isoften described in such terms as a deep booming noise, a dull heavyrumble, a grating roaring noise, or a deep groan or moan; more rarelyas a rustling or a loud hissing rushing sound. As a rule, it beganfaintly, increased gradually in strength, and then as gradually diedaway; and this no doubt is the reason why it sometimes appeared as ifan underground train or waggon were approaching quickly, rushingbeneath the observer, and then receding in the opposite direction. Occasionally, the sound was very loud, being compared to the noise ofmany traction-engines heavily laden passing close at hand, or to aheavy crash or peal of thunder. But its chief characteristic was itsextraordinary depth, as if it were almost too low to be heard. According to one observer, it was a low rumbling sound, much lowerthan the lowest thunder; and another compared it to the pedal notes ofa great organ, only of a deeper pitch than can be taken in by thehuman ear, a noise more _felt_ than heard. It will be seen presentlyhow the sound, from its very depth, was inaudible to many persons. A few observers described the sound in terms like those quoted above, but by far the larger number compared it to some more or lesswell-known type, and in many cases the resemblance was so close thatthe observer at first attributed it to the object of comparison. Thedescriptions, which present great varieties in detail, may beclassified as follows: (1) One or several traction-engines passing, either alone or heavily laden, sometimes driven furiously past; asteam-roller passing over frozen ground or at a quicker pace thanusual; heavy waggons driven over stone paving, on a hard or frostyroad, in a covered way or narrow street, or over hollow ground or abridge; express or heavy goods trains rushing through a tunnel or deepcutting, crossing a wooden bridge or iron viaduct, or a heavy trainrunning on snow; the grating of a vessel over rocks, or the rolling ofa lawn by an extremely heavy roller; (2) a loud clap or heavy peal ofthunder, sometimes dull, muffled or subdued, but most often distantthunder; (3) a moaning, roaring, or rough, strong wind; the rising ofthe wind, a heavy wind pressing against the house; the howling of windin a chimney, a chimney or oil-factory on fire; (4) the tipping of aload of coal, stones, or bricks, a wall or roof falling, or the crashof a chimney through the roof; (5) the fall of a heavy weight or tree, the banging of a door, only more muffled, and the blow of a wave onthe sea-shore; (6) the explosion of a boiler or cartridge of dynamite, a distant colliery explosion, distant heavy rock-blasting and the boomof a distant cannon; (7) sounds of a miscellaneous character, such asthe trampling of many men or animals, an immense covey of partridgeson the wing, the roar of a waterfall, the passage of a party ofskaters, and the rending and settling together of huge masses of rock. The total number of comparisons made was 1, 264. Of these, 45. 4 percent. Refer to passing waggons, etc. , 15. 0 per cent. To thunder, 15. 5to wind, 3. 9 to loads of stones falling, 2. 7 to the fall of a heavybody, 7. 2 to explosions, and 10. 3 per cent. To miscellaneous sounds. Generally, the sound adhered throughout to one of the types mentionedabove, and, if it varied at all, varied only in intensity. At someplaces, however, the character of the sound was observed to change. For instance, one person described it as like the rumbling of a traingoing over a bridge, with a terrific crash, such as is heard in athunderstorm at the instant when the shock was strongest, the rumblingdying away afterwards for some seconds. _Inaudibility of the Sound to some Observers. _--The total number ofobservers who give a detailed account of the earthquake is 2, 681, and, of these, 59 per cent. State that they heard the sound, 23 per cent. Give no information, while 18 per cent. Distinctly say that theyheard no sound; that is, roughly, out of every five observers, threeheard the sound, one made no reference to it, and one failed to hearthe sound. In a few cases, no doubt, this failure was due to the distance of theobserver, but this is far from being a complete explanation; for, inHerefordshire, six out of 179, and in Gloucestershire 17 out of 227, observers heard no sound. Nor is the peculiarity a local one, for atClifton two out of five observers who were awake did not hear thesound, at Birmingham four out of 23, and in London, eight out of 18. Even in the same house, it would happen that one observer would hear asound as of a heavily-laden traction-engine passing, while to anotherit was quite inaudible. Again, a large number of observers who heard the sound expressly statethat they were unconscious of any while the shock lasted. The noise atfirst resembled the approach of a steam-roller or traction-engine upthe street, it became gradually louder, and then ceased more or lesssuddenly as the shock began; while, to others in the same places, thesound continued to grow in loudness until the strongest vibrationswere felt. Even when observers in the same place agreed in hearing the sound, itpresented itself to them under different aspects. Thus, at Hereford, acrash or bomb-like explosion was noticed by some, but not by all, observers; at Ledbury, the sound according to one began like a rushingwind and culminated in a loud explosive report, another heard a noiselike distant thunder, which ended when the shock began, while a thirdheard no sound at all. At places more distant from the epicentre, thesame diversity, both in character and intensity, is manifested. Thus, at Birmingham, the accounts refer on the one hand to the distantapproach of a train and the rising of the wind, on the other to thereports of large cannons and to a noise as if tons of _débris_ hadbeen hurled against the wall of the house; at Bangor, to muffledthunder, wind through trees, and a loud rumbling sound. The first explanation of these apparent anomalies which presentsitself is inattention on the part of the observers; but it is one thatwill not bear examination, though it may apply in some cases. Thesound is too loud, at any rate near the epicentre, to escape notice, and it is generally heard before the shock begins to be felt. Moreover, as described in the last chapter, three out of every fourearthquakes in Japan are unaccompanied by recorded sound, and theJapanese as a race cannot be accused of such constant inattention. Thedefect, it can hardly be doubted, is inherent to the observer, and notdependent on the conditions in which he is placed. That the higher limit of audibility varies with different persons haslong been known; and there can be no reason for doubting that there isa similar variability in the lower limit. Thus, to some observers, thesound remains inaudible throughout, however intently they may belistening. Again, it is found that, the deeper the sound, the greatermust be the strength of the vibrations required to render themaudible. As the vibrations which reach an observer increase in period, it may therefore happen that, sooner or later, the strength of somedoes not attain or exceed that limiting value, and, at that moment, the sound will cease to be heard. Moreover, for vibrations of a givenperiod, this limiting value varies for different persons. Thus, to oneobserver, the sound may become inaudible, while another may continueto hear it. Lastly, the vibrations which affect an observer at anymoment are of various strength and period. One may hear all perhaps, while a second may be able to hear some and not others. Thus, to oneobserver, the sound may be like a rising wind, to another like a heavytraction-engine passing; one may hear the crashes which accompaniedthe strongest part of the shock, while a second may be deaf to thesame vibrations; to one the sound may become continually louder andcease abruptly, to another it may increase to a maximum and then dieaway. _Sound-Area. _--While the sound was a very prominent feature of theearthquake in and near the epicentral area, records at a greatdistance are naturally difficult to obtain, and, on this account, thenumber of stations for determining the boundary of the sound-area istoo small to allow of it being accurately drawn. As a rule, however, it must lie between the isoseismals 5 and 4, but it is less nearlycircular than either of these lines. Its length, from north-west tosouth-east, is 320 miles, its breadth 284 miles, and the areacontained by it about 70, 000 square miles, or roughly two-thirds thatof the disturbed area. _Isacoustic Lines. _--The dotted lines in Fig. 60 represent isacousticlines--that is, lines which pass through all places where thepercentage of observers who recorded their perception of the sound isthe same. For instance, if we take any point in the line marked 80 anddescribe a small circle with that point as centre, then 80 per cent. Of the observers within that circle would hear the earthquake-sound. The isacoustic lines thus show how the audibility of the sound variesthroughout the sound area. To draw the curves with a close approach toaccuracy, the unit of area should be small and of constant dimensions;but, in the present case, owing to the comparative paucity of theobservations, a smaller unit than the county would give unreliableresults. [65] At the centre of each county, the sound audibility may beregarded as proportional to the percentage of the total number ofobservers within the county who distinctly heard the sound. To drawthe curve marked 50, the centre of every county in which the averagepercentage is less than 50 is joined to the centres of those adjoiningcounties in which it is above 50, and these lines are then divided inthe proper ratio so as to give a point where the percentage would beexactly 50. A number of points at which the percentage is 50 is thusobtained, and the curve drawn through them is the required isacousticline. The percentage of audibility varies from 87 in Herefordshire to23 in Essex and the east of Ireland, but the only isacoustic lineswhich can be completely drawn are those that correspond to thepercentages between 80 and 50 inclusive. The peculiar form of the isacoustic lines will be evident at a glance. They bear little relation to the isoseismal lines. Their greatestextensions are not along the axes of those lines, but in twodirections which are a little east of north-east and south ofsouth-west. They lie indeed along a hyberbolic line which, towardsthe south-west, agrees closely with the curvilinear axis of thehyperbolic band represented by the broken line in Fig. 60. Towards thenorth-east, the coincidence is not so close, but this is chiefly owingto the magnitude of the northern counties, which causes a deflectionof the isacoustic lines towards the north. It will be remembered that the hyperbolic band is the area withinwhich the vibrations from the two foci were superposed. Now, the soundaccompanied each part of the shock, and ceased entirely during theinterval between them. Also, the stronger series of vibrations wasaccompanied by the louder sound; but, while the difference in strengthwas considerable between the two parts of the shock, it was veryslight between the two sounds. There is therefore no marked distortionof the isoseismal lines when crossing the hyperbolic band, while theisacoustic lines are completely diverted from their normal course. Thus, the study of the isacoustic lines strongly confirms theconclusions at which we have arrived above (p. 223)--namely, thatthere were two distinct foci arranged in a north-west and south-eastline, and that the impulse at the former focus occurred a few secondsearlier than that at the latter. [66] _Variations in the Nature of the Sound throughout the Sound-area. _--Inone respect, the sound exhibited a marked uniformity all over thesound-area--namely, in its great depth; the word "heavy" being used inone out of every four accounts of the sound, whether close to theepicentre or near the boundary of the sound-area. The type of comparison employed varies in different parts of thesound-area. As we recede from the origin, the sound becomes on theaverage less like thunder or explosions and more like wind. Thereferences to passing waggons, etc. , are so numerous that it ispossible to draw curves, in the same way as isacoustic lines, whichrepresent equal percentages of comparison to this type out of thetotal number of comparisons. The curves are somewhat incomplete, butit is noteworthy that those corresponding to the higher percentagescling to the extremities of the hyperbolic band, probably because theuninterrupted duration of the sound is greater there than elsewhere. The effect of distance from the epicentre, however, is most noticeablein connection with changes in the character of the sound. It is onlyon the immediate neighbourhood of the origin that the explosivereports or crashes were heard in the midst of the rumbling sound. At amoderate distance, the sound before and after the shock becamesmoother, while the sound which accompanied the shock retained to acertain extent its rougher and more rumbling or grating character. Close to the boundary of the sound-area, the irregularities were stillfurther smoothed away, and the only sound heard was like the low rollof distant thunder. The explanation of these changes depends on the fact that, as werecede from the epicentre, the vibrations of every period tend tobecome inaudible. The limiting vibrations of the whole series will bethe first to be lost, especially those of the longest period. Thus, near the epicentre, sound-vibrations of many different periods will beheard, and the sound will be more complex than it is elsewhere. Thegreater the distance, the narrower are the limits with regard toperiod between which the audible vibrations lie, until, near theboundary of the sound-area, the sound becomes an almost monotonousdeep growl of nearly uniform intensity. _Time-relations of the Sound and Shock. _--The principal epochs to becompared are the beginning, the epoch of maximum intensity, and theend. The beginning of the sound preceded that of the shock in 82 percent. Of the observations on this epoch, coincided with it in 12, andfollowed it in 6 per cent. ; the epoch of maximum intensity precededthat of the shock in 21 per cent. Of the records, coincided with it in73, and followed it in 6 per cent. ; while the end of the soundpreceded that of the shock in 22-1/2 per cent. , coincided with it in27-1/2, and followed it in 50 per cent. Thus, as a general rule, thebeginning of the sound preceded that of the shock, the sound wasloudest when the shock was strongest, and the end of the soundfollowed that of the shock. In other words, the duration of the soundwas in most cases greater than that of the shock. MINOR EARTHQUAKES. Of the twelve undoubted minor earthquakes, nine occurred before, andthree after, the principal shock, the times of the first eleven lyingbetween limits about seven hours apart. With three exceptions, therecords are insufficient to determine the positions of the epicentrewith any approach to exactness. The first occurred at about 11 or 11. 30 P. M. On December 16th. Theboundary of the disturbed area, which coincides nearly with that ofthe fifth shock (E, Fig. 63), is 97 miles long from north-west tosouth-east, 83 miles wide, and contains about 6, 300 square miles. Thefocus was apparently situated between the two foci of the principalearthquake and partly coincided with them. [Illustration: FIG. 63. --Map of minor shocks of Hereford earthquake. (_Davison. _)] Then came three slight shocks (at about 1 A. M. On December 17th, 1. 30or 1. 45 A. M. , and 2 A. M. ), about which little is known except thatthey probably originated somewhere near the Ross focus. The fifth shock (E, Fig. 63) occurred at about 3 A. M. , and disturbedan area 104 miles in length, 79 miles in width, and about 6, 400 squaremiles in area. Its boundary occupies approximately the position thatwould be taken by an isoseismal of intensity between 7 and 6 of theprincipal earthquake. We may therefore infer that this shock and theprincipal earthquake were caused by slips along the same fault and inabout the same region of the fault. Also, as there is no evidence ofdiscontinuity in the vibrations of the minor shock, it is probablethat the focus was continuous, and occupied the space between the twofoci of the principal earthquake, as well as part or the whole of boththese foci. The next four shocks occurred at about 3. 30, 4, 5, and 5. 20 A. M. , andwere more closely associated with the Ross than with the Herefordfocus, and then followed the principal earthquake at 5. 32 A. M. A few minutes later, at 5. 40 or 5. 45 A. M. , a very slight shock wasfelt, the focus of which was possibly situated in the central regionbetween the two foci. The next, at about 6. 15 A. M. (K, Fig. 63), disturbed an area 41 miles long, 27 miles broad, and containing about870 square miles. Its focus must have coincided approximately with theRoss focus of the principal earthquake, and this was also the caseprobably with the last shock of all, which occurred on July 19th, 1897, at 3. 49 A. M. ORIGIN OF THE EARTHQUAKES. The greater part of the epicentral district is covered by a sheet ofOld Red Sandstone (Fig. 64), but, just to the north-east of theposition laid down for the originating fault (indicated by thestraight broken line), is the well-known Woolhope anticlinal, by whichSilurian beds are brought to the surface. The anticlinal axis runsapproximately north-west and south-east, and is thus roughly parallelto the earthquake-fault. Moreover, the thinning-out and occasionaldisappearance of some of the Silurian beds on the south-west side ofthe anticlinal (as compared with those on the north-east side) issuggestive of a north-west and south-east fault or rapid flexure at ornear the south-west junction of the Old Red Sandstone and theSilurian strata. If it be a fault, it must hade to the north-east, andwould therefore satisfy two of the conditions determined by theseismic evidence. It would lie, however, about two miles too far tothe north-east, being in fact to the north-east of the villages whichsuffered most from the earthquake. [Illustration: FIG. 64. --Geology of meizoseismal area of Hereford earthquake. (_Davison. _)] But only a few miles to the south-east of the Woolhope anticlinal, andalmost in the same line with it, there is a second anticlinal, that ofMay Hill. This is a triangular area, and is known to be bounded on allthree sides by faults. The fault on the north-east side has an averagenorth-west and south-east direction, and, if it were continued throughthe Old Red Sandstone towards the north-west, but bending at first afew degrees more to the west, it would pass through a point about1-1/2 miles west of Hereford. It is worthy of notice that both thisfault and another nearly parallel to it, about half-a-mile farthernorth-east, stop, according to the Geological Survey map, at thepoints where they enter the Old Red Sandstone. The latter is an areawhich has never been investigated with thoroughness by modernstratigraphical methods, and in which it is difficult to trace faults. It therefore appears not improbable that the earthquakes were due toslips along a continuation of this fault. Whether this be the case or not, however, it is clear that theearthquake-fault must pass between the anticlinal areas of Woolhopeand May Hill, the former being on the north-east, and the latter onthe south-west, side of the fault. At the Hereford focus, the faultmust hade to the north-east; and, at the Ross focus, it is probable, from the distribution of places where damage occurred to buildings, that it hades to the south-west If this be the case, the fault mustchange in hade between the two foci. How long a time had elapsed since the last sign of growth in theearthquake-fault took place, it is impossible to say; but it must bemany years in length. During this interval, the stresses tending toproduce movement along the fault-service had been graduallyincreasing, until they were sufficient to overcome the resistanceopposed to them. It is worthy of notice that the earliest perceptiblemovements were slight. Their function seems to have been to preparethe way for the great slips by equalising the difference betweenstress and resistance over a large area of the fault-surface. Wecannot trace with accuracy the transference of the seat of movementfrom one part of the fault-surface to another. The first slip seems tohave taken place chiefly in the region between the two foci of theprincipal earthquake; possibly it overlapped both of them partly. Thenext three slips were apparently in the neighbourhood of the Rossfocus, and were followed by a fifth in the same area as the first. Then came a series of small movements that we cannot locate furtherthan by saying that they were more closely connected with the Rossfocus than the other. In consequence of the preliminary slips within and near the Rossfocus, the effective stress in that portion of the fault wasdiminished; and this may be the reason why the first great slip tookplace at the Hereford focus. The immediate result of such a movementwould naturally be an increase of stress in and beyond the terminalregions, and the next slip might have been expected in an area partlyoverlapping the Hereford focus, and either to the north-west orsouth-east of it. Instead of this, for a distance of two miles in thelatter direction, there was not the least perceptible movement duringthe principal earthquake, and the second great slip occurred in theregion beyond occupied by the Ross focus. This second slip, moreover, occurred within two or three seconds after the other; that is, beforethe earth-waves had time to travel from the Hereford to the Rossfocus. In other words, the slip at the Ross focus was not aconsequence of the slip at the Hereford focus; but both were due to asingle generative effort. Now, a section drawn parallel to the earthquake-fault and on thenorth-east side of it, would show an anticline near the Hereford focusand a corresponding syncline near the Ross focus, with an undisplacedportion in the intermediate region; while a parallel section on theother side of the fault would show a syncline near the Hereford focus, an anticline near the Ross focus, and again an undisplaced portion inthe intermediate region. If further movements tending to accentuatesuch a structure were to occur (that is, if the anticlinals were to bemade more anticlinal and the synclines more synclinal), there wouldtherefore be two slips, one in each focus; while, along thefault-surface between, there would be practically no displacement. Atany rate, the earlier stresses in that region may have been fullyrelieved by two slight preliminary slips (those causing the first andfifth minor earthquakes), and those resulting from the greatdisplacements by the first after-slip which followed in about tenminutes. Half-an-hour later, another slip took place at the Ross focus, and bythis the equilibrium of the rock-masses was almost completelyrestored; for we have no certain evidence of any further movementsuntil seven months have elapsed (July 19th, 1897), when there was afinal slip in the same region of the fault. THE INVERNESS EARTHQUAKE OF SEPTEMBER 18TH, 1901. Between the north-east end of Loch Ness and the Moray Firth atInverness, there lies a tract of land not more than seven miles inlength, which is notable as one of those most frequently shaken byearthquakes in the British Islands. In the intensity of its shocks itis inferior to the south-east of Essex and the centre ofHerefordshire, and, in mere number, to the celebrated village ofComrie in Perthshire. But, in the interest of its seismic phenomena, in the light which they cast on the development of the earth's crust, the neighbourhood of Inverness has no equal in Great Britain, and notmany superiors in any part of the world. For this importance from a seismological point of view, the districtis indebted to the great fault which traverses Scotland along the lineof the Caledonian Canal, and to the fact that this fault, although itdates from Old Red Sandstone times, has not yet finished growing. Asresults of its formation, we have the almost straight cliff along thesouth-east coast of Rossshire, and the long chain of lakes, beginningwith Loch Dochfour and Loch Ness, and ending with Loch Oich, LochLochy, and Loch Linnhe. As evidences of its persistent thoughintermittent growth, we have the slight tremors and earth-soundsoccasionally observed at and near Fort William, and the much strongershocks felt in the neighbourhood of Inverness. During the nineteenth century there were three strong earthquakeshocks in this district. The first and most severe occurred on August13th, 1816, and was felt over the greater part of Scotland; the secondon February 2nd, 1888; and the third and weakest on November 15th, 1890. This last shock was followed by several slighter ones, theseries ending with a rather smart shock on December 14th. Between thisdate and the summer of 1901 no earthquakes seem to have been felt ator anywhere near Inverness. PREPARATORY SHOCKS. The date of the first shock of 1901 is not quite certain. One is saidto have been felt at Aldourie (see Fig. 66) some time in June, and asecond at Dochgarroch in July. These may have been succeeded by otherstoo slight to attract much notice, but the first to be generallyobserved occurred on September 16th at 6. 4 P. M. A weak tremor, accompanied by a faint sound, was perceived over a nearly circulararea about 12 miles in diameter, and with its centre about 1-1/2 milessouth of Dochgarroch. On the next day, at 11 P. M. , a quivering lastingtwo seconds was felt at Inverness, and a weak tremor, accompanied bysound, at Dochgarroch at 1. 15 A. M. On September 18th. Nine minuteslater, at 1. 24 A. M. , occurred the principal earthquake, the shock ofwhich would be called a strong one, even in Italy and Japan. EFFECTS OF THE SHOCK. In Inverness, the damage to buildings, though seldom serious, was byno means inconsiderable. One brick building used as a smithy wasdestroyed, several chimneys or parts of them fell, and manychimney-cans were displaced or overthrown. At Dochgarroch and otherplaces within the meizoseismal area, walls were cracked, chimneysthrown down, and lintels loosened. But, for this country, an unusual effect of the earthquake was a longcrack made in the north bank of the Caledonian Canal near DochgarrochLochs. It occurred in the middle of the towing-path, and could betraced at intervals for a distance of 200 yards to the east of theLochs, and 400 yards to the west, being often a mere thread, and in noplace more than half-an-inch wide. Soon after its formation, however, the fissure was obliterated by heavy showers of rain. ISOSEISMAL LINES AND DISTURBED AREA. The map (Fig. 65) shows the area over which the earthquake wasperceptible. The isoseismal lines are drawn partly continuous andpartly dotted--continuous where some confidence can be placed in theiraccuracy, and dotted where their course must be regarded as doubtful, owing to the rarity or absence of observations. The innermost isoseismal (shown on a larger scale in Fig. 66)corresponds to the intensity 8 of the Rossi-Forel scale, and includesthe places where the shock was strong enough to cause slightstructural damage to buildings. It is elliptical in form, 12 mileslong, 7 miles broad, and 67 square mile in area, with its centre at apoint about 1-1/2 mile east-north-east of Dochgarroch, and its longeraxis running N. 33° E. And S. 33° W. [Illustration: FIG. 65. --Isoseismal lines of the Inverness earthquake. (_Davison. _)] The remaining isoseismals are less accurately drawn, owing to thescarcity of observations made in the west of Scotland. Except towardsthe west, however, the course laid down for the isoseismal 7 may betrusted. Its length is 53-1/2 miles, width 35 miles, and area 1, 500square miles. Its longer axis is almost exactly parallel to that ofthe preceding isoseismal, but the distance between the two curves is 9miles on the north-west, and 14 miles on the south-east, side. Theisoseismal 6 is 105 miles long, 87 miles wide, and contains 7, 300square miles; and the isoseismal 5, 157 miles long, 143 miles wide, and about 17, 000 square miles in area. The isoseismal 4 may be regarded as the boundary of the disturbed areaof the earthquake, for, so far as known, the shock was not noticed atany point outside it. Towards the north, it was felt at Wick, Castletown, and other intermediate places; towards the west atTobermory in the island of Mull; and, towards the south, at Skelmorlie(in Ayrshire), Paisley, Belsyde (near Linlithgow), Gullane (near NorthBerwick), and Dunbar. Along the east coast of Scotland, between Wickand Dunbar, there are few places of any size where the shock was notfelt. The disturbed area of the earthquake is thus 215 miles long fromnorth-east to south-west, 198 miles wide, and contains about 33, 000square miles. _Position of the Originating Fault. _--The only isoseismals which aredrawn accurately enough to determine the earthquake-fault are the twoinner ones, those marked 8 and 7; but these are sufficient for thepurpose. It is clear, from the direction of their longer axes, thatthe average direction of the fault must be N. 33° E. And S. 33° W. Again, the isoseismals are farther apart towards the south-east thantowards the north-west, implying that the fault hades to thesouth-east. Lastly, as the intensity of the shock is greater on theside towards which the fault hades, it follows that the fault-linemust lie a short distance (about a mile or so) on the north-west sideof the centre of the isoseismal 8. Now, the great fault alluded to above occupies almost exactly theposition indicated by the seismic evidence. Its mean direction fromTarbat Ness to Loch Linnhe is N. 35° E. And S. 35° W. , it hades to thesouth-east, and the fault-line passes through a point aboutthree-quarters of a mile to the north-west of the centre of theisoseismal 8 (Fig. 66). There can be little doubt, therefore, that theearthquake was caused by a slip of this fault; and the evidence of theafter-shocks, as will be seen, offers additional support to thisconclusion. The region in which the slip took place may be determined roughly fromthe position and form of the innermost isoseismal. Its centre musthave been close to the point marked A in Fig. 66, which corresponds toa point about 1-1/2 mile east-north-east of Dochgarroch. In ahorizontal direction, its length must have been at least five or sixmiles; otherwise, the isoseismal 8 would have been less elongated. Itmust therefore have reached from about half-a-mile north-east of LochNess to about half-a-mile south-west of Inverness. Its width, measuredalong the dip of the fault-surface is unknown; but the small distancebetween the centre of the isoseismal and the fault-line shows that theprincipal movement took place at a depth which was probably under, rather than over, one mile. NATURE OF THE SHOCK. We come now to the evidence afforded by the nature of the shock, inwhich there was but little variation throughout the disturbed area. AtInverness, a gentle movement was first felt, followed by anextraordinary quivering, which increased in force for two or threeseconds, and then decreased for two or three seconds; just as thequivering was about to cease, there was a distinct lurch or heave, after which the vibration was much more severe than before and lastedseveral seconds longer than the first part of the shock. Dalarossielies about fourteen miles south-east of Inverness, and here the firstindication was a loud sound, as of an express train, coming from theeast, rushing close to, and then under, the house; this lasted for afew seconds, and towards the end of it the house vibrated. Thensucceeded an interval of quietness for about a second, followed by aterrific burst or crash, not unlike the crash of a loud thunder peal, of about two seconds' duration, during which the house distinctlyheaved up once and then sank back. After another brief interval ofquietness, there was a low rumble, like the sound of a dying peal ofthunder. It will be noticed, in this account, that the two parts of the shockwere no longer consecutive. There was a short interval of rest betweenthem, the intermediate vibrations observed at Inverness being too weakto be felt at Dalarossie. Still farther away, the extinction becamemore marked. At Aberdeen, for instance, the shock consisted of twoparts, the first a tremble, followed, after an interval of a fewseconds, by a swinging movement of longer duration than the tremble. In all parts of the disturbed area, the shock maintained the samecharacter of division into two parts, the second of which was ofgreater duration and intensity than the first and consisted ofvibrations of longer period. A phenomenon of such wide occurrence wasclearly not due to local influences. It must have been caused by twoseparate initial impulses, the stronger succeeding the other after aninterval of a few seconds and taking place in nearly the same regionof the fault. [67] SOUND-PHENOMENA. Outside the isoseismal 5, there are but few records of theearthquake-sound; but it was heard faintly at Skelmorlie (inAyrshire), Belsyde (near Linlithgow), and Gullane (near NorthBerwick). Towards the north, it was not observed beyond Wick andWathen (in Caithness). The boundary of the sound-area cannot be laiddown with any approach to accuracy, but it must have included adistrict containing about 27, 000 square miles. Throughout the whole disturbed area, 84 per cent. Of the observersheard the sound. The percentage varies in different counties, from 93in Inverness-shire to 77 in the counties of Perth and Aberdeen; butthe records in the more distant regions are too few to allow of theconstruction of isacoustic lines. In its character, the sound resembled that usually heard with strongearthquakes, 39 per cent. Of the observers having compared it topassing waggons, traction-engines, etc. , 25 per cent. To thunder, 14to wind, 8 to loads of stones falling, 3 to the fall of heavy bodies, 4 to explosions or the firing of heavy guns, and 7 per cent. Tomiscellaneous sounds. The intensity of the sound gradually diminishedoutwards from the epicentre, and most rapidly near the isoseismal 7, which abounds approximately the area in which the sound was very loudfrom that in which it was distinctly fainter, and also includes nearlyall the places at which loud explosive crashes were heard with thestrongest vibrations. In the time-relations of the sound and shock, the Inverness earthquakeresembles the Hereford earthquake of 1896. The beginning of the soundpreceded that of the shock in 72 per cent. Of the records, coincidedwith it in 20, and followed it in 8 per cent. ; the epoch of maximumintensity of the sound preceded that of the shock in 20 per cent. Ofthe records, coincided with it in 73, and followed it in 7 per cent. ;while the end of the sound preceded that of the shock in 15 per cent. Of the records, coincided with it in 34, and followed it in 52 percent. Somewhat similar proportions hold over the greater part of thedisturbed area, the percentages being nearly the same in the countiesof Inverness, Ross, Nairn, Elgin, Banff, and the most distantcounties. But in Aberdeenshire an exception occurs, the three epochsof sound and shock in most cases coinciding with one another. Themajority of the observations in this county come from the southernpart, and the line joining this district to the epicentre is nearlyperpendicular to the line of the earthquake-fault. This result has animportant bearing on the origin of the sound-vibrations. For, if thegeneral precedence of the sound with respect to the shock were due toits superior velocity, the percentage of records in which thebeginning of the sound preceded that of the shock would vary only withthe distance, and not with the direction from the origin. Indeed, with increasing distance from the origin, this percentage shouldcontinually approach 100; while that in which the end of the soundfollowed that of the shock should diminish to zero. There is, however, no trace of either tendency, the sound being heard after the shock atplaces close to the boundary of the sound-area. On the other hand, itthe sound-vibrations were to start simultaneously, or nearly so, fromall parts of the focus, but especially from its marginal regions, then, in the greater part of the disturbed area, the sound would beheard both before and after the shock; for the lateral margins of thefocus would be the portions nearest to, and farther from, mostobservers; while, at places near the line through the epicentre atright angles to the earthquake-fault, the three principal epochs ofthe sound and shock should approximately coincide. The inference that the sound-vibrations heard before and after theshock come from the margins of the focus is also supported by theobservations on the relative duration of the sound and shock. If wetake only those records which are free from doubt, in 78 per cent. Ofthe total number, the duration of the sound was greater than that ofthe shock; while, in Aberdeenshire, according to 93 per cent. Of theobservers, the durations of sound and shock were equal. We may imagine, then, that the slip within the seismic focus would begreatest in a central region, and that it would die outwards in alldirections towards the edges. The friction arising from the slippingin the central region would produce chiefly the comparatively largeoscillations that formed the perceptible shock; the evanescent creepwithin the marginal regions would produce the small and rapidvibrations that were sensible only as sound. ORIGIN OF THE EARTHQUAKE. While the seismic evidence enables us to determine thesurface-position and the horizontal dimensions of the seismic focus, it unfortunately throws no light whatever on a point of someimportance--namely, the direction of the movement which caused theearthquake. We cannot infer from it whether it was the rock on thesouth-east or north-west side of the fault that slipped or whetherboth sides slipped at once; nor, if that point had been settled, do weknow if the movement of the displaced side was upward or downward. Inthe formation of the fault, however, it is clear that either thesouth-east side has been depressed or the north-west side elevated;and, as the bed of Loch Ness is below the level of the sea, that theformer movement has predominated. If the displacements which gave riseto the earthquake were merely a continuation of the original series ofmovements--and this is, to say the least, a very probable view totake--then we may imagine that, for a distance of five or six miles, and at a depth of about a mile or less, there was a sudden sagdownwards of the rock on the south-east side of the fault through adistance which perhaps in no part exceeded a fraction of an inch. Fig. 66 is an attempt to represent roughly the displacement whichcaused the principal earthquake. The diagram makes no pretence toaccuracy, and the scale in the vertical direction is enormouslygreater, perhaps a hundred thousand times greater, than that in thehorizontal direction. The straight line is supposed to represent astraight line drawn before the earthquake on the surface of the rockadjoining the fault on the south-east side and at a depth of about amile, and the curve the form of the same line after the earthquake. [Illustration: FIG. 66. --Diagram to illustrate supposed fault-displacement causing Inverness earthquake. ] The effect of this great slip would obviously be to relieve the stressin the central region A, and to increase it suddenly in the partsdenoted by the letters B and C. It is, therefore, in these partsespecially that we should expect future slips to occur. Each slipwould of course give rise to an after-shock, and would in like mannerresult in an increase of stress in its own terminal regions, thoughchiefly on the side remote from the centre A. THE AFTER-SHOCKS AND THEIR ORIGIN. It is difficult to form any estimate of the total number ofafter-shocks. The list, compiled from the records of careful observersonly, includes forty-six shocks and ten earth-sounds, the last of alloccurring on November 21st. But the list is certainly incomplete. Itcontains, for instance, only one entry on September 18th between 3. 56and 9 A. M. ; whereas, during the same interval, no fewer than eighteenslight shocks were felt by one observer at Dochgarroch, while anothernear Aldourie estimates the number of shocks up to October 23rd atabout seventy. The total number probably did not fall short of onehundred. The majority were certainly very slight, and, at another time, wouldhardly have attracted any notice. There were, however, three of muchgreater importance than the rest. These occurred on September 18th at3. 56 and 9 A. M. , and on September 30th at 3. 39 A. M. The isoseismallines of all three are elongated ovals, their longer axes are parallelto the fault, and their centres lie on the south-east side of thefault-line. The shocks were therefore evidently due to slips severalmiles in length along the fault. At present, we are concerned morewith the position of their epicentres. These are indicated by the dotslettered B, C, D in Fig. 67; the dot marked A denoting the centre ofthe principal earthquake, and the continuous line the path of thefault. Thus, within two and a half hours, the great slip was followed by onewith its centre at B, near the south-west margin of the principalfocus. About five hours later, the scene of action was suddenlytransferred to a region with its centre at C on the north-east margin. Both slips affected a portion of the fault-surface several miles inlength, and must therefore have increased the area of displacement, slightly towards the north-east and considerably towards thesouth-west. Only small movements occurred during the next twelve daysuntil 3. 39 A. M. On September 30th, when another long slip took place, with its centre at D, still farther to the south-west, and thereforeagain extending the area and amount of displacement in this direction. [Illustration: FIG. 67. --Map of epicentres of after-shocks of Inverness earthquakes. (_Davison. _)] Turning now to the weaker after-shocks and earth-sounds, we find themaffecting chiefly three regions of the fault. One of these is close toDochgarroch, another near Inverness, and the third between Aldourieand Drumnadrochit; the effects of the slips in the last two districtsbeing, as before, to extend the area of displacement a short distance(perhaps half a mile) to the north-east and not less than six miles tothe south-west underneath Loch Ness. The unequal division of the after-shocks between the two sides of theprincipal centre (A, Fig. 67) is very marked. The positions of theepicentres of forty-four shocks and earth-sounds can be determinedwith more or less accuracy, and, of these, only ten lie to thenorth-east of the principal centre, while thirty-four lie to thesouth-west, six or seven of the latter being beneath Loch Ness. One other point may be referred to before leaving these minor shocks. So far as regards the stronger shocks, there was a continual decreasein the depths of the seismic foci. This is shown by the progressiveapproach of their epicentres towards the fault-line; the distances inthe three chief after-shocks being 1. 7, 1. 0, and 0. 5 milesrespectively; and in one of the latest shocks (that of October 13th at4. 24 P. M. , E, Fig. 67) the distance is no more than one-tenth of amile. The focus of this shock must, indeed, have been quite close tothe surface near Dochgarroch. This constant diminution in the depth ofthe foci shows that the great slip was followed by a sudden increaseof stress upwards as well as laterally, and explains why that slip didnot leave any perceptible trace, either as fault-scarp or fissure, atthe surface. SYMPATHETIC EARTHQUAKES. It is remarkable that, of the 56 recorded after-shocks, at least sixwere felt or heard only at Dalarossie and other places in the valleyof the Findhorn, a valley which lies about 13 or 14 miles to thesouth-east of the great fault. That they had no connection with thatfault is certain, for two of them were so strong that, if they were soconnected, they could not have escaped the notice of one or more ofthe watchful observers between Drumnadrochit and Inverness. Theprobable explanation of these after-shocks is that they were due toslips of a fault running along the Findhorn valley;[68] and that thegreat displacement near Inverness on September 18th led to a suddenincrease of stress within the rocks for many miles around, which, atand near Dalarossie, was sufficient to precipitate the slips referredto. CONCLUSION. At first sight, two earthquakes could hardly be more unlike than theJapanese earthquake of 1891 and the Inverness earthquake of 1901. Inthe rice-fields of central Japan, as we have seen, the roads for manyleagues were edged with ruins, the fault-slip was prolonged up to thesurface and visible as a scarp forty, if not seventy, miles in length, plots of ground were compressed and their boundaries altered, thehillsides were scored by landslips, places can now be seen from oneanother that formerly were hidden by a mountain ridge, and the totalnumber of after-shocks within little more than two years amounted toabove three thousand. On the other hand, when we examine thedistribution of the after-shocks in space, we find that, though nopart of the fault was exempt from slips, they favoured three regionsin particular--one, the most important, a central region, yet notcoincident with that in which the principal shock was most intense;and the other two surrounding the extremities of the fault. With thelapse of time, the after-shocks on the whole became weaker andoccurred less frequently, and the average depth of the foci graduallydiminished. Moreover, in two districts distant forty-five andfifty-five miles from the fault, the frequency of the shocks duringthe month succeeding the earthquake was suddenly increased to ten andsixteen times the normal rate. It is interesting to notice so close a similarity in character, subsisting with so vast a difference in the scale of intensity. Theidentity of the powers at work in shaping the structure of bothislands Is manifest. In Japan, we see the mountain-making forcesacting with violence and producing effects that are only too apparentto the eye. In Scotland, whatever may have happened in formergeological epochs, the changes in surface-structure are now takingplace with almost infinite slowness, and hundreds or thousands ofyears must elapse before Loch Ness makes any visible progress in itsmarch towards the sea. REFERENCES. 1. DAVISON, C. --_The Hereford Earthquake of December 17, 1896. _ (Birmingham, 1899. ) 2. ---- "The Inverness Earthquake of Sept. 18, 1901, and its accessory shocks. " _Quart. Journ. Geol. Soc. _, vol. Lviii. , 1902, pp. 377-397. FOOTNOTES: [61] The study of the Hereford earthquake is based on 2, 902 records, coming from 1, 943 places; that of the Inverness earthquake on 710records from 381 places. [62] The disturbed area of the Hereford earthquake of 1896 wasprobably greater than that of any other British earthquake of thenineteenth century; that of the Pembroke earthquake of 1892 being morethan 56, 000 square miles, of the Pembroke earthquake of 1893 about63, 600 square miles, while that of the Essex earthquake of 1884 (a farstronger shock in the meizoseismal area) is estimated at about 50, 000square miles. [63] The approximate circularity of the two outer isoseismals is dueto the fact that the vibrations propagated to such great distances arethose which start from the comparatively small central region of thefocus. [64] The above statement summarises the evidence of the majority ofthe observers in each portion of the disturbed area. In this, as inother similar cases, discrepancies in the observations areunavoidable; but it is important to notice that they are leastfrequent in the observations evidently made with the greatest care. [65] Except in the case of Yorkshire, where the three Ridings areregarded as separate counties. [66] The Derby earthquake of March 24th, 1903, was also a twinearthquake. The centres of the two foci were situated near Ashbourneand Wirksworth, above eight or nine miles apart, along a line runningN. 33° E. And S. 33° W. The two parts of the shock coalesced along arectilineal band about five miles wide running centrally across thelower isoseismals in a direction at right angles to their longer axes. The isacoustic lines are also elongated in the direction of this band. In this case, the impulses at the two foci must have taken place atthe same instant. (_Quart. Journ. Geo. Soc. _, vol. Lx. , 1904, pp. 215-232. ) [67] If the foci of the two impulses had been detached, there would, with so small an interval between the two parts, have been a variationin the nature of the shock like that observed during the Herefordearthquake. [68] This part of Inverness-shire has not yet been mapped by theGeological Survey, but a fault is known to exist in the Findhornvalley near Drysachan Lodge, which lies about eleven miles down thevalley from Dalarossie. CHAPTER IX. THE INDIAN EARTHQUAKE OF JUNE 12TH, 1897. Very different from the shocks of Britain was the earthquake thatoverwhelmed so large a part of its great dependency on June 12th, 1897--an earthquake which, if it is not without a rival, is certainlyone of the most disastrous and most widely-felt of which we possessany record. That it was of the first magnitude was evident at once inCalcutta from the extensive injury to buildings, and its investigationwas undertaken without delay by the members of the Geological Surveyof India. The four officers who were at the headquarters in Calcuttawere despatched to the area of greatest damage, letters and circularswere distributed as widely as possible, a large number of observerswere induced to co-operate by keeping records of the after-shocks, and, later on, during the cold weather of 1897-98, Mr. R. D. Oldham, one of the superintendents of the Survey, made a tour through theepicentral district. To him, moreover, fell the much harder task ofdiscussing the very numerous observations collected by himself andothers; and the least that can be said of the valuable report preparedby him is that it is worthy of a great subject. Professor Omori alsospent several months in studying the earthquake on behalf of theJapanese Government; but the account, which is written in his ownlanguage, unfortunately remains a sealed book to westernseismologists. [Illustration: FIG. 68. --Isoseismal Lines of Indian Earthquake. (_Oldham. _)] ISOSEISMAL LINES AND DISTURBED AREA. In Fig. 68, which shows the area disturbed by the earthquake, Mr. Oldham has drawn two series of curves. In the absence of detailedrecords of the intensity--records that could not have been obtainedfrom some parts of the disturbed area, and would have been difficultto procure in sufficient number from others--he has represented by thedotted curves a group of isoseismals in the form which he believesthey would have assumed had the earth-waves been propagated in ahomogeneous medium. The first includes all places, such as Shillongand Goalpara, where the destruction of brick and stone buildings waspractically universal; the second, those, like Darjiling, in whichdamage to buildings was universal and often serious; the third, places, like Calcutta, where the earthquake was strong enough toinjure all or nearly all brick buildings. Inside the fourthisoseismal, the shock was strong enough to disturb furniture and looseobjects, but not to cause more than slight damage; within the fifth, it was generally noticed; and, beyond this, and as far as the sixthisoseismal, the earthquake was perceived only by a small number ofsensitive persons at rest. The approximation of the curves towards theeast and south-east, Mr. Oldham believes to be partly real, and notdue to imperfect information. The continuous curves represent more closely the actual variation ofintensity. The innermost curve A indicates the probable boundary ofthe epicentral tract, which is about 200 miles in length and more than6000 square miles in area. This will be referred to afterwards ingreater detail. The next curve B bounds the region within whichserious damage to brick houses was common. Its irregular course isclosely connected with the geological structure of the country, and isdue to the fact, of which we have already met with several examples, that earthquakes are more destructive to houses built on alluvialground than to those founded on rock. The area included within thiscurve is not less than 145, 000 square miles; and, if we include theparts from which reports were not obtainable, it must amount to about160, 000 square miles. The curve C represents the boundary of the disturbed area, so far asknown, for about one-third of the area lies in regions from which noinformation was procurable, while another third is inhabited byignorant and illiterate tribes. But, notwithstanding this, the shockis known to have been felt over an area of at least 1, 200, 000 squaremiles. If we include the detached region to the west, near Ahmedabad, the portion of the Bay of Bengal in which the shock would have beenfelt had the sea been replaced by land, and a large part of Thibet orWestern China, from which no reports have come, but in which the shockwas certainly sensible, this estimate, great as it is, must be raisedto about 1, 750, 000 square miles. [69] Figures, such as those given above, convey but little idea of thevastness of the area concerned. Transferring them to countries withwhich we are more familiar, we may say that the disturbed area wasonly a little less than half the size of Europe; the region in whichserious damage occurred to masonry was more than twice as large as thewhole of Great Britain; while, if the centre of the epicentral tracthad been in Birmingham, nearly every brick and stone building betweenYork and Exeter would have been levelled with the ground. NATURE OF THE SHOCK. Few and slight were the forerunners of the greatest of modernearthquakes. Early in June, faint tremors were felt by sensitivepersons at Shillong. Others at the same place heard a rumbling soundfor ten or fifteen seconds before the shock began, and at Silcharbirds were seen to rise suddenly from trees before the movement becamesensible to man. Except for these almost imperceptible warnings, theearthquake broke abruptly over the whole district. "At 5. 15, " writes one observer at Shillong, "a deep rumbling sound, like near thunder commenced, apparently coming from the south orsouth-west.... The rumbling preceded the shock by about two seconds... And the shock reached its maximum violence almost at once, in thecourse of the first two or three seconds. The ground began to rockviolently, and in a few seconds it was impossible to stand upright, and I had to sit down suddenly on the road. The shock was ofconsiderable duration, and maintained roughly the same amount ofviolence from the beginning to the end. It produced a very distinctsensation of sea-sickness.... The feeling was as if the ground wasbeing violently jerked backwards and forwards very rapidly, everythird or fourth jerk being of greater scope than the intermediateones. The surface of the ground vibrated visibly in every direction, as if it was made of soft jelly; and long cracks appeared at oncealong the road.... The road is bounded here and there by low banks ofearth, about two feet high, and these were all shaken down quite flat. The school building, which was in sight, began to shake at the firstshock, and large slabs of plaster fell from the walls at once. A fewmoments afterwards the whole building was lying flat, the wallscollapsed, and the corrugated iron roof lying bent and broken on theground. A pink cloud of plaster and dust was seen hanging over everyhouse in Shillong at the end of the shock.... My impression at the endof the shock was that its duration was certainly under one minute, andthat it had travelled from south to north.... The violence of theshock may be imagined when it is stated that the whole of the damagedone was completed in the first ten or fifteen seconds of the shock. " Other estimates of the duration are generally higher than that givenabove, ranging from three to five or even more minutes at Tura, Dhubri, Silchar, Calcutta, and other places. In some cases, it ispossible that the immediately succeeding tremors were included as partof the great shock; but, in the central area, it is probable that theaverage duration of the shock did not differ much from three or fourminutes. In this district, the movement was most complicated. Changes ofdirection were frequently noticed. At Silchar, for instance, theearthquake began with an undulatory movement from north to south, likethe swinging of a suspension bridge; it closed with a motion likethat of a boat tossed in a choppy sea, or by the crossing of greatwaves which, whatever their dominant direction may have been, certainly did not travel from north to south. The vertical componentof the motion must have been considerable; for, at Shillong, loosestones lying on the roads were tossed in the air "like peas on adrum, " But this was even less pronounced than the horizontal movement, the range of which was at least eight or nine inches, and during whichpeople felt as if they were being shaken like a rat by a terrier. Theperiod of these vibrations was estimated at about a second. As they left the central region, the period of the waves lengthened, so that, at a distance, the shock no longer consisted of short jerks, but became a gentle rocking motion, causing in some people a sensationof nausea. At Calcutta, the undulations were regular and resembled therolling of a mighty ship, the period being between one and twoseconds. At Balasor, the motion was a long rolling one, such as wouldbe felt on the deck of a ship in a fairly heavy sea; and, farther tothe south as far as the limit of the disturbed area, the sameundulatory movements were observed, gradually decreasing in intensity, and usually compared to the easy motion of a ship in a gentle sea. _Visible Earth-Waves. _--A few examples have already been given of theobservation of visible waves on the surface of the ground. They wereseen at Charleston during the earthquake of 1886 (p. 110), and atAkasaka and other places in the meizoseismal area during the Japaneseearthquake of 1891 (p. 186). But they were more than usuallyprominent in the Indian earthquake; indeed, much of the difficultyexperienced in standing during the shock seems to have been due to thepassage of these surface-waves. At Shillong, according to an observer quoted above (p. 266), thesurface of the ground vibrated visibly in every direction, as if itwere made of soft jelly. Another describes it as presenting "the aspectof a storm-tossed sea, with this difference that the undulations wereinfinitely more rapid than any seen at sea. " Near Maimansingh, earth-waves were watched approaching, exactly like rollers on thesea-coast, and, as they passed, the observers had a difficulty instanding. At Nalbari, the rice in the fields could be seen rising andfalling at intervals during the transit of the waves. In the Assamvalley, near Mangaldai, there were seen "waves coming from oppositedirections and meeting in a great heap and then falling back; each timethe waves seemed to fall back the ground opened slightly, and each timethey met, water and sand were thrown up to a height of about 18 inchesor so. " Even as far as Midnapur, the ground was "distinctly billowy, "and at Allahabad a series of waves was observed to cross the groundfrom south-south-west to north-north-east. It is obviously difficult to judge in any case of the magnitude ofsuch waves. In the epicentral area, Mr. Oldham believes that, on anaverage, they were probably about thirty feet long and one foot inheight, though some may have been both shorter and higher. Thesemovements must have been comparatively slow, for their progress couldbe easily followed by the eye; indeed, their rate, as one witnessremarks, "though decidedly faster than a man could walk, was not sofast as he could run. " ELEMENTS OF THE WAVE-MOTION. In his study of the Neapolitan earthquake, Mallet showed how theamplitude and maximum velocity of the vibrations could be determinedroughly from the displacement, projection, or overthrow of variousbodies by the earthquake. Somewhat similar methods were employed byMr. Oldham in the absence of seismographs from the epicentral area. His results are of course only approximate, but they lead neverthelessto a conclusion of great value and interest. [Illustration: FIG. 69. --Section of Tombs in the Cemetery at Cherrapunji. (_Oldham. _)] _Amplitude. _--The best measure of the amplitude was obtained at thecemetery at Cherrapunji, situated near the southern margin of theepicentral area. Here were two oblong masonry tombs (Fig. 69), standing close together with their longer axes pointing north andsouth. Their inner sides were partially destroyed. "On the outersides, they are almost intact, but the tombs have been driven bodilydown into the ground, and on either side to east and west, there is adepression with a vertical side parallel to the outer surface of thetomb and a smooth flat bottom over which the base of the tomb hasslid.... The edge of the western depression has the grass growingundisturbed up to the edge of it, and along the edge small fragmentsof lime and plaster show that this was originally in contact with theedge of the tomb, which has now moved away to a distance of 18 inches. On the east the edge of the depression is raised and the grass andearth forced upwards by the thrust of the tomb against it; the breadthof this depression is 10 inches. " During the movement of the ground, the tombs, owing to their inertia, remained comparatively stationary, and the depressions were formed bythe backward and forward movement of the ground against them. Themovement on the east side was clearly arrested in some manner, and therange therefore cannot have been less than 10 inches. It may have beenas much as 18 inches, and was probably, in Mr. Oldham's opinion, themean of these two amounts--namely, 14 inches. This would give anamplitude of about 7 inches, a value which may be in excess of theaverage amount elsewhere in the district, as the cemetery is situatednear the edge of a high sandstone scarp. At Tura, also within the epicentral area, a range of not less than 10inches was given by the sliding of a wooden house over the posts onwhich it rested. Six months after the shock, Mr. Oldham frequentlynoticed vacant spaces four or five inches across by the side of largeboulders scattered over the Khasi hills, and he infers that"throughout the whole tract lying west of Shillong and Gauhati, as faras the hills extend, and probably over a large area of the plainsbesides, the amplitude of the wave-motion was nowhere less than 3inches, while in many places it was over 6 inches. " _Maximum Velocity. _--The most trustworthy measure of the maximumvelocity are those obtained from the projection of bodies. Mr. Oldhamselects the following as most deserving of notice:--At Goalpara, anobelisk surmounting a tomb was broken off and thrown to one side, giving a maximum velocity of not less than 11 feet per second. AtGauhati, the coping of a small gate-pillar was shot off and fell at adistance of 4 feet 4 inches from the centre of the pillar; in thiscase the maximum velocity must have exceeded 8 feet per second. Thehighest velocity, of more than 16 feet per second, was measured atRambrai, where a small group of monoliths were shot out of the ground, one of them to a distance of 6-1/2 feet. Lastly, at Silchar, a bulletwas projected from the corner of a wooden post, acting as a rough formof seismometer, from which a maximum velocity of at least 1-1/2 feetper second was deduced. _Maximum Acceleration. _--Estimates of the maximum horizontalacceleration were made from 28 overthrown pillars by means ofProfessor West's formula (p. 184, footnote). The measures obtained atthe same place show some variation, but Mr. Oldham considers as fairaverage values those of 14 feet per second per second at Goalpara, 12at Gauhati, Shillong, and Sylhet, 10 at Cherrapunji, 9 at Dhubri, and4 feet per second per second at Silchar. Of the vertical component of the acceleration, not even the roughestnumerical estimate can be formed. We know, however, that at Shillong, Gauhati, and indeed throughout the epicentral area, stones wereprojected upwards, and this is evidence that the vertical componentwas greater than that of gravity--namely, 32 feet per second persecond. Violent as the shock was at the places just mentioned, it must havebeen still greater in certain parts of the epicentral area. At Dilma, in the Garo hills, the shock seems to have been strong enough todisable men; and, in the neighbourhood of the faults that will bedescribed in a later section, forest trees were snapped in two. Fortunately, as Mr. Oldham remarks, there were in these districts notowns or populous settlements to feel the full power of the earthquaketo destroy. _Anomalies in the above Measurements. _--If the movements of the groundfollowed the law of simple harmonic motion, any two of the fourelements (period, amplitude, maximum velocity, and maximumacceleration) would suffice to determine the others (p. 4). Applyingthe usual formulæ to the quantities obtained at Gauhati--namely, 8feet per second for the maximum velocity and 12 feet per second persecond for the maximum acceleration, it follows that the amplitudewould be 5 feet and the period 4 seconds--values, which are evidentlyinadmissible. Or, taking the maximum vertical component at 32 feet persecond per second, the corresponding values would be 2 feet and 1-1/2seconds, that of the amplitude being still too great. Again, atRambrai, the maximum velocity was found to exceed 16 feet per second. The other elements are unknown, but, if the amplitude were one foot, Mr. Oldham shows that the maximum acceleration would be 256 feet persecond per second; or, taking the amplitude at the impossible amountof two feet, that the maximum acceleration would be 128 feet persecond per second. It follows, therefore, that only part of the high velocities atRambrai and elsewhere can be due to the elastic waves provoked by theinitial disturbances. The remaining portion must be attributed to abodily displacement of the earth's crust within the epicentral area--adisplacement of which the fault-scarps and other distortions of thatregion furnish ample evidence. SOUND-PHENOMENA. In the epicentral area, the sound that accompanied the earthquake wasremarkable for its extraordinary loudness. At Shillong, the crash ofhouses falling within thirty yards was completely drowned by the roarof the earthquake. The sound was generally compared to distant thunder, the passage of atrain or cart, etc. ; but, whatever the type may be, it always impliesa sound of deep pitch, close to the lower limit of audibility--acontinuous rumbling or rattling noise, as a rule gradually becominglouder and then dying away. There was the usual conflict in theevidence of different observers due to the depth of the sound. InCalcutta, which lies well within the sound-area, some persons assertedthat they heard a rumbling noise; others were positive that the onlynoise was that caused by falling buildings and furniture. Some, again, noticed that the shock was preceded by a loud roar; while others werecertain that there was no sound of any kind until the earthquake hadbecome severe. As in the case of the disturbed area, it is impossible to define theboundary of the region over which the sound was heard. Like the shock, also, it seems to have been observed farther to the west than towardsthe east. Leaving out of account records that are possibly doubtful, the sound was heard for a distance of 330 miles to the west andsouth-west, and 290 miles to the east of the epicentral area--that is, allowing for the dimensions of that area, it must have beenperceptible over a region measuring not less than 800 miles from eastto west. VELOCITY OF THE EARTH-WAVES. It is somewhat doubtful whether a more accurate estimate of thevelocity is to be obtained from a violent earthquake or from one ofmoderate intensity. In the former case, the vast distances to whichthe shock is noticed lessen the effects of errors in thetime-determinations, but this advantage is to a great extentcompensated by the considerable duration of the shock and theconsequent uncertainty whether all observers have timed the same phaseof the movement. Also, in the Indian earthquake, there are furthersources of error in the variety of standard times employed throughoutthe country and in the magnitude of the epicentral area. Of the numerous time-records collected by Mr. Oldham, the best arethose which were obtained from a few self-recording instruments, fromthe more busy telegraph offices, from the larger railway stations, andin some cases from private individuals. All records were in the firstplace subjected to a rigid process of selection; a large number wererejected on various grounds, and those only were retained which boreinternal evidence of accuracy, due either to the conditions of thereporter's occupation or to the care taken by him to ensure exactness. To guard against any unconscious bias in making the selection, thisprocess was carried out before the distances were calculated, and evenbefore the position of the epicentral area was known. The boundary of this area is shown by the continuous line A in Fig. 68. Its greatest length being about 200 miles from east to west, it isnecessary in the first place to fix upon an equivalent centre withinit, which may be regarded for this special purpose as the point ofdeparture of the earth-waves. The more natural course perhaps would beto assume this point to coincide with the centre of the area. But, asthe rate at which the initial movement spread over that area wouldprobably differ little from the velocity of the earth-wave, and as allthe time-stations lie towards the west, Mr. Oldham regards a pointnear the western boundary of the area (in lat. 25° 45' N. And long. 90° 15' E. ) as a sufficiently exact approximation to the position ofthe equivalent centre. The nearest place at which good time-observations were made isCalcutta, distant 255. 5 miles from the assumed centre. One isindicated on the recording tide-gauge by a sudden rise of the water, while the others were obtained from the central telegraph office, theterminal railway stations, and from two careful readings by interestedobservers. They vary from 4h. 27m. 0s. To 4h. 28m. 37s. P. M. , allbeing liable to an error of half-a-minute. The arithmetic mean for thebeginning of the shock is 4h. 27m. 49s. , and this is probably asaccurate an estimate as the conditions allow. [70] Bombay lies outside the disturbed area, 1208. 3 miles from theequivalent centre; and, for the time of arrival in that city, we haveto depend on the records of the barograph and the three magnetographs. The horizontal force magnet was set in motion two and a half minutesbefore the others, no doubt by the advance tremors. The times given bythe barograph and the vertical force-instrument differ by only oneminute, and the best result seems to be that obtained by taking theirmean--namely, 4h. 35m. 43s. , which is probably accurate to within aminute. Assuming, then, that the time-interval between Calcutta and Bombaydoes not err by more than half-a-minute, it follows that theintervening velocity must lie between 2. 8 and 3. 2 kilometres persecond, its probable value being 3 kilometres, or 2 miles, per second. The remaining records, which are of less value than those obtained inthese cities, fall into two groups, the first consisting of a numberof stations along a line running north and south between Calcutta andDarjiling or within a hundred miles on either side of the same, andthe second a long series of stations crossing Northern India in anearly westerly direction. The observations made at the Burmesestations were unfortunately affected by an error arising from theretardation of the Madras time-signals through frequent repetitionalong the line. [Illustration: FIG. 70. --Time-curve of Indian earthquake. (_Oldham. _)] Individually, these records are not exact enough to be used indetermining the velocity, but they may be employed collectively forthe construction of the time-curve in Fig. 70. In this diagram, distances in hundreds of miles from the equivalent centre arerepresented along the horizontal line, and the time of occurrence inminutes past 4 P. M. Along the perpendicular line. The small circlesrepresent the observations at Calcutta and Bombay, the dots those atplaces lying nearly west of the origin, and the crosses those atplaces situated to the south or north-west. The continuous curvepasses in an average manner through the series of points, and probablydoes not differ much from the true curve of the time of arrival of theshock at different places. The curve, it will be noticed, is at firstconcave, and afterwards convex, upwards; indicating that the timesrequired to traverse successive equal distances at first increased, and then decreased. Thus, if the curve is an accurate representationof the facts, it would follow that the surface-velocity was subject toa continual decrease outwards from the centre, until it was a minimumat a distance of about 280 miles, after which it increased. The deviation of the curve from a straight line is, however, so slightthat we cannot feel much confidence in this conclusion. If we join thepoints corresponding to Calcutta and Bombay by a straight line (drawndotted in Fig. 70), it does not in any part vary from the continuousline by a distance equivalent to more than half-a-minute. Indeed, if avery few discordant records are excluded, and if less weight is givento those times which are multiples of five minutes, the straight linerepresents the mean quite as fairly as the curved line does; and thatthis is the more probable interpretation will appear from theobservations on the unfelt earthquake described in the next section. We may therefore conclude that the earth-waves travelled along thesurface at an approximately uniform rate of 3 kilometres per second, or about 120 miles a minute--a result which Mr. Oldham considers maybe accepted as accurate to within five per cent. If the two time-curves in Fig. 70 are continued to the right untilthey meet the time-scale, it will be seen that they intersect it nearthe point corresponding to 4. 26 P. M. , implying that this would beapproximately the time at which the shock was felt within theepicentral area. This agrees closely with the observed times of about4. 25 at Parbatipur and Kuch Bihar, 4. 26 at Siliguri, and 4. 27 atShillong and Goalpara; and it is probable that the error is not morethan a quarter of a minute in defect or half-a-minute in excess. Thus, the time of arrival of the first sensible waves at the surface wouldlie between 4h. 25m. 45s. , and 4h. 26m. 30s. P. M. , Madras time, orbetween 11h. 4m. 45s. And 11h. 5m. 30s. A. M. , Greenwich mean time. THE UNFELT EARTHQUAKE. Of the crowd of vibrations that agitate the ground during anearthquake, part only combine to form the perceptible shock. Some areinsensible owing to their small amplitude, others to the slowness ofthe motion. An interesting observation belonging to the latter classwas made by an engineer near Midnapur, a place which lies just withinthe area of damage. At the time of the earthquake, he was takinglevels on a railway bank, and was about to take a reading when henoticed the bubble of the level oscillating. In five or ten secondsthe shaking began and appeared to last three or four minutes; but, formore than five minutes after it had apparently ceased, the levelshowed that the ground continued to rock. Again, in Burmah, at a place nineteen miles east of Tagaung and closeto the border of the disturbed area, the water in a shallow tank, about 300 yards in length, was seen lapping up against the side in amanner that was at first attributed to elephants bathing. No shock wasfelt, but the shaking of the trees at the same time showed that thedisturbance was due to the earthquake. Far beyond the limits of the disturbed area, however, the earthquakewas recorded by many of the delicate instruments which have beenemployed during the last few years for the registration of distantshocks. Among the more important of these instruments are longvertical pendulums, horizontal pendulums of various forms, andmagnetographs. In the vertical, and some of the horizontal, pendulums, especially in those used in the Italian observatories, themasses carried are heavy, and the movements of the ground aremagnified by lightly-balanced levers ending in points which tracetheir records on bands of smoked paper driven by clockwork. In theother horizontal pendulums and in the magnetographs, the method ofregistration is photographic. The paper required for the mechanicalrecords being inexpensive, a high velocity (half-an-inch or more perminute) can be given to it, and the resulting diagrams are open anddetailed. The Italian instruments also respond more readily than theothers to the earlier and slighter tremors: while the apparatus inwhich photographic methods are used are sometimes so violentlydisturbed by the later undulations that the spot of light fails toleave any trace on the photographic paper. It is therefore from theItalian observatories that the more interesting records come. One ofthese, given by a horizontal pendulum at Rocca di Papa near Rome, isreproduced in Fig. 71; while the curve of the bifilar pendulum atEdinburgh (Fig. 72) is a good example of those obtained by thephotographic method of registration. [71] All over Italy, from Ischia and Catania in the south to Pavia in thenorth, the different instruments employed began, one after the other, to write their records of the movement as the unfelt earth-waves spedoutwards from the centre. Italy passed, the tale was taken up bymagnetographs at Potsdam and Wilhelmshaven, Pawlovsk (near St. Petersburg), Copenhagen, Utrecht, and Parc St. Maur (near Paris); byhorizontal pendulums at Strassburg and Shide (in the Isle of Wight), and by a bifilar pendulum at Edinburgh. Shide is 4, 891 miles from thecentre of disturbance, but, as we shall see, the movement could betraced for a distance greater even than this. [Illustration: FIG. 71. --Seismographic Record of Indian Earthquake at Rocca di Papa. (_Cancani. _)] In the more complete records, and especially in those given by theItalian apparatus, Mr. Oldham distinguishes three phases of motion. The first consists of rapid and nearly horizontal movements of theground. In Italy, it begins at about 11. 17 A. M. --that is, about 12-1/2minutes after the commencement of the shock at the epicentre (Fig. 71, _a_). Without any break in the movement, and after a further intervalof about 8-1/2 minutes, the second phase begins; the vibrations aresimilar to the preceding, but they are larger and more open, and areaccompanied by an unmistakable tilting of the surface of the ground(Fig. 71, _b_). Lastly, after the lapse of about twenty minutes more, the second phase gives place, without interruption, to the third (Fig. 71, _c_), [72] consisting of well-marked slow undulations, which havebeen aptly compared by Professor Milne to the movements caused by anocean-swell. As they travelled across Europe, the surface of theground was thrown into a series of flat waves, 34 miles in length, and20 inches in maximum height, the complete period of each wave being 22seconds. This phase is by far the longest of the three; in the moresensitive instruments, two or three hours elapsed before their tracesceased to show any sign of movement. [Illustration: FIG. 72. --Seismographic Record of Indian Earthquake at Edinburgh. (_Heath. _)] Knowing the distances of the different observatories from theepicentre, and the times taken by each phase to reach them, we canform some idea of the rates at which they travelled. If the earlytremors moved in straight lines, their mean velocity for the firstphase was 9. 0, and for the second 5. 3, kilometres per second; but, ifthey moved along curved paths through the body of the earth, theirmean velocities must have exceeded these amounts. For the firstundulations of the third phase, the velocity would be 2. 9 kilometresper second if they travelled along straight lines, and 3. 0 kilometresper second if they were confined to the surface of the earth. The existence of the second phase was noticed for the first time byMr. Oldham in the records of the Indian earthquake, but he has sincedetected it in those of other shocks. He believes, in common with mostseismologists, that the first phase corresponds to waves of elasticcompression, or longitudinal waves, travelling through the body of theearth; and the second phase he attributes to waves of elasticdistortion, or transversal waves, travelling in the same way, in whichthe particles move at right angles to the direction in which the wavetravels, thus causing a slight tilting of the surface. It is probablethat the waves of both phases move along curved, rather than straight, lines through the earth, that the curves are concave towards thesurface, and that the velocity of the waves increases with the depthof their path below the surface. On the other hand, the surface-velocity of the first undulations ofthe third phase is practically constant for all distances from theepicentre, and, in the case of the Indian earthquake, it agrees almostexactly with that obtained for the velocity within the disturbed area, and as far as Bombay. It is therefore difficult to resist theconclusion that the third phase consists of undulations which travelalong the surface of the earth. Diverging in two dimensions only, theyfade away much more slowly than the vibrations of the other twophases. We may thus imagine these surface-undulations speeding outwards fromthe epicentre in ever-widening circles until they have passed over aquarter-circumference of the earth, when they should begin to convergetowards the antipodes. Here they should cross each other, and againspread out as circular waves, once more in their course passing thesame observatories where they were first recorded, but in the oppositeorder. It has been reserved for the most violent earthquake of moderntimes to verify this interesting conclusion. Faint, but decided, arethe traces of the second crossing. At Edinburgh, they occur at 2. 6P. M. , at about the same time at Shide, at Leghorn 2. 10, Catania2. 12-3/4, while at Ischia there are several movements between 2 and 3P. M. At Rocca di Papa, near Rome, the time is slightly earlier, butthe undulations, like those at the first crossing, have a completeperiod of about 20 seconds. The distances traversed by the waves aremore than 20, 000, instead of less than 5000 miles; but the meanvelocity with which they travelled is almost exactly the same as atfirst--namely, 2. 95 kilometres per second. EARTH-FISSURES, SAND-VENTS, ETC. _Earth-Fissures. _--Among the superficial effects of the earthquake, none take a more important place than the fissures formed in alluvialplains. Not only were they remarkably abundant, more so than in anyother known earthquake, but they occurred over an unusually wide area. Wherever the necessary conditions prevailed, they were found to benumerous over a district bounded approximately by the isoseismal 1(Fig. 68), and measuring about 400 miles from east to west, and about300 miles from north to south; and they were present, though insmaller numbers, over an area nearly 600 miles long in aneast-north-east and west-south-west direction. They were naturallymore frequent near river-channels and reservoirs, on account of theabsence of lateral support, and as a rule were parallel to the edge ofthe bank, a few hundred yards in length, and in width varying fromsome inches to four or five feet. Fissures in such positions are formed with every violent earthquake, and even with some of those more moderate shocks that visit theBritish Islands (see p. 247). But an interesting point established bythe Indian earthquake is that they also occurred at a distance fromany water-channel or excavation, often running parallel to, and alongeither side of, a road or embankment. In other situations, they showeda distinct tendency to range themselves parallel to one another; and, in these cases, it is possible that their formation was connected withthe passage of the visible surface-waves. In an account already quoted(p. 247), it is stated that these waves came from opposite directionsand that, as they separated after meeting, the ground opened slightly. Among the Khasi and Garo hills (see Fig. 75), wherever the alluvium ofthe plains runs up to the foot of the hills, another form of fissure, represented in Fig. 73, was constantly noticed. Close to thejunction, there was a sudden drop, as at _a_, of from one to fivefeet, the vertical face having the appearance of a fault, butdistinguished from one by following the windings of the hills. Thencame a depressed band _b_, from ten to twenty feet wide, and outsidethis a low rounded ridge _c_ raised above its former level, andmerging beyond at _d_ into the undisturbed plain. When Mr. Oldhamvisited the district in March 1898, the natives had flooded therice-fields, and the features described were clearly depicted by thegathering of the water in the depression and the isolation of theridge. [Illustration: FIG. 73. --Displacement of alluvium at foot of a hill. (_Oldham. _)] The explanation of these peculiarities is evidently that given by Mr. Oldham. During the passage of repeated waves of compression, thethrust of the hill and plain against one another caused the heaping upof the alluvium in the ridge _c_; while the return movements resultedin the tearing of the alluvium away from the hillside, leaving thescarp _a_ and the depression _b. _ _Displacements of Alluvium. _--Many other remarkable evidences ofcompression were observed. Telegraph posts, originally set up in astraight line, were displaced, occasionally as much as ten or fifteenfeet; sometimes without any apparent connection with neighbouringriver-channels. In one part of the Assam-Bengal Railway, for nearlyhalf a mile, the whole embankment, including borrow-pits and trees oneither side, was shifted laterally without any sign of wrenching fromthe adjoining ground, the maximum distance amounting to 6-3/4 feet. Asthe displacement took place parallel to the only river-course in theneighbourhood, Mr. Oldham attributes it to the sliding of thesurface-layers over some yielding bed beneath. Again, throughout largeareas of Northern Bengal, Lower Assam, and Maimansingh, rice-fields, which had been carefully levelled so that they might be uniformlyflooded, were thrown into gentle undulations, the crests of which wereoccasionally two or three feet above the hollows. The piers of bridgeswere also moved parallel to, as well as towards, the streams, showingthat the displacements extended to the depth of the foundations. The buckling of railway lines was often violent and took place over alarge area. In the Charleston earthquake, every such bend wasaccompanied by a corresponding extension elsewhere (p. 113); but, inthe Baluchistan earthquake of 1892, the neighbouring fish-joints werejammed up tight. [73] In the one case, there was merely localcompression; in the other, a permanent displacement of the earth'scrust. The distortion of the Indian lines seems to belong to theformer class. Repairs were of course generally made without delay; butall the information that could be obtained on this point showed thatthe compression causing the crumpling of the lines was accompanied bya compensating expansion, generally at a distance of about 300 yards. _Sand-Vents. _--Shortly after the earthquake, large quantities of waterand sand issued from fissures in the ground. At Dhubri, "innumerablejets of water, like fountains playing, spouted up to heights varyingfrom 18 inches to quite 3-1/2 or 4 feet. Wherever this had occurred, the land was afterwards seen to occupy a sandy circle with adepression in its centre. These circles ranged from 2 to 6 and 8 feetin diameter, and were to be seen all over the country. In some places, several were quite close together; in others they were at a distanceof several yards. " Near Maimansingh, they seem to have been almost asnumerous, fifty-two, of four feet and less in diameter, being countedwithin an area 100 yards long and about 20 feet wide. The sand and water were ejected from the vents with some force. A fewobservers estimated the height of the spouts at about 12 feet, butthis probably refers to stray splashes. It is clear, however, that thesand and water were forced not only up to the surface, but even in acontinuous stream to heights of from two to ten feet above it. In manydistricts, trunks of trees or lumps of coal and fossil resin werewashed up with the water, and even, in one or two cases, pebbles ofhard rock weighing as much as half-a-pound. The origin of the sand-vents is to be sought in the presence of awater-bearing bed situated not far below the surface. In the centralarea, where there was a marked vertical component in the motion, thisbed during the earthquake was compressed between those above and belowit, and the resulting pressure was in places sufficient to force thewater and sand, through the fissures formed by the earthquake, up toand beyond the surface. The gradual settling of the upper layer, cutup by the fissures, into the underlying quicksand, prolonged theprocess for some time after the shock was over; and, when the pressurewas at last relieved, some of the water was sucked back and soproduced the crateriform hollows. _Rise of River-Beds, etc. _--Over a large area, river-channels, tanks, wells, etc. , were filled up, partly by the outpouring of the sand fromvents, but chiefly, as shown by the forcing up of the central piers ofbridges, by the elevation of the beds of the excavations. In thelowlands which lie between the Garo hills and the Brahmaputra, therewere numerous channels from 15 to 20 feet in depth, the beds of whichwere pressed up until they became level with the banks, while acompensating subsidence took place close to the streams on eitherside. The general tendency of the earthquake was thus to obliteratethe surface inequalities, so that, when the rivers rose later on, thedistrict was extensively flooded. Besides these deferred floods, there occurred immediately after theearthquake a sudden rise in many rivers, amounting to from two to tenfeet, followed by a gradual decline to the former state in two orthree days. At Gauhati, for instance, the river-gauge showed that, atabout three-quarters of an hour after the earthquake, the water stood7 feet 7 inches higher than on the morning of June 12th; at 7 A. M. OnJune 13th it had fallen to 5 feet 8 inches, and at the same time onthe two following days to 2 feet 7 inches and 6 inches, showing thatthe water had returned nearly to its original level after the lapse oftwo and a half days. In most of the large rivers, the rise of water was due to theformation of partial dams formed by the local elevation of theriver-beds described above. As the barriers were composed of loosesand, they were gradually scoured away and the material was spreadover the bottom so as to leave the water at a level slightly higherthan that which it maintained before the earthquake. LANDSLIPS. The distribution of landslips shows that their formation dependsalmost as much on local conditions as on the violence of the shock. The effect of the latter is manifested by their limitation to acertain central area. To the east of the North Cachar hills, few, ifany, were to be seen; but, as far as Kohima, cracks or incipientlandslips were formed on the hillsides. The Sylhet valley and a lineto the west of Darjiling form the southern and western boundaries ofthe landslip area, which was therefore not less than 300 miles inlength from east to west. Within this area, however, local conditions asserted theirsuperiority. Among the more important may be mentioned theconstitution of the hills and the presence of a thick superficiallayer of subsoil or rock with an inner bounding surface of weakcohesion, the slope of the hillsides, and their height from base tocrest. Thus, though the epicentral area was situated chiefly to thesouth of the Brahmaputra valley (Fig. 75), the east and west range ofthe landslips was more extensive in the Himalayas on the north sidethan in the Garo and Khasi hills on the south. In many places, thesteep sides of the Himalayan valleys exist always in a criticalcondition of repose, and the effect of the Indian earthquake was suchthat all along the north side of the Brahmaputra valley, the range isscarred by landslips, even to the east of Tezpur. Again, along the southern edge of the Garo and Khasi hills, landslipswere unusually prevalent. "Viewed from the deck of a steamer sailingup to Sylhet, " says Mr. Oldham, "the southern face of these hillspresented a striking scene. The high sandstone hills facing the plainsof western Sylhet, usually forest-clad from crest to foot, werestripped bare, and the white sandstone shone clear in the sun, in anapparently unbroken stretch of about 20 miles in length from east towest. " At Cherrapunji, also, the deep valleys were so scored that, from a distance, there appeared to be more landslip than untouchedhillside. But in no part, probably, were landslips more strikingly developedthan in the small valley of the Mahádeo, which forms an amphitheatreabout four miles long from east to west, and a mile and a half across, lying to the south of the Bálpakrám and Pundengru hills. "Here, "remarks Mr. Oldham, "everything combined to favour the formation oflandslips. The hills were composed of soft sandstone, they weresteep-sided, high, and narrow from side to side, and consequently weredoubtless thrown into actual oscillation as a whole; while the rangeof motion of the wave particle was not less than eight inches near theedge of the precipices. The result ... Has been to produce anindescribable scene of desolation. Everywhere the hillsides facing thevalley have been stripped bare from crest to base, and the seams ofcoal and partings of shale could be seen running in and out of theirregularities of the cliffs with a sharpness and distinctness whichrecalled the pictures of the cañons of Colorado. At the bottom of thevalley was a piled-up heap of _débris_ and broken trees, while the oldstream had been obliterated and the stream could be seen flowing overa sandy bed, which must have been raised many feet above the level ofthe old watercourse. " In the sandstone districts of the area here considered, the landslipshad some important secondary effects. Along the southern edge of theGaro and Khasi hills, great sand-fans spread over the fields, and theexposure of the hillsides formerly protected by forest left free scopefor future denudation. Every stream of any size has in this waydevastated many square miles of country. Among the hills themselves, more sand was brought down than the streams could carry away, andeverywhere their beds were raised. "Ordinarily, the beds of theserivers, which are raging torrents when in flood, consist of asuccession of deep pools separated by rocky rapids. After the rains of1897, it was found that the pools had been filled up, and the rapidsobliterated by a great deposit of sand, over which the rivers flowedin a broad and shallow stream. " A few valleys were for a short time barred across by landslips. Inone, on the northern foot of the Garo hills, a landslip crossed thedrainage channel and formed a shallow pond, which was not filled up bysand until the end of January 1898. Near Sinya, in the northern Khasihills, an unusually large landslip formed a barrier, of which theremains are more than 200 feet above the level of the river-bed. Behind this, the water accumulated in a great lake until the beginningof September 1897, when the barrier burst and a flood of water rusheddown the valley. ROTATION OF PILLARS, ETC. A curious effect of earthquakes strong enough to damage buildings isthat pillars, monuments, etc. , may be fractured and the upper partrotated over the lower without being overthrown. Even in Hereford andthe surrounding villages, several pinnacles and chimney-stacks weretwisted by the earthquake of 1896. The interest of the phenomenon, which has been known, since 1755, [74] is mainly historical, for theendeavour to discover its cause was the origin of Mallet's views onthe dynamics of earthquakes. Partly, also, it lies in the difficultyof finding a satisfactory explanation, or rather in deciding which ofthree or four possible explanations is the true one in any particularcase. [Illustration: FIG. 74. --Twisting of monument at Chhatak. (_Oldham. _)] The Indian earthquake offered exceptional opportunities for studyingthe phenomenon in the large number of examples observed and thevariety of objects rotated. None could be more striking than thetwisted monument to George Inglis, represented in outline in Fig. 74. Chhatak, where this is situated, lies close to the southern boundaryof the epicentral area. The monument is an obelisk, built of broadflat bricks or tiles on a base of 12 feet square, and originally morethan 60 feet high. It was split by the earthquake into four portions. The two upper, about six and nine feet long, were thrown down; whilethe third, 22 feet high, remains standing, but is twisted through anangle of 30° with respect to the lowest part, which is unmoved. Theupper of these two parts had evidently rocked on the lower, as thecorners and edges were splintered, and below the fracture a slice ofmasonry about 15 inches thick, which was not bonded into the mainmass, was split off by the pressure on its upper end. The plan of theparts still standing is shown in the lower part of Fig. 74. The possible explanations of the phenomenon are at least three innumber. According to the first, which was given by Mallet in 1846, theadhesion of the twisted portion to its base is not uniform, and theresultant resistance to motion is not in the same vertical plane asthe wave-movement. [75] Some years later, Mallet offered anotherexplanation. The body, he imagined, might be tilted on one edge by theearthquake, and, while still rocking, a second shock oblique to thefirst might twist it about that edge. [76] In 1880, Professor T. Graysuggested that the column might be tilted on one corner and thentwisted round it by later vibrations of the same shock. [77] None of these theories, Mr. Oldham argues, can give by itself acomplete explanation of the phenomena observed in the central districtof the Indian earthquake; and he therefore favours an extension of thesecond theory, which, though first proposed in 1882, [78] was thoughtout independently and in greater detail by himself. When the focus isof considerable dimensions, the shock at neighbouring places isconstantly varying in direction, owing to the arrival of vibrationsfrom different parts of the focus. Thus, instead of the two separateshocks required by Mallet's second explanation, we have a number ofclosely successive impulses frequently changing in direction andgiving rise to what is known in the South of Europe as a vorticoseshock. And, instead of a single twist of the pillars about one centreonly, we have a series of small twists round a number of differentcentres, accompanied in consequence by a much smaller displacement ofthe centre of gravity than would have occurred had the same rotationbeen accomplished in one operation. The theory, it will be seen, accounts for the twisting of the pillarwithout overthrow, and for the splintering of the edges during therocking of the column. It explains why in any district a number ofsimilarly placed objects are generally twisted in the same direction. Moreover, a low column rocks to and fro more rapidly than a tall onesimilar in form and position, so that, at the instant when a laterimpulse comes from a different direction, two such columns mighthappen to be tilted on opposite edges, and would then be twisted inopposite directions. In certain cases, then, as occurred at severalplaces during the Indian earthquake, an object may rotate in onedirection, while others, similar in every respect but size, may betwisted in the opposite direction. AFTER-SHOCKS. _Frequency of After-Shocks. _--For some days after the greatearthquake, the after-shocks by their very frequency and by their widedistribution baffled close inquiry. During the first 24 hours, hundreds were felt at all points of the epicentral area; indeed, it isnot too much to say that for several days the ground was neveractually at rest. At the Bordwar tea-estate, which is traversed by oneof the great fractures to be described in the next section, thesurface of a glass of water on a table was for a whole week in aconstant state of tremor; and at Tura a hanging lamp was keptcontinually swinging for the first three or four days. Most of these shocks were, of course, very slight; but, interspersedamong them, were others of greater strength, and a few of considerableviolence. One, on June 13th, about eight hours after the earthquake, was sensible beyond Allahabad--that is, for more than 520 miles fromthe epicentre; and another on the same day was felt in Calcutta, distant 255 miles. On June 14th, 22nd, and 29th, and again on August2nd and October 9th, shocks were noticed in that city; but, after thelatter date, the disturbed area of no shock reached to so great adistance. To form any estimate of the total number of after-shocks isimpossible, even for any one station. At first, lists were kept atisolated places, such as Shillong, Maimansingh, Dhubri, and a fewothers. Then, from July 15th, through Mr. Oldham's efforts, therecords became more numerous until the end of the year, after whichinterest in the subject declined. Mr. Oldham's catalogue closes withthe year 1898; but the register of a roughly-constructed seismograph, erected at Shillong in July 1897, continues to the present day. Imperfect as all non-instrumental registers must be, they neverthelessfurnish some idea of the frequency of the after-shocks. Thus, untilthe end of June, 679 shocks were recorded at Rangmahal (NorthGauhati), 135 at Maimansingh, 89 at Kuch Bihar, and 83 at Kaunia(omitting those on June 12th). Again, from August 1st to 15th, 182were felt at Goalpara, 151 at Darangiri, 124 at Tura, 105 at Bijni, 94at Lakhipur, 94 at Krishnai, 48 at Dhubri, 28 at Rangpur, and 12 atKuch Bihar; while at Borpeta, 113 shocks were reported during thefirst nine days of August. Turning to the registers of longerduration, we find that at Maophlang (near Shillong) 1, 194 shocks werefelt by one observer from September 12th, 1897, to October 7th, 1898;at the neighbouring station of Mairang, 1, 065 from September 7th, 1897, to December 31st, 1898; and at Tura, in the Garo hills, 1, 145shocks from July 21st, 1897, to December 31st, 1898. The total numberof earthquakes registered by the seismograph at Shillong from August1897 to the end of 1901 amounts to 1, 274, and all of these wereprobably strong enough to arouse the observer from sleep. Outside theepicentral area, Mr. Oldham's list includes 88 shocks from June 12thto July 15th, about 950 from July 16th to December 31st (the periodwhen the after-shocks were most carefully observed), and 296 shocksduring the year 1898. _Geographical Distribution of After-Shocks. _--When we endeavour tocompare the lists of after-shocks at different places, we are at oncemet by two serious difficulties, --the imperfection of the records andthe approximate character of the times of occurrence. Making everyallowance, however, for these deficiencies, it is evident that veryfew of the shocks felt at any one station were perceptible at itsneighbours; in other words, that the shocks originated in a largenumber of foci scattered over a very wide area. For instance, two of the most carefully kept registers of after-shocksare those compiled at Maophlang (near Shillong), and at Mairang, only11 miles to the north-west. Now, between September 12th and September28th, 1897 (both dates inclusive), 92 shocks were felt at Maophlangand 83 at Mairang. Of the former, 37 were described as smart, 45slight, and 10 feeble; of the latter, 6 as smart, 9 slight, 65 feeble, and 3 very feeble. But, of the total number, only 20 were felt at bothplaces at recorded times that were not more than fifteen minutesapart; 13 being described as smart--one at both places, one at Mairangalone, and the remaining 11 at Maophlang alone. When shocks occur sofrequently, as in these cases, it is inevitable that, even if all wereindependent, some should coincide approximately in time of occurrence. It is therefore probable that only one in every eight shocks, andpossibly only one in every twelve, was felt at both places. The actual numbers of shocks felt within stated periods at differentplaces are perhaps hardly comparable, owing to the obviousimperfection of the records and the probably varying standards adoptedby the reporters. But there can be little doubt that certain districtswere more subject to after-shocks than others, especially such placesas North Guahati, Shillong, and neighbouring villages, Tura, Darangiri, Goalpara, Bijni, Borpeta, Kaunia, and Rangpur. On the otherhand, they seem to have been unusually scarce at Dhubri and in thedistrict to the north-west, and they became rare at Gauhati longbefore they ceased to be frequent at Borpeta. In the plain to thesouth of the Garo and Khasi hills, they were also uncommon, thecombined records for Sylhet and Sonamganj for August 1-15 giving only20 shocks, and, neither to the east nor to the west of these places, is there any sign of greater frequency. _Sound-Phenomena of After-Shocks. _--Many of the after-shocks wereaccompanied by sound, or else consisted of sound-vibrations only; andMr. Oldham notices that such sounds were equally frequent both on therocky ground of the hills and on alluvial plains nearly all the shocksthat originated under the Borpeta plain being attended by distinctlyaudible rumblings. During his tour in the epicentral area in the winter of 1897-98, Mr. Oldham had many opportunities for observing these earth-sounds. Theywere, he says, close to the lower limit of audibility, less a notethan a rumble, and very like distant thunder, though sometimes theyconsisted of a rapid succession of short sounds, such as is caused bya cart when driven rapidly over a rough pavement. "As a rule, theybegan as a low, almost inaudible rumble, gradually increasing inloudness, though to a very varying degree, and then gradually dyingout after having lasted anything from 5 to 50 seconds. It cannot besaid that there was any connection between the duration and theloudness of the sounds, some of the most prolonged never becomingloud, and some of those which lasted a shorter period being as loud asordinary thunder at a distance of two or three miles. " Mr. Oldham records an interesting fact in connection with thedistribution of the earth-sounds. At Naphak, in the Garo hills andabout five miles south of Samin, 48 distinct rumbles were heard during23 hours on January 21-23, 1898, only seven of them being accompaniedby a perceptible shock. At Samin, which was visited next, they weremuch less frequent, not more than 8 or 10 a day, and most of themattended by tremors. At Damra, a few miles to the north-east, theyagain became frequent; while, in the Chedrang valley, very few wereheard, and only a small proportion of them were unaccompanied bysensible shocks. In the next section, it will be seen that the mostconspicuous fault-scarps known in the epicentral area pass close bySamin and along the Chedrang valley. Thus, though the statementperhaps requires further confirmation, it would appear thatearth-sounds were more common where the surface of the ground had beenmerely bent than where fractures extended right up to the surface. STRUCTURAL CHANGES IN THE EPICENTRAL AREA. We come now to the important features which assign the Indianearthquake to a small class apart from nearly every other shock. Mostearthquakes are due to movements that are entirely deep-seated. Ifstrong enough, they may precipitate landslips or fissure the alluvialsoil near river-channels. In the Neapolitan, Andalusian, andCharleston earthquakes, there were many such effects of the shockwithin the meizoseismal areas. In all three, however, the disturbancesproduced were superficial; no structural change, no fissuring that didnot die out rapidly downwards, was in any place perceptible. In theRiviera earthquake, the seismic sea-waves point to a smalldisplacement of the ocean-bed; but it is only in the long fault-scarpof the central Japanese plain that we find a rival of themountain-making movements that gave rise to the Indian earthquake. The boundary of the epicentral area, to the growth of which thesedistortions contributed, is represented by the curve marked A in Fig. 68, and on a larger scale by the continuous line A in Fig. 75. A greatpart of the district is occupied by a group of hills known by variousnames locally, but which are conveniently included under the generalterm of the Assam range. To avoid the confusion of hill-shading, onlythe boundary of the range is indicated (by the broken line) in the mapin Fig. 75. The Garo hills form the western part, and the Khasi andJaintia hills the central and western parts, of the range as theredepicted. They are formed chiefly of crystalline gneissic and graniticrocks and some metamorphic schists and quarzite, with cretaceous andtertiary rocks of varying thickness along its southern edge. Three stages have been distinguished in the history of the range. During the earliest, an old land-surface was worn down by rain andrivers till they were almost incapable of producing any furtherchange. Traces of this surface are still visible in the plateaucharacter of the mass. It was then elevated, not uniformly, but alonga series of faults, so that it now consists of a succession of ranges, the face of each range being a fault-scarp, and its crest the edge ofan adjoining plateau sloping away from the summit. With this elevationbegan the third and last stage. The streams were able to work again, and deep gorges were cut out of the range, so that in parts itsoriginal character was nearly effaced. But the retention of thatcharacter in other districts is of course evidence of thecomparatively recent date of the final elevation. [Illustration: FIG. 75. --Epicentral Area of Indian Earthquake. (_Oldham. _)] Owing to the great size of the epicentre and to the thickness of theforests which cover so much of its area, a comparatively small part ofit could be traversed by Mr. Oldham during his tour in the winter of1897-98. The positions of the more important structural changes areindicated in Fig. 75. Of these, the fault-scarps are represented bycontinuous straight lines, the Bordwar fracture by the dotted straightline, pools and lakes not due to faulting by black ovals, reportedchanges in the aspects of the hills by circles, and the principalstations of the revised trigonometrical survey by crosses. _Fault-Scarps. _--The most important fault-scarp is that called by Mr. Oldham the Chedrang fault, after the stream which coincides roughlywith a great part of its course. The longer straight line in Fig. 75represents its position and general direction, and the sketch-map inFig. 76 gives the plan of its southern half. From these, it will beseen that the fault follows on the whole a nearly straight path fromsouth-south-east to north-north-west for not less than twelve miles, and that its throw, as indicated by the numbers to the right in Fig. 76; is very variable, being zero in some places, and in one as much as35 feet or more. The upthrow is uniformly on the eastern side of thefault. At its southern end, as mapped in Fig. 76, there is no perceptiblethrow at the surface, but various marks of violence are manifested inthe fissuring of the hillside and the snapping of small trees. About aquarter of a mile from this point, the fault crosses a tributarystream, where the throw amounts to two feet, and the same distancefarther on it meets the Chedrang river, the bed of which it crossesmany times in its short course. [Illustration: FIG. 76. --Plan of Chedrang fault. (_Oldham. _)] Mr. Oldham describes the fault in detail, as observed by him inFebruary 1898. Here, it will be sufficient to refer to its moreimportant features, and to its effects on the superficial drainage ofthe district. At the spot marked _a_ (Fig. 76) the river, afterrunning on the west or down-throw side of the fault for nearly half amile, meets the scarp, and is ponded back by it for about a quarter ofa mile upstream. For the next half-mile, the river keeps to theupthrow side of the fault, the scarp of which blocks the tributarystreams from the west, forming a number of small pools. At the last ofthese, the total throw is not less than 25 feet. A little farther on, the fault crosses the Chedrang and causes the waterfall at _b_, theheight of which, owing to the fall of dislodged fragments, does notexceed nine feet. The fault then runs along the old and now dry bed ofthe river, while the stream itself flows in a depression on thedown-throw side. About a quarter of a mile below the waterfall, thefault crosses the river, and soon after enters a large sheet of waterat _c_, half a mile long, from 300 to 400 yards wide, and with amaximum depth of 18 feet. At first, the pool spreads on both sides ofthe fault, but the inequalities due to the scarp are evidenced bysoundings. At the point where the fault leaves the pool, its throw isreduced to nothing, and it is just here that the water attains itsgreatest depth. To the north the throw increases rather rapidly, to 25feet in a quarter of a mile. But the peculiarity of this pool is thatit is not, like the others mentioned above, dammed back by thefault-scarp. There is no barrier at its northern end, where the riverescapes, except that formed by the gradually increasing throw of thefault. The pool is simply due to the reversal of the natural slope ofthe river-bed, caused by the formation of a roll or undulation in theground on the upthrow side of the fault. Its recent origin is evidentfrom the number of dead trees and bamboo clumps still standing in thewater. For a mile after the fault leaves the pool, its throw variesconsiderably. It rises, as already mentioned, from zero to 25 feet. Alittle farther on, the fault runs up the side of a spur, the throwincreasing to 31 feet; and, in this part, the violence of the shockwas shown by the dislodgment of blocks of granite as much as 20 feetin diameter, and by the overthrow or destruction of many trees. Aftercrossing the spur, the fault returns to the neighbourhood of theriver, and crosses its bed four times, forming pools (_e_, _g_) orwaterfalls (_d_, _f_) according as the scarp occurs on the downstreamor upstream side. The throw of the fault then changes considerablywithin little more than half a mile, from 18 feet to zero and again to20 feet, the undulation so formed producing a large pool (_h_)entirely on the upthrow side of the fault. At the point marked _i_ on the map, the river once more crosses thefault; but the bottom of the valley is filled with alluvium, and, instead of a waterfall, a large sandy delta spreads down the stream. The scarp is, however, readily traced on the east side of the river, athrow of 32 feet being measured. After this, the alluvium becomes ofconsiderable thickness, and the continuation of the fault is marked bya short slope, which tilts over the trees when it traversesforest-land. Leaving the valley of the Chedrang, the fault crosses anopen plain, and is followed with some difficulty to the neighbourhoodof Jhira, where, owing to the thick bed of alluvium, it forms a gentleroll or undulation of the surface, crossing the main channel of theKrishnai to the north-east of Jhira. On the west side of this barriera large sheet of water, a mile and a half in length, three-quarters ofa mile wide, and 12 feet in depth, gathered over the village of Jhira. "On the east side of the Jhira lake, " says Mr. Oldham, "there is ampleevidence of change of level, for part of the dry land was formerly ... Perpetually under water, and at one place the remains of an oldirrigation channel can be seen.... At the northern end of the lake thedrainage now makes its escape in a broad and shallow sheet of waterover what was once high land covered with _sal_ forest. " This is the last marked feature due to the Chedrang fault. Beyond thenorth of Jhira the throw rapidly diminishes, and perhaps dies outaltogether before reaching the low hills lying to the north of thatvillage. In several ways, this fault-scarp differs from that formed with theJapanese earthquake of 1891. Throughout its course the down-throw, wherever it is perceptible, is invariably to the west; in no placecould any trace of horizontal shifting be detected; and the plane ofthe fault, when it traversed rock, is practically vertical. Whether the scarp was formed by the elevation of the rock to the eastof the fault, or by the depression of that to the west, or by bothsuch movements at once, there is no decisive evidence; but there arevery good reasons for believing the first alternative to be the trueone. The undulations in the ground which gave rise to the large poolsat _c_ and _h_ (Fig. 76) occur on the east side of the fault. Also, between the outlet of the lake at Jhira and the point where theKrishnai rejoins its original channel, the gradient of the riverapproaches that of a mountain stream, although the new bed consists ofalluvium, and not of rock. Now, the alluvial plain of this district israised so slightly above the sea-level that no subsidence great enoughto have caused the existing gradient could have occurred without thedepressed area being flooded with water. Though some movements mayhave taken place on the west side of the fault, it seems clear, then, that elevation of the rock on the east side was the predominant, ifnot the sole, cause of the fault-scarp. As the Chedrang fault has been described somewhat fully, a briefreference to the rest will be sufficient The only other known scarp ofany consequence lies about ten miles to the south of the Chedrangfault, and runs by the village of Samin, with an average course fromE. 30° S. To W. 30° N. Its total length does not exceed 2-1/2 miles. The down-throw is uniformly to the north, and the throw, whichamounts to ten feet near its centre, gradually diminishes to zero ateither end. Several pools are formed along the course of thefault-scarp by the blocking of small streams. _The Bordwar Fracture. _--In the map of the epicentral area (Fig. 75), this remarkable fracture is represented by a dotted straight line. Itis apparently an incipient fault. Though traceable for a distance ofabout seven miles, at no point is there any decisive evidence ofeither vertical or horizontal displacement; and, even if some doubtfulindications of a change of level should be real, the throw mustcertainly be less than one foot. Yet, in the immediate neighbourhoodof the fracture, the violence of the shock was extreme. "Trees havebeen overthrown or killed as they stood; a huge mass of rock, dislodged from near the crest of the hills, has rolled down the slope, scoring the side of the hill. On the opposite side an equally largeblock has been dislodged, and in its downward course cleared astraight track down the hill; and on the summit a gap has been clearedby the overthrow of trees along the line of fracture. " Being only afew inches in width where it has rent the solid rock, the fracture wasdifficult to follow in many parts of its course. But, throughforest-clad land, its track was marked by "a well-defined band ofabout half a mile broad, in which overturned trees are much moreabundant than on either side, and towards the centre of this band theoverturned trees are not only more numerous, but many of the smallerones, up to six inches in diameter, have been snapped across by theviolence of the shock. " _Lakes and Pools not due to Faulting. _--A few miles to the south ofthe Chedrang and Samin faults, and also of the Bordwar fracture, occurs a group of lakes or pools, represented on the map of theepicentral area (Fig. 75) by small black ovals. In the gradualincrease in depth from either end, they resemble the two large sheetsof water along the course of the Chedrang fault (_c_ and _h_, Fig. 76), but they differ from them in having no direct connection with anyapparent fault. One of these pools lies in the valley of the Rongtham river, to thesouth of the Samin fault. It seemed, at first sight, to be nothingmore than an ordinary pool, such as may be seen on any mountainstream. On the bottom, and close to the outlet, however, are coarse, partially rounded boulders, exactly resembling those farther down theriver; and, as the old bed was followed up, these became coated with aslight deposit of sand and mud, pointing clearly to a change in theconditions under which they were formed. The water gradually deepened, until trees were met standing in the water, but killed by the recentsubmergence of their roots. The pool is nearly a quarter of a milelong, and its greatest depth (12 feet) occurs near the middle, justwhere the former stream, with an average depth of about a foot, wascrossed by the track from Darangiri. Towards the upper end, the watershallows as gradually as it deepens at the other, and ends in a deltaof boulders brought down by the stream above. As no fault could bediscovered in the neighbourhood of the pool, it is evident that itsformation was due to a bend of the river-bed, the maximum change oflevel, taking into account the river-slope, being not less than 24feet. Similar features characterise the other pools that were examined, someof which are smaller, and others larger, than that described above. One, higher up the valley of the Rongtham, has a length of about 1-1/2mile and a maximum depth of 18 feet. Others of the same type, but ofsmaller size, were observed among the Khasi hills, about fifteen milessouth of the Bordwar fissure; and it is probable that many otherswould have been found in the intermediate district, which Mr. Oldhamwas unable to visit. _Changes in the Aspects of the Hills. _--There are, again, other factsof considerable interest which point to changes of level over a widearea; the places where they were noticed being indicated by smallcircles in Fig. 75. For instance, from Maophlang, near Shillong, aroad leads to the neighbouring station of Mairang. Before theearthquake, only a short stretch of this road could be seen from theformer place, as it rounded a spur about three miles away. Now, a muchlonger stretch is visible, and it can also be seen passing round thenext, and previously hidden, spur. In this district the movements seemto have continued with the after-shocks; for, before the earthquake, the crest only of a ridge about a mile and a half to the west wasvisible; while, after it, a considerable portion could be seen, andmuch more some months later than immediately after the shock. Again, from a spot near the southern end of the Chedrang fault, itused to be only just possible to see the Brahmaputra over anintervening hill; whereas, now, the whole width of the river has comeinto view. Lastly, at Tura, which is 95 miles west of Maophlang, a battalion ofmilitary police were accustomed to signal by heliograph with anotherstation, Rowmari, 15 miles farther to the west. This, formerly, couldjust be done by means of a ray which grazed a hill between the twoplaces; it can now be done quite easily, and, in addition, a broadstretch of the plains east of the Brahmaputra is visible from the samespot. _Revision of the Trigonometrical Survey. _--The movements described inthe preceding pages are of course referred to points which maythemselves have been displaced, and only a revision of thetrigonometrical survey of the epicentral area and of part of thesurrounding district could determine their absolute magnitude. Duringthe cold weather of 1897-98, some of the triangles were re-measured bya member of the trigonometrical survey; but, as the time at hisdisposal was short, they were confined to the eastern part of theepicentral area, as the focus at that time was supposed to lie underthe Khasi hills. The positions of some of these stations are indicatedby crosses in Fig. 75; and in Fig. 77 the more important triangles areshown. In the revised work, all tower stations, consisting of bricktowers built on alluvium, were omitted, as it could not be assumedthat they had been undisturbed by displacements of the superficialbeds. In re-calculating the lengths of the sides, the sideRangsanobo-Taramun Tila was adopted as the initial base, and theheight of Rangsanobo as the initial height; a choice which laterexperience showed to be unfortunate, for Taramun Tila probably liesjust outside, and Rangsanobo within, the epicentral area. Of the 16sides, whose old and new lengths were compared, only one was found tobe apparently unchanged, two were shortened by an inch or two, whilethe others were all lengthened by amounts varying from one to eightor nine feet, the numbers affixed to the sides in Fig. 77 denoting thecalculated increases in feet. Assuming the new base-line to beunaltered by the earthquake movements, these changes imply thefollowing displacements of the principal stations:--Thanjinath 6 feet, Mun 4, and Laidera 2, feet to the north; Mopen 5, Dinghei 9, LandauModo 12, and Umter 11, feet to the north-west; and Mosingi 3, andMautherrican 5, feet to the west. At the same time, the height ofmost of the stations was found to be increased with reference to thatof Rangsanobo: Mun by 2 feet, Thanjinath and Umter by 3, Mosingi by 4, Taramun Tila and Laidera by 6, Dinghei by 7, Landau Modo by 17, andMautherrican by 24, feet; while the height of Mopen seems to have beendiminished by 4 feet. Thus, at first sight, these calculations appearto indicate "a general elevation and extension of the hills, such asmight follow on a bulging upwards of the surface due to the extensionof a large mass of molten matter underground. " [Illustration: FIG. 77. --Re-triangulation of Khasi hills. (_Oldham. _)] Unfortunately, as Mr. Oldham shows, a very different, and moreprobable, interpretation may be given of these results; for all thecalculated changes are rendered uncertain by the choice of the twostations which form the ends of the new base-line. One at least mayhave been displaced by the structural movements within the epicentralarea; and, moreover, the line joining them runs nearly north andsouth. As compression in this direction is to be expected, it isprobable that this line was shortened; and the assumption that itslength was unchanged would therefore lead to an apparent expansion ofall the other sides. The calculated changes seem to favour this explanation to a greatextent. The sides joining Mopen, Rangsanobo, and Thanjinath run nearlyeast and west, and are apparently lengthened by 4. 9 and 3. 4 feetrespectively; while, of the four sides joining these stations toMosingi and Mun, lying next to the north, two are nearly or quiteunchanged, and the others increased by 2. 3 and 3. 2 feet. Again, theestimated increase of the Mosingi-Mun line is 4. 4 feet; while the foursides joining these stations to the next northerly group areincreased by small amounts--namely, 1. 2, 2. 6, -0. 3, and 2. 4 feet. Thus, the apparent expansion that should have occurred in these more or lessnortherly sides is lessened, or roughly compensated, probably by acompression of the whole region in a meridianal direction. For a similar reason, the slight general upheaval of the hillsindicated by the repeated calculations, must be regarded as doubtful, for it depends on the assumed fixity of the station of Rangsanobo, whereas it is more probable that it was the height of Taramun Tilathat remained unchanged. Reducing the calculated heights of all theother stations by six feet (the assumed rise of the latter), itfollows that, on the whole, the height of the Khasi hills underwentbut little change, except at Mautherrican and Landau Modo, and thesecondary stations of Mairang and Kollong Rock, near Maonoi. Theapparent elevations of 24, 17, 11, and 15 feet at these places exceedthe probable error of the observations; and it is worthy of noticethat all four stations lie close to the edge of fault-scarps, whileLandau Modo is not far from two of the pools formed by distortion ofthe surface unaccompanied by faulting. If, then, the revised triangulation of the Khasi hills has failed toprovide absolute measures of the displacements in the epicentral area, it has, nevertheless, proved that important movements, both horizontaland vertical, have taken place. _Distribution of the Structural Changes. _--The boundary of theepicentral area, as drawn in Figs. 68 and 75, lays no claim to greataccuracy; but its departure from the true line is probably in no placeconsiderable. It must evidently include all the districts wheremarked structural changes occurred, and must therefore extend east ofMaophlang and west of Tura. Towards the north, these changes have beentraced to the foot of the Garo hills, and there is some, though notvery certain, evidence of alterations of level along the course of theBrahmaputra. The very large number of after-shocks recorded at Borpetaand Bijni also points to an extension of the epicentral area beyondthese places. To the east, the course of the boundary becomesdoubtful, but it must pass close to Gauhati and east of Shillong, andprobably ends a short distance beyond Jaintiapur. The southernboundary must coincide nearly with the north edge of the alluvialplains of Sylhet, for there is no evidence of its intrusion into theplains. On the west side, the epicentral area includes the Garo hillsand part of the alluvial plain to the west; and, from the large numberof after-shocks felt at Rangpur and Kaunia, and the great violence ofthe shock at the former, we may infer that both places lie within theboundary-line. If, then, there is no great error in the mapping ofthis line, it follows that the epicentre was about 200 miles long fromeast to west, not less than 50, and possibly as much as 100, miles inmaximum width, and contained an area of at least 6000 square miles. Near the boundary, the permanent displacements must have beencomparatively small; but they were certainly marked in the northernpart of the Assam hills for a distance of 100 miles from east to west. At the limits of the latter area, as Mr. Oldham remarks, "the evidencepoints to the changes being of the nature of long, low rolls, thechange of slope being insufficient to cause any appreciable change inthe drainage channels. Then comes a zone in which the surface changesare more abrupt, the slopes of the stream beds have been altered so asto cause conspicuous changes in the nature of the streams, but anyfracture or faulting which may have taken place has died out beforethe surface was reached. And north of this, close to the edge of thehills, the rocks have been fractured and faulted right up to thesurface. " ORIGIN OF THE EARTHQUAKE. Almost every feature of the great earthquake points to an origin verydifferent from that of the others described in this volume. Thesuddenness with which the shock began, its unusual duration, and theoccurrence of many maxima of intensity, are inconsistent with a simplefault-displacement. Again, the excessive velocities of projection atRambrai and elsewhere, the existence of isolated fault-scarps andfractures, the local changes of level, the compression indicated bythe revised trigonometrical survey, the wide area over which thesestructural changes took place, and the numerous distinct centres ofsubsequent activity, all these phenomena demonstrate the intense andcomplex character of the initial disturbances, as well as thewidespread bodily displacement of the earth's crust within theepicentral area. There may, it is conceivable, have been a number offoci, nearly or quite detached from one another, and giving rise to agroup of nearly concurrent shocks. Or--and this is a far more probablesupposition--there may have been one vast deep-seated centre, fromwhich off-shoots ran up towards the surface, each partaking to agreater or less degree in the movement within the parent focus. As Mr. Oldham points out, we have recently become acquainted with astructure exactly corresponding to that which is here inferred. Thegreat thrust-planes, so typically developed in the Scottish Highlands, are only reversed faults which are nearly horizontal instead of beinghighly inclined; and they are accompanied by a number of ordinaryreversed faults running upwards to the surface. In Fig. 78, the mainfeatures of a section drawn by the Geological Survey of Scotland arereproduced; T, T, representing thrust planes, and _t_, _t_, minorthrusts or faults. A great movement along one of the mainthrust-planes would carry with it dependent slips along many of thesecondary planes. Direct effects of the former might be invisible atthe surface, except in the horizontal displacements that would berendered manifest by a renewed trigonometrical survey; whereas thelatter might or might not reach the surface, giving rise in the onecase to fissures and fault-scarps, in the other to local changes oflevel, and in both to regions of instability resulting in numerousafter-shocks. [Illustration: FIG. 78. --Diagram of Thrust-planes. ] The enormous dimensions of the parent focus will be obvious from thephenomena that have been described above. Mr. Oldham has traced theprobable form of the epicentre. It may in reality be neither sosimple nor so symmetrical as is represented in Fig. 75, but there aregood reasons for thinking that it does not differ sensibly either insize or form from that laid down. The part of the thrust-plane overwhich movement took place must therefore have been about 200 mileslong, not less than 50 miles wide, and between 6000 and 7000 squaremiles in area. With regard to its depth, we have no decisiveknowledge. It may have been about five miles or less; it can hardlyhave been much greater. It is a strain on the imagination to try and picture the displacementof so huge a mass. We may think, if we will, of a slice of rock threeor four miles in thickness and large enough to reach from Dover toExeter in one direction and from London to Brighton in the other; notslipping intermittently in different places, but giving way almostinstantaneously throughout its whole extent; crushing all before it, both solid rock and earthy ground alike; and, whether by the suddenspring of the entire mass or by the jar of its hurtling fragments, shattering the strongest work of human hands as easily as thefrailest. Such a thrust might well be sensible over half a continent, and give rise to undulations which, unseen and unfelt, might wendtheir way around the globe. REFERENCES. 1. AGAMENNONE, G. --"Notizie sui terremoti osservati in Italia durante l'anno 1897 (Terremoto dell' India poco dopo il mezzogiorno del 12 giugno). " _Ital. Sismol. Soc. Boll. _, vol. Iii. , pte. Ii. , 1897, pp. 249-293. 2. ---- "Il terremoto dell' India del 12 giugno 1897. " _Ibid. _, vol. Iv. , 1898, pp. 33-40. 3. ---- "Eco in Europa del terremoto indiano del 12 giugno 1897. " _Ibid. _, vol. Iv. , 1898, pp. 41-67. (See also the same volume, pp. 167-172. ) 4. BARATTA, M. --"Il grande terremoto indiano del 12 giugno 1897. " _Ital. Soc. Geogr. Boll. _, vol. X. , 1897, fasc. Viii. 5. CANCANI, A. --"I pendoli orizzontali del R. Osservatorio geodinamico di Rocca di Papa, ed il terremoto indiano del 12 giugno 1897. " _Ital. Sismol. Soc. Boll. _, vol. Iii. , 1897, pp. 235-240. 6. HEATH, T. --"Note on the Calcutta Earthquake (June 12th, 1897) as recorded by the bifilar pendulum at the Edinburgh Royal Observatory. " _Edinb. Roy. Soc. Proc. _, 1897, pp. 481-488. 7. OLDHAM, R. D. --"Report on the Great Earthquake of 12th June 1897. " _Mems. Geol. Surv. Of India_, vol. Xxix. , 1899, pp. I. -xxx. , 1-379, with 44 plates and 3 maps. 8. ---- "List of After-shocks of the Great Earthquake of 12th June 1897. " _Ibid. _, vol. Xxx. , pt. I. , 1900, pp. 1-102. 9. ---- "On Tidal Periodicity in the Earthquakes of Assam. " _Journ. Asiat. Soc. _, vol. Lxxi. , 1902, pp. 139-153. FOOTNOTES: [69] According to some reports, the earthquake was felt in Italy. AtLivorno, the first movements were registered by seismographs at 11. 17A. M. (G. M. T. ), and tremors were noticed by some persons at rest atabout 11. 15 A. M. At Spinea, a sensible undulatory shock fromsouth-east to north-west, and lasting about four seconds, was felt atthe moment when all the seismographs were set in motion by the Indianearthquake. In spite of the great distance, the perception of theearthquake in Italy is not impossible, but the records seem to me torefer to local tremors rather than to the very slow evanescentoscillations of a very distant earthquake. [70] All the times in this section are referred to Madras mean time, which is 5h. 20m. 59. 2s. In advance of Greenwich mean time. In thenext section it will be found convenient to use the latter standard. [71] It may be useful to give references to works in English in whichthe principal instruments for registering distant earthquakes aredescribed. For Cancani's vertical pendulum, see _Brit. Assoc. Rep. _, 1896, pp. 46-47; Darwin's bifilar pendulum, _Brit. Assoc. Rep. _, 1893, pp. 291-303, and _Nature_, vol. 1. , 1894, pp. 246-249; Milne'shorizontal pendulum, _Seismology_, pp. 58-61; Rebeur-Paschwitz'shorizontal pendulum, _Brit. Assoc. Rep. _, 1893, pp. 303-308. [72] The beginnings of the second and third phases are shown moreclearly in the record of the vertical pendulum at Catania, a record, however, that will not bear the reduction necessary for these pages. [73] _Geol. Mag. _, vol. X. , 1893, pp. 356-360. [74] _Irish Acad. Trans. _, vol. Xxi, 1848, p. 52. [75] _Irish Acad. Trans. _, vol. Xxi. , 1848, pp. 55-57. [76] _Neapolitan Earthquake of 1857_, vol. I. , 1862, pp. 376-378. [77] _Japan Seismol. Soc. Trans. _, vol. I. , pt. II. , 1880, pp. 33-35. [78] _Geol. Mag. _, vol. Ix. , 1882, pp. 257-265. CHAPTER X. CONCLUSION. In this concluding chapter, I propose to give a summary of the resultsat which we have arrived from the study of recent earthquakes, andthis can, I think, be done best by describing what may be regarded asan average or typical earthquake, though it may be convenientoccasionally to depart slightly from such a course. Few shocks havecontributed more to our knowledge than the majority of those describedin this volume; but, on certain points, we gain additional informationfrom the investigation of other earthquakes, and these are referred towhen necessary for the purpose in view. FORE-SHOCKS. At the outset, we are met by a question of some interest and greatpractical importance--namely, whether there are any constant signs ofthe coming of great earthquakes by means of which their occurrencemight be predicted and their disastrous effects mitigated. Excluding the Ischian earthquakes, which belong to a special class, itis evident that there is generally some slight preparation for a greatearthquake. For a few hours or days beforehand, weak shocks andtremors are felt or rumbling noises heard within the futuremeizoseismal area. But, unfortunately, it has not yet been foundpossible to distinguish these disturbances from others of apparentlythe same character which occur alone, so that for the present theyfail to serve as warnings. In Japan, where the organisation of earthquake-studies is morecomplete than elsewhere, it is possible that a vague forecast might bemade, if the distribution of the fore-shocks of the earthquake of 1891should prove to be a general feature of all great earthquakes. It wasat first supposed that this earthquake occurred without preparation ofany kind; but a closer analysis of the records shows that during theprevious two years there was a very decided increase in the seismicactivity of the district, and also that the distribution of theepicentres marked out the future fault-scarp, and at the same timeexhibited a tendency to comparative uniformity over the wholefault-region. For the present, then, the only warning available is that given by thepreliminary sound, which may precede the strongest vibrations by asmuch as five or ten or even more seconds. Though two or three secondsmay elapse before its character is recognised, the fore-sound thusallows time for many persons to escape from their falling houses. Someraces, however, are less capable of hearing the sound than others, andthis may be one reason why Japanese earthquakes are so destructive ofhuman life. DISTURBED AREA. It is usual with some investigators to measure the intensity of anearthquake roughly by the extent of its disturbed area. The depth ofthe seismic focus must of course have some influence on the size ofthis area, and this condition is only neglected because we have noprecise knowledge of the depth in any case. Thus, Mr. Oldham regardsthe Indian earthquake of 1897 as rivalling the Lisbon earthquake of1755, which is generally considered to hold the first place, becauseits disturbed area was not certainly exceeded by that of the latter. That disturbed area is, however, an untrustworthy measure of intensitywill be evident from the following table, in which the earthquakesdescribed in this volume (omitting those of Ischia) are arranged asnearly as may be in order of intensity, beginning with thestrongest:-- Earthquake. Disturbed Area in Sq. Miles. Indian 1, 750, 000 Japanese 330, 000 Neapolitan 39, 200 Charleston 2, 800, 000 Riviera 219, 000 Andalusian 174, 000 Hereford 98, 000 Inverness 33, 000 Here we see that the Charleston earthquake was perceptible over agreater area than the Indian earthquake, while the Neapolitanearthquake was inferior to that of Hereford in this respect. Theexplanation of course is that the boundaries of the disturbed areasare isoseismal lines corresponding to different degrees of intensity, the inhabitants of Great Britain and the United States being evidentlymore sensitive to weak tremors, or more observant, than those ofItaly, Spain, or Central Asia. The only disturbed areas that arebounded by isoseismals of the same intensity are the two last. Veryroughly, then, we may say that the intensity of the Herefordearthquake was three times as great as that of the Invernessearthquake. POSITION OF THE EPICENTRE. One of the first objects in the investigation of an earthquake is todetermine the position and form of the epicentre. In a few rare cases, as in the Japanese and Indian earthquakes, when the fault-scarp isleft protruding at the surface, only careful mapping is required toascertain both data. But, in the great majority of earthquakes, thefault-slip dies out before reaching the surface and the position ofthe epicentre is then inferred by methods depending chiefly on thetime of occurrence or on the direction or intensity of the shock. At first sight, methods that involve the time of occurrence atdifferent places seem to be of considerable promise. No scientificinstruments are so widely diffused as clocks and watches; but, on theother hand, few are so carelessly adjusted. It is the exception, rather than the rule, to find a time-record accurate to the nearestminute; and, as small errors in the time may be of consequence, methods depending on this element of the earthquake are seldomemployed. If, however, the number of observations is large for thesize of the disturbed area, the construction of coseismal lines maydefine approximately the position of the epicentre. In the Herefordearthquake of 1896, the centre of the innermost coseismal line (Fig. 62) is close to the region lying between the two epicentres. The method of locating the epicentre by means of the intersection oftwo or more lines of direction of the shock was first suggested byMichell in 1760, [79] and has been employed by Mallet in investigatingthe Neapolitan earthquake, by Professors Taramelli and Mercalli intheir studies of the Andalusian and Riviera earthquakes, as well as byother seismologists. The diversity of apparent directions at one andthe same place caused its temporary neglect, until Professor Omorishowed in 1894 that the mean of a large number of measurements gives atrustworthy result (p. 19). His interesting observations shouldreinstate the method to its former place among the more valuableinstruments at the disposal of the seismologist. No observations, however, are at present so valuable for the purposein view as those made on the intensity of the shock. For many years, it has been the custom to regard the epicentre as coincident with thearea of greatest damage to buildings; and, when the area is small, theassumption cannot be much in error. It is of course merely a rough wayof obtaining a result that is generally given more accurately by meansof isoseismal lines; but there are exceptional cases, such as theNeapolitan and Ischian earthquakes, when the destruction wrought bythe earthquake furnishes evidence of the greater value. A single isoseismal accurately drawn not only gives the position ofthe epicentre with some approach to exactness, but also by thedirection of its longer axis determines that of the originating fault. When two or three such lines can be traced, the relative positionsupplies in addition the hade of the fault (p. 219). The successfulapplication of the method requires, it is true, a large number ofobservations, and these cannot as a rule be obtained except indistricts that are somewhat thickly and uniformly populated, such asthose surrounding the cities of Hereford and Inverness. In theCharleston earthquake, also, the position and form of the epicentreswere deduced from the trend of isoseismal lines based on the damage torailway-lines and various structures within a sparsely inhabitedmeizoseismal area. In a few cases, of which the Indian earthquake may be regarded astypical, a fourth method has recently been found of service. Thenumerous after-shocks which follow a great earthquake originate forthe most part within the seismic focus of the latter; and, as theyusually disturb a very small area, it is not difficult to ascertainapproximately the positions of their epicentres. Some, as in theInverness after-shocks of 1901, result from slips in the very marginof the principal focus; but, as a rule, the seat of their activitytends to contract towards a central region of the focus. Bearing inmind, then, that some of the succeeding shocks originate at and beyondthe confines of the focus, and that others may be sympathetic shocksprecipitated by the sudden change of stress, it follows that theshifting epicentres of the true after-shocks map out, in part at anyrate, the epicentral area of the principal earthquake. DEPTH OF THE SEISMIC FOCUS. It is much to be regretted that we have no satisfactory method ofdetermining so interesting an element as the depth of the seismicfocus. That it amounts to but a few miles at the most is certain fromthe limited areas within which slight shocks are felt or disastrousones exhibit their maximum effects. Nor can we suppose that the rocksat very great depths are capable of offering the prolonged resistanceand sudden collapse under stress that are necessary for the productionof an earthquake. The problem is evidently beyond our present powers of solution, andits interest is therefore mainly historical. All the known methods arevitiated by our ignorance of the refractive powers of the rockstraversed by the earth-waves. But, even if this ignorance could bereplaced by knowledge, most of the methods suggested are open toobjection. Falb's method, depending on the time-interval between theinitial epochs of the sound and shock, is of more than doubtful value. Dutton's, based on the rate of change of surface-intensity, isdifficult to apply, and in any case gives only an inferior limit tothe depth. Time-observations have been employed, especially in NewZealand; but the uncertainty in selecting throughout the same phase ofthe movement, and the large errors in the estimated depth resultingfrom small errors in the time-records, are at present most seriousobjections. There remains the method devised by Mallet, and, though heclaimed for it an exaggerated accuracy, it still, in my opinion, holdsthe field against all its successors. When carefully applied, as ithas been by Mallet himself, by Johnston-Lavis and Mercalli, weprobably obtain at least some conception of the depth of the seismicfocus. Professor Omori and Mr. K. Hirata have recently[80] lessened the chiefdifficulty in the application of Mallet's method. They have deducedthe angle of emergence from the vertical and horizontal components ofthe motion as registered by seismographs, instead of from theinclination of fissures in damaged walls. In two recent earthquakesrecorded at Miyako in Japan, they find the angle of emergence to be7. 2° and 9° respectively, the corresponding depths of the foci being5. 6 and 9. 3 miles. These are probably the most accurate estimates thatwe possess, and it will be noted that they differ little from the meanvalues obtained for the Neapolitan, Andalusian, and Rivieraearthquakes--namely, 6. 6, 7. 6, and 10. 8 miles. NATURE OF THE SHOCK In one respect, the earthquakes described above fail to represent theprogress of modern seismology. They furnish no diagrams made byaccurately constructed seismographs within their disturbed areas. Thecurve reproduced in Fig. 36, as already pointed out, is no exceptionto this statement. For another reason, the records that were obtainedin Japan of the earthquake of 1891 are trustworthy for little morethan the short-period initial vibrations; for, owing to the passage ofthe surface-waves, visible in and near the meizoseismal area, theJapanese seismographs registered the tilting of the ground rather thanthe elastic vibrations that traversed the earth's crust. Notwithstanding this defect, personal impressions of anearthquake-shock give a fairly accurate, if incomplete, idea of itsnature. Nearly all observers placed under favourable conditions agreethat an earthquake begins with a deep rumbling sound, accompanied, after the first second or two, by a faint tremor which gradually, andsometimes rapidly, increases in strength until it merges into theshock proper, which consists of several or many vibrations of largeramplitude and longer period, and during which the attendant sound isgenerally at its loudest; the earthquake dying away, as it began, withtremors and a low rumbling sound. [Illustration: FIG. 79. --Seismographic Record of Tokio Earthquake of 1894. (_Omori. _)] The vibrations that produce the sensible shock are by no means allthat are present during an earthquake. The Indian earthquake, forinstance, seemed to last about three or four minutes at Midnapur; butthe movements of the bubble of a level showed that the groundcontinued to oscillate for at least five minutes longer (p. 280). Manyof these unfelt waves are rendered manifest by seismographs, althoughthere are still others that elude registration either from the extremeshortness or the great length of their periods. In Fig. 79 is shown the principal part of a diagram obtained at Tokioduring the Japanese earthquake of June 20th, 1894 (p. 18), the curverepresenting the N. E. -S. W. Component of the horizontal motion duringthe first 25 seconds of the record. The instrument employed is onespecially designed for registering strong earthquakes, and isunaffected by very minute tremors. Those which formed the commencementof this earthquake lasted for about 10 seconds, as shown by ordinaryseismographs, and the vibrations had attained a range of a fewmillimetres before they affected the instrument in question. For thefirst 2-1/2 seconds, they occurred at the rate of four or five asecond. The motion then suddenly became violent, and the ground wasdisplaced 37 mm. In one direction, followed by a return movement of 73mm. , and this again by one of 42 mm. , the complete period of theoscillation being 1. 8 seconds. The succeeding vibrations were ofsmaller amplitude and generally of shorter period for a minute and ahalf, then dying out during the last three minutes as almostimperceptible waves with a period of two or more seconds. [81] Though incomplete in some respects, this diagram illustrates clearlythe division of the earthquake-motion into three stages--namely, thepreliminary tremors, the principal portion or most active part of anearthquake, and the end-portion or gradually evanescent slowundulations. In all three stages, however, both tremors and slowundulations may be present; and, as the latter, owing to their longperiod, are more or less insensible to human beings, the ripples ofthe final stage give the impression of a tremulous termination asdescribed above. The duration of each stage varies considerably indifferent earthquakes. Thus, in a valuable study of 27 earthquakesrecorded at Miyako, in Japan, during the years 1896-98, Messrs. Omoriand Hirata show[82] that the duration of the preliminary stage variesfrom 0 to 26 seconds, with an average of about 10 seconds; that of theprincipal portion from 0. 7 to 26 seconds, also with an average ofabout 10 seconds; and that of the end portion from 28 and 105 seconds, with an average of about one minute. The total apparent duration, however, depends on the instrument employed; one of the earthquakes, that of April 23rd, 1898, disturbing the seismograph at Miyako for twominutes; while, at Tokio, a horizontal pendulum designed by ProfessorOmori oscillated for at least two hours. The periods of both ripplesand slow undulations, again, vary from one earthquake to another; butit is worthy of notice that the average period of the undulations isalmost constant in all three stages of the motion, being 1. 1, 1. 3, and1. 3 seconds, respectively, for the east-west component of thehorizontal motion, and 1. 0 second throughout for the north-southcomponent. For the ripples, the average period is . 08 second in thepreliminary stage, . 10 second in the principal portion, and . 08 secondagain in the end portion; those of the principal portion beingslightly larger in amplitude, as well as longer in period, than theripples of the first and third stages. SOUND-PHENOMENA. Besides the ripples already mentioned, there are others of stillsmaller amplitude and shorter period that are sensible, but as a ruleonly just sensible, to us as sounds. All the known evidence points tothe extraordinary lowness of the earthquake-sound. According to someobservers, it seems as if close to their lower limit of audibility;while others, however intently they may listen, are unable to hear theslightest noise. In other words, the most rapid vibrations present inan earthquake do not recur at a rate of much more than about 30 to 50per second; or, if they do, they are not strong enough to impress thehuman ear. To most observers, the sound seems to increase and decrease inintensity with the shock, and so gradually and smoothly does thischange take place that the sound is frequently mistaken for that of anunderground train approaching the observer's house, passing beneathit, and receding in the opposite direction. Some persons, especiallyif situated within the meizoseismal area, hear also loud crashes inthe midst of the rumbling sound and simultaneously with the strongestvibrations. At a moderate distance, say from 30 to 40 miles, the soundbecomes more harsh and grating while the shock is felt; and, at agreater distance, even this change disappears, and nothing is heardbut an almost monotonous sound like the low roll of distant thunder. The explanation of this is that the sound-vibrations are of differentperiods and varying amplitude, and the limiting vibrations tend tobecome inaudible with increasing distance, the lower on account oftheir long period, the higher owing to their small amplitude. The magnitude of the sound-area depends, even more than that of thedisturbed area, on the personal equation of the observers. The lowerlimit of audibility varies not only in different individuals, but alsoin different races. In Great Britain, it is doubtful whether anearthquake ever occurs unaccompanied by sound; and in the meizoseismalarea the noise is heard by nearly all observers. With Italians, theaverage lower limit of audibility is higher than with the Anglo-Saxonrace; slight shocks frequently occur without noticeable sound, butwith strong ones, the larger number of observers is sure to includeone or more capable of hearing the rumbling noise. The Japanese are, however, seldom affected by the most rapid earthquake-vibrations, andthe strongest shocks may be unattended by any recorded sound. Theresult is manifest in the size of the sound-area in differentcountries. In the Hereford earthquake, the sound-area contained 70, 000square miles; in the Neapolitan earthquake, about 3, 300 square miles;while, in Japanese earthquakes, the sound is rarely heard more than afew miles from the epicentre. Another effect of this personal equation of the observers is that thesound-vibrations apparently outrace those of longer period. TheItalians, for instance, generally hear the sound that precedes theshock, and more rarely the weaker sound that follows it. In Japan, only the earlier sound-vibrations, if any, seem to be audible. InGreat Britain, on the contrary, the fore-sound is perceptible to four, and the after-sound to three, out of every five observers; and theseproportions are maintained roughly to considerable distances from theepicentre. It follows, therefore, that the sound-vibrations and thosewhich constitute the shock must travel with nearly, if not quite, thesame velocity; and that the greater duration of the sound is dueeither to the prolongation of the initial movement or to theoverlapping of the principal focus by the sound-focus. Neitheralternative can be regarded as improbable, but observations made onBritish earthquakes point to the latter explanation as the true one. It will be sufficient to refer to two phenomena in support of thisstatement. In the first place, the percentage of observers who hearthe fore-sound varies with the direction from the epicentre. Thus, during the Inverness earthquake of 1901, the majority of observers inAberdeenshire regarded the sound as beginning and ending with theshock; while, in counties lying more nearly along the course of thegreat fault, the sound was generally heard both before and after theshock (p. 253). In this case, then, the initial and concluding soundvibrations must have come chiefly from the margins of the seismicfocus; and those from the margin nearest to an observer would be moresensible than those from the farther margin. Again, in slightearthquakes, such as the Cornwall earthquake of April 1, 1898, [83] thecurves of equal sound intensity, while their axes are parallel tothose of the isoseismal lines, are displaced laterally with respect tothese curves, owing to the arrival of the strongest sound-vibrationsfrom the upper margin of an inclined seismic focus. When a fault-slip occurs, the displacement is obviously greatest inthe central region, and dies out gradually towards the margins of thefocus. The phenomena described above show that the evanescentdisplacement within these margins generate sound-vibrations only; andthat the greater slip within the central region produces also the moreimportant vibrations that compose the shock. As the former areperceptible over a limited district, while the latter may be feltthrough half a continent, it is clear that the sound-area should bearno fixed relation in point of size to the disturbed area, but shouldbe comparatively greater for a slight shock than for a strong one. VELOCITY OF THE EARTH-WAVES. If we consider only the earthquakes here described, we see at once howgreat is the diversity in the estimated velocity of the earth-waves. On the one hand, we have a value as high as 5. 2 kms. Per sec. For theCharleston earthquake, and, at the other end of the scale, a value of0. 9 km. Per sec. For the Hereford earthquake. Between them, andequally trustworthy, lie the estimates of 3. 0 km. Per sec. For theIndian earthquake, and 2. 1 kms. Per sec. For the Japanese earthquakeand its immediate successors. It is difficult to account entirely for such discordance. Errors ofobservation may be responsible for a small part of the differences. The initial strength of the disturbance appears to have some effect, and the nature of the rocks traversed must be a factor of consequencewhen the distances in question are not very great. In the Japanese andHereford earthquakes, all three may have combined to produce thedivergent results, the distance in these cases being only 275 and 142kms. Respectively. In the Indian and Charleston earthquakes, the distances are muchgreater (1944 and 1487 kms. ), and the variety of rocks traversed musttend to give a truer average. In the former, the result obtained (3. 0kms. Per sec. ) agrees so closely with the velocity of the long-periodundulations of distant earthquakes as to suggest that it was thesewaves that were timed at the stations west of Calcutta and disturbedthe magnetographs at Bombay. [84] Omitting, then, the Indian estimate, we find that, for the Japaneseand Charleston earthquakes, the velocity increases with the distanceas measured along the surface. To a certain extent, such a resultmight have been expected, had we assumed the earthquake-waves totravel along the chords joining the focus to very distant places ofobservation. The wave-paths that penetrate the earth are straight lines, however, only when the conditions that determine the velocity are uniformthroughout, and such uniformity we have no reason to expect. From whatwe know of the earth's interior, there can, indeed, be little doubtthat the velocity of earthquake-waves increases with the depth belowthe surface, and that the wave-paths in consequence are curved lineswith their convexity downwards. It would be out of place to state morethan the principal result of the recent investigations by Dr. A. Schmidt[85] and Prof. P. Rudzki[86] on this subject. These are based onthe assumptions that the velocity increases with the depth below thesurface, and that it is always the same at the same depth. From thefocus of the earthquake, wave-paths diverge in all directions. Thosewhich start horizontally curve upwards, and intersect the surface ofthe earth in a circle dividing the whole surface into two areas of veryunequal size. Within the small area, the surface-velocity is infiniteat the epicentre, and decreases outwards until it is least on theboundary-circle. In the larger region beyond, the surface-velocityincreases with the distance from the epicentre, until, at the antipodesof that point, it is again infinite. But, as the depth of the focus isalways slight compared with the radius of the earth, the small circulararea surrounding the epicentre is practically negligible, and we mayregard the surface-velocity of the waves that traverse the body of theearth as a quantity that continually increases with the distance fromthe epicentre. How fully this interesting theoretical result has been confirmed iswell shown in Mr. Oldham's recent and very valuable investigation onthe propagation of earthquake-motion to great distances. [87] A studyof the records of the Indian earthquake revealed the existence ofthree series of waves, the first two consisting in all probability oflongitudinal and transversal waves travelling through the body of theearth, and the third of undulations spreading over its surface (pp. 282-285). Extending his inquiries to ten other earthquakes originatingin six different centres, Mr. Oldham distinguishes the same threephases in their movements; the third phase being the most constantlyrecorded, the second less so, while the first phase is the mostfrequently absent. With the exception of a few very divergent records, the initial times of these phases and the maximum epoch of the thirdphase are plotted on the accompanying diagram (Fig. 80), in whichdistances from the epicentre in degrees of arc are represented alongthe horizontal line and the time-interval in minutes along theperpendicular line. The dots near the two lower curves refer to therecords of the heavily weighted Italian instruments, and the crossesto those of the light horizontal pendulums, which respond somewhatirregularly to the motion of the first two phases (p. 282). In thethird phase, there is less divergence between the indications of thetwo classes of instruments, and dots are used in each case for theinitial, and crosses for the maximum epoch. [Illustration: FIG. 80. --Time-curves of principal epochs of earthquake-waves of distant origin. (_Oldham. _)] Of the smoothed curves drawn between these series of points, thosemarked A, B, and C represent the time-curves of the beginnings of thefirst, second, and third phases respectively, while D is thetime-curve for the maximum of the third phase. The concavity of the two lower lines towards the horizontal base-lineshows that the surface-velocity of the corresponding waves increasesrapidly with the distance, far more so than would be possible withrectilinear motion. The rates at which these waves travel through theearth therefore increase with the depth, and the wave-paths must inconsequence be curved lines convex towards the centre of the earth. If the time-curves A and B were continued backwards to the origin, their inclinations at that point to the horizontal line give theinitial velocities of the corresponding waves, which prove to be about5 and 3 kms. Per sec. Respectively. Now, according to recentexperiments made by Mr. H. Nagaoka on the elastic constants ofrocks, [88] the mean velocity of seven archaean rocks is 5. 1 kms. Persec. For the longitudinal waves, and 2. 8 kms. Per sec. For thetransversal waves--values which agree so closely with those obtainedfor the first two series of earthquake-waves as to leave little doubtwith regard to their character. The other time-curves, C and D, corresponding to the initial andmaximum epochs of the third phase, are practically straight lines. Some of the records are slightly discordant for the average curve, especially for the initial epoch; but it is often difficult to definethe commencement of this phase with precision. At any rate, theobservations show no distinct sign of an increase in thesurface-velocity of these waves with the distance from the origin. Itmay therefore be concluded that they travel along the surface withvelocities which are practically constant for each individualearthquake, the largest waves at the rate of about 2. 9 kms. Per sec. , and the advance waves with a velocity of about 3. 3 kms. Per sec. , rising occasionally to over 4. 0 kms. Per sec. STRUCTURAL CHANGES IN THE EPICENTRAL AREA. Changes of elevation have long been known as accompaniments of greatearthquakes, though many of the earlier observations and measurementsleft much to be desired in accuracy and completeness. The Japaneseearthquake of 1891, however, placed the reality of such movementsbeyond doubt, and revealed the existence of a fault-scarp, with aheight in one place of 18 or 20 feet, and a length of at least 40, ifnot of 70, miles. In the Indian earthquake of 1897, the fault-scarpswere shorter, though more pronounced in character, the largest known(the Chedrang fault) being about 12 miles long, and having a maximumthrow at the surface of 35 feet. In some other recent earthquakes, also, remarkable fault-scarps have been developed. After the greatshocks felt in Eastern Greece on April 20th and 27th, 1894, a fissurewas traced for a distance of about 34 miles, running in aneast-south-east and west-north-west direction through the epicentraldistrict, and varying in width from an inch or two to more than threeyards. That it was a fault, and not an ordinary fissure, was evidentfrom its great length, its uniform direction, and its independence ofgeological structure. The throw was generally small, in no placeexceeding five feet. [89] Again, in British Baluchistan, after thesevere earthquake of December 20th, 1892, a fresh crack was observedin the ground running for several miles in a straight line parallel tothe axis of the Khojak range. It coincided almost exactly with a lineof springs, and was clearly produced by a fresh slip along an old lineof fault, for before the earthquake it had the appearance of an oldroad, and the natives assert that the ground has always cracked alongthis line with every severe shock. In 1892, the change in relativeheight of the two sides of the fault was small, in one place where itwas measured being only two inches. [90] But other changes, besides those in a vertical direction, occasionallytake place; though, owing to their recent discovery, comparatively fewexamples are as yet known. While the throw of the Japanese faultvaried greatly in amount, and once even in direction, there was also aconstant shift towards the northwest of the ground on the north-eastside of the fault, the displacement at one spot being as much as 13feet. In the fault-scarp formed in 1894 in Eastern Greece, a similarshift took place, though to what extent is unknown. There is, moreover, evidence of actual compression of the earth's crust at rightangles to the fault-line. The Neo valley, traversed by the Japanesefault, was apparently narrower after the earthquake than it wasbefore, and plots of ground were reduced from 48 to 30 feet inlength--_i. E. _, by nearly 40 per cent. In British Baluchistan, theformation of the fissure referred to above was accompanied both bycompression perpendicular, and by shifting parallel, to the fault. Theactual displacement in each direction is unknown, but the resultantwas not less than 27 inches. There can be no doubt that a fault-scarp is formed in the first placewith great rapidity. So abrupt, indeed, were the structuraldisplacements in the epicentral area of the Indian earthquake, thatthey contributed very materially to the intensity of the shock, givingrise to the excessive velocities observed at Rambrai and elsewhere (p. 273). The growth of the scarp does not, however, always cease with thefirst great earthquake, though it may take place in a contrary sense, as in the elevation connected with the Conception earthquake of 1835. The principal shock, according to Darwin, was followed during the fewsucceeding days "by some hundred minor ones (though of noinconsiderable violence), which seemed to come from the same quarterfrom which the first had proceeded; whilst, on the other hand, thelevel of the ground was certainly not raised by them; but, on thecontrary, after an interval of some weeks, it stood rather lower thanit did immediately after the great convulsion. "[91] AFTER-SHOCKS. A series of after-shocks, more or less long, is a constant attendanton every great tectonic earthquake, and few are the earthquakes ofany degree of strength that can be regarded as completely isolated. Even in those which visit this country, after-shocks are seldomabsent. For instance, confining ourselves to the last few years, thePembroke earthquake of 1892 was followed by 8 shocks, the Invernessearthquake of 1890 by at least 10, and possibly by 19 shocks, and thatof the same district in 1901 by 15 well-defined after-shocks inaddition to many others recorded by one observer. Of 300 Italianearthquakes strong enough to cause some damage to buildings, Dr. Cancani finds that every one was either preceded or followed, andchiefly followed, by its own train of minor shocks. For some hours, and even for days, after a great earthquake, theshocks are so numerous that it is often impossible to keep count ofthem. Many local centres spring into activity in different parts ofthe epicentral area; and, though only the strongest shocks can beidentified elsewhere, it is clear that as a rule the shocks felt atany one station are quite distinct from those observed at another. The enormous number of after-shocks that follow some earthquakes canonly be realised when they are subjected to continuous seismographicregistration; and, even then, countless earth-sounds and the slightesttremors must escape detection. The shocks may, indeed, succeed oneanother so rapidly that one begins before another ends, and the resultis an almost incessant tremulous motion rendered manifest by thequivering of water-surfaces or the swinging of chandeliers. Of thetotal number of after-shocks, we may form some idea from recentrecords in Japan. After the Mino-Owari earthquake of 1891, 3, 365shocks were recorded within little more than two years at Gifu, and1, 298 at Nagoya, but neither of these figures includes the shocks feltwithin the first few hours. Of the Kumamoto earthquake of July 28th, 1889, the after-shocks recorded at Kumamoto until the end of 1893amount to 922; and those of the Kagoshima earthquake of September 7th, 1893, recorded at Chiran until the end of January 1894, to 480. Duringthe first 30 days, the numbers recorded were 1, 746 at Gifu, 340 atKumamoto, and 278 at Chiran; showing, as Professor Omori remarks, thatthe after-shocks diminish in frequency with the size of the disturbedareas, [92]--_i. E. _, roughly with the initial intensity of the shocks. Next to absolute number, the rapid decline in general frequency is themost marked characteristic of after-shocks. Professor Omori has shownthat, excluding minor oscillations, it follows the law representedgeographically by the curves in Fig. 51, and algebraically by theequation y = k / (h + x), where _y_ is the frequency at time _x_ and_h_ and _k_ are constants for one and the same earthquake. By means ofthis formula, it is possible to estimate roughly the interval of timethat must elapse before the seismic activity of the central districtresumes its normal value. For the Mino-Owari earthquake, this provesto be about forty years, for the Kumamoto earthquake about seven oreight years, and for the Kagoshima earthquake about three or fouryears. In a recent memoir on Italian after-shocks, [93] Dr. Cancani has urgedthat other factors besides initial intensity determine the duration ofa seismic period, and prominently among these he places the depth ofthe seismic focus. When the depth is very small, the duration of theperiod is short, not much more than ten days; when the depth ismoderate, the duration may extend to three months; and, when great, itmay amount to several years. The principal law that governs the distribution of after-shocks intime may be regarded as well-established. It is otherwise with regardto their distribution in space. This has been examined only in thecases of the Japanese earthquake of 1891 and the Inverness earthquakeof 1901. So far as we can judge from the evidence which they furnish, after-shocks appear to be most numerous within and near the centralportion of the seismic focus; though the area of maximum activity issubject to continual oscillation. In this region, also, there isevidence of a gradual decrease in the depths of the after-shock foci;while, near the extremities of the epicentral area, there occurdistricts of slightly greater frequency than elsewhere. With the lapseof time, there seems therefore to be a constant extension, bothupwards and longitudinally, of the area over which the principalfault-slip took place. ORIGIN OF EARTHQUAKES. In the introductory chapter, a brief sketch is given of the differentcauses to which earthquakes are assigned. With those due to rock-fallsin subterranean channels, we need have little to do. The shocks areinvariably slight, and the part they play in the shaping of theearth's crust is insignificant. Volcanic earthquakes possess a higherdegree of interest. They represent, no doubt, incipient orunsuccessful attempts to produce an eruption. They may be theforerunners of a great catastrophe. Of far higher importance in the history of our globe is the thirdclass of earthquakes, including all those connected with the manifoldchanges which the crust has undergone. In the slow annealing process, to which it has been subjected from the earliest times, the crust hasbeen crumpled and fractured, elevated into the loftiest mountainranges or depressed below the level of the sea. Every sudden yieldingunder stress is the cause of an earthquake. It is chiefly, perhapsalmost entirely, in the formation of faults that this yielding ismanifested. The initial fracturing may be the cause of one or manyshocks, but infinitely the larger number must be referred to the slowgrowth of the fault, the intermittent slips, now in one part, now inanother, which, after the lapse of ages, culminate in a greatdisplacement. Of the length of time occupied in the formation of asingle fault, we can make no estimate in years. The anticlinal faultof Charnwood Forest dates from a pre-carboniferous period. In 1893 ithad not ceased to grow. [94] Still less can we conceive, however faintly, the number of elementalslips that constitute the history of a single fault. We may think, ifwe please, of the 143 tremors and earth-sounds noted at Comrie inPerthshire during the last three months of 1839, of the 306earthquakes felt in the Island of Zante during the year 1896, or the1, 746 shocks recorded at Gifu during thirty days in 1891; but we shallbe as far as ever from realising the vast number of steps involved inthe growth of a fault, let alone a mountain-chain. Yet, all over the land-surface of the globe, the crust is intersectedby numberless faults, and hardly any portion is there in which some ormany of these faults are not growing. One country, indeed, such asGreat Britain, may have reached a condition of comparative stagnancy;the fault-slips are few and slight, and earthquakes in consequence arerare and generally inconspicuous. In another, like Eastern Japan andthe adjoining ocean-bed, the movements are frequent, occasionallyalmost incessant, and few years pass without some great convulsion bywhich cities are wrecked and hundreds of human lives are lost. At suchtimes, we magnify the rôle of earthquakes, and are in some danger offorgetting that, in the formation of a mountain-chain or continent, they serve no higher purpose than the creaking of a wheel in thecomplex movements of a great machine. FOOTNOTES: [79] _Phil. Trans. _, vol. Li. , pt. Ii. , 1761, pp. 625-626. [80] _Journ. Sci. Coll. Imp. Univ. _, Tokyo, vol. Xi. , 1899, pp. 194-195. [81] _Journ. Coll. Sci. Imp. Univ. _, Tokyo, vol. Vii. , pt. V. , 1894, pp. 1-4; _Ital. Sismol. Soc. Boll. _, vol. Ii. , 1896, pp. 180-188. [82] _Journ. Coll. Sci. Imp. Univ. _, Tokyo, vol. Xi. , 1899, pp. 161-195. [83] _Quart. Journ. Geol. Soc. _, vol. Lvi. , 1900, pp. 1-7. [84] There is no reason why the surface-undulations of the Indianearthquake should not have produced a sensible shock even as far asItaly. Taking their amplitude in that country at 508 mm. And theirperiod at 22 sec. (p. 283), the maximum acceleration would be about 40mm. Per sec. , corresponding to the intensity 2 of the Rossi-Forelscale. (_Amer. Journ. Sci. _, vol. Xxxv. , 1888, p. 429. ) [85] _Nature_, vol. Lii. , 1895, pp. 631-633. [86] Gerland's _Beiträge zur Geophysik_, vol. Iii. , pp. 485-518. [87] _Phil. Trans. _, 1900A, pp. 135-174. [88] _Publ. Of Earthq. Inves. Com. In For. Langs. _ (Tokyo), No. 4, 1900, pp. 47-67. [89] S. A. Papavasiliou, Paris, _Acad. Sci. , Compt. Rend. _, vol. Cxix. , 1894, pp. 112-114, 380-381. [90] _Geol. Mag. _, vol. X. , 1893, pp. 356-360. [91] _Geol. Soc. Trans. _, vol. V. , 1840, pp. 618-619. [92] The disturbed areas of these earthquakes contained, respectively, 221, 000, 39, 000, and 30, 000 square miles. [93] _Boll. Sismol. Soc. Ital. _, vol. Viii. , 1902, pp. 17-48. [94] _Roy. Soc. Proc. _, vol. Lvii. , 1895, pp. 87-95. INDEX. Acceleration, maximum, of wave-motion in Japanese earthquake, 184, 185; in Indian earthquake, 272 After-shocks, definition, 4; frequency, 198, 256, 296, 344; distribution in space, 200, 203, 298, 326, 345; sound-phenomena, 207, 300; connection with fault-scarps, 300; outlining of epicentre by, 326; origin of, 257; of Neapolitan earthquake, 40; of Ischian earthquakes, 56, 65; of Andalusian earthquake, 97; of Charleston earthquake, 133; of Riviera earthquake, 167; of Japanese earthquake, 198; of Hereford earthquake, 240; of Inverness earthquake, 256; of Indian earthquake, 296; of British earthquakes, 343; of Italian earthquakes, 343; of Japanese earthquakes, 344 Agamennone, G. , 93, 94, 101, 319 Alluvium, displacement of, by Indian earthquake, 287 Amplitude of wave-motion, definition, 4; in Neapolitan earthquake, 34; in Japanese earthquake, 185; in Indian earthquake, 270 Andalusian earthquake, preparation for, 75; investigation of, 76; damage caused by, 77; isoseismal lines and disturbed area, 78; the unfelt earthquake, 82; position of epicentre, 84; depth of focus, 85; nature of shock, 87; sound-phenomena, 91; velocity of earth-waves, 92; connection between geological structure and intensity of shock, 95; fissures, 96; landslips, 97; effect on underground water, 97; after-shocks, 97; origin of, 99; bibliography, 101 Animals, effects of earthquakes on, 143 Baluchistan earthquake of 1892, 288, 341 Baldacci, L. , 70, 73 Baratta, M. , 320 Barrois, C. , 76 Bergeron, C. , 76 Bertelli, T. , 175 Bertrand, M. , 76 Birds, effects of earthquakes on, 143 Bordwar, crust-fracture at, 309 Bréon, R. , 76 Burton, W. K. , 214 Cancani, A. , 281, 282, 320, 343, 345 Castro, M. F. De, 76, 101 Charleston earthquake, investigation of, 102; damage caused by, 103; isoseismal lines and disturbed area, 104; preparation for, 107; nature of shock, 108; double epicentre, 111; origin of double shock, 120; depth of foci, 122; velocity of earth-waves, 126; fissures, 130; sand-craters, 130; effects on human beings, 131; feeling of nausea, 132; after-shocks, 133; origin of, 134; bibliography, 137 Charlon, E. , 175 Chedrang, fault-scarp at, 304 Clocks, untrustworthiness of time-records of stopped, 39, 94, 121, 127 Conder, J. , 177, 213 Coseismal lines, 227, 324 Covelli, N. , 67, 69 Damage caused by Neapolitan earthquake, 10, 24; by Ischian earthquakes, 50, 56; by Andalusian earthquake, 77; by Charleston earthquake, 103; by Riviera earthquake, 139; by Japanese earthquake, 181; by Hereford earthquake, 217; by Inverness earthquake, 247 Darwin, H. , 281 Daubrée, A. , 73 Davison, C, 202-206, 208, 210, 213, 215-261, 295 Death-rate of Neapolitan earthquake, 24; of Ischian earthquakes, 50, 56; of Andalusian earthquake, 77; of Charleston earthquake, 104; of Riviera earthquake, 140; of Japanese earthquake, 182 Denza, F. , 155, 175 Depth of seismic focus, methods of determining, 25, 86, 122, 326; of Neapolitan earthquake, 28; of Ischian earthquakes, 54, 61; of Andalusian earthquake, 86; of Charleston earthquake, 122, 125; of Riviera earthquake, 150; of Japanese earthquakes, 328 Derby earthquake of 1903, 236 Direction of shock, 22, 33, 186, 225, 325 Disturbed area, definition of, 3; of Neapolitan earthquake, 10; of Ischian earthquakes, 51, 58; of Andalusian earthquake, 80; of Charleston earthquake, 107; of Riviera earthquake, 145; of Japanese earthquake, 183; of Hereford earthquake, 219; of Inverness earthquake, 249; of Indian earthquake, 265; connection between intensity of shock and, 323 Dolomieu, 11 Du Bois, F. , 73 Dutton, C. E. , 103-137 Dutton's method of determining depth of seismic focus, 122, 327 Earthquake-motion, nature of, 280, 282, 328, 330, 337; propagation of, to great distances, 337 Earth-sound, definition of, 4 Edinburgh, record of Indian earthquake at, 281, 283, 285 Ellis, W. , 83 Emergence, angle of, 13 Epicentre, definition of, 3; methods of determining position of, 14, 52, 60, 324; of Neapolitan earthquake, 22, 23; of Ischian earthquakes, 53, 60, 67; of Andalusian earthquake, 84; of Charleston earthquake, 111; of Riviera earthquake, 146; of Hereford earthquake, 224; of Inverness earthquake, 248; of Indian earthquakes, 264, 276, 302 Epomeo, 45, 61, 71 Falb's method of determining depth of seismic focus, 86, 327 Fallen pillars, evidence of, 17, 19 Fault, originating, of Hereford earthquake, 219; of Inverness earthquake, 249 Fault-scarp of Japanese earthquake, 189; general appearance, 189; length, 192; throw, 193; horizontal shift, 193; course, 193; swamp formed by it, 194 Fault-scarps of Indian earthquakes, 273, 304; Chedrang fault, 304; Samin fault, 308; of Greek earthquake of 1894, 340, 341; of Baluchistan earthquake of 1893, 341, 342; formation and growth of, 342 Fault-slips, tectonic earthquakes due to, 5, 43, 100, 135, 174, 211, 219, 224, 241, 249, 255, 317, 346 Fishes, destruction of, by Riviera earthquake, 162 Fissures, caused by Andalusian earthquake, 96; by Charleston earthquake, 130; by Inverness earthquake, 247; by Indian earthquake, 285 Focus, seismic, definition of, 3 Focus, seismic, depth of, methods of determining, 25, 86, 122, 326; of Neapolitan earthquake, 28; of Ischian earthquakes, 54; of Andalusian earthquake, 86; of Charleston earthquake, 122, 125; of Riviera earthquake, 150; of Japanese earthquakes, 328 Focus, dimensions of seismic, of Hereford earthquake, 224; of Inverness earthquake, 250 Fore-shocks, 321; of Neapolitan earthquake, 40; of Ischian earthquake, 57; of Andalusian earthquake, 76; of Charleston earthquake, 107; of Riviera earthquake, 142; of Japanese earthquake, 201; of Hereford earthquake, 239; of Inverness earthquake, 246 Fouqué, F. , 76, 84, 101 Fracture, crust-, at Bordwar, 309 Fractures in buildings, evidence of, 14, 15, 26 Fuchs, C. W. C. , 102 Galli, I. , 82 Geological structure and intensity of shock, connection between, 95, 106, 113, 115, 135, 164, 265 Gifu, records of Japanese after-shocks at, 183, 197 Gray, T. , 295 Great Glen fault and Inverness earthquakes, connection between, 245 Greek earthquake of 1894, fault-scarp of, 340 Hayden, E. , 103 Heath, T. , 283, 320 Hereford earthquake, investigation of, 215; preparation for, 215, 238; isoseismal lines and disturbed area of, 216; damage caused by, 217, 294; position of originating fault, 219; nature of shock, 220; origin of double series of vibrations, 223; position and dimensions of the two foci, 224; direction of the shock, 225; coseismal lines and velocity of earth-waves, 227; sound-phenomena, 229; isacoustic lines and sound-area, 234; fore-shocks, 238; after-shocks, 240; origin of earthquake, 240; bibliography, 261 Hills, changes in aspect of, after Indian earthquake, 311 Hirata, K. , 327, 331 Human beings, effects of Charleston earthquake on, 131 Hypocentre, 3 Iberian peninsula, earthquakes of, 75 Indian earthquake, investigation of, 262; isoseismal lines and disturbed area, 264; nature of shock, 266; visible earth-waves, 268; elements of wave-motion, 270; sound-phenomena, 274; velocity of earth-waves, 275; the unfelt earthquake, 280; earth-fissures, 285; displacements of alluvium, 287; sand-vents, 288; rise of river-beds, etc. , 290; landslips, 291; rotation of pillars, 293; after-shocks, 296; structural changes in epicentral area, 301, 315; structure of epicentral district, 302; fault-scarps, 304; crust-fractures, 309; lakes and pools not due to faulting, 310; changes in aspects of hills, 311; revision of trigonometrical survey, 312; origin of earthquake, 317; bibliography, 319 Inverness earthquake, preparation for, 246; damage caused by, 247; fissure in ground, 247; isoseismal lines and disturbed area, 247; position of originating fault, 249; nature of shock, 250; sound-phenomena, 253; origin of earthquake, 255; after-shocks and their origin, 256; sympathetic earthquakes, 259; comparison with Japanese earthquake, 260; bibliography, 261. Investigation, Mallet's methods of, 12, 21 Isacoustic lines, 234; of Hereford earthquake, 235; of Derby earthquake, 236 Ischia, volcanic history of, 45, 70; characteristics of eruptions, 49; seismic history, 49 Ischian earthquake of 1881, investigation of, 50; isoseismal lines and disturbed area, 51; position of epicentre, 52; depth of focus, 54; nature of shock, 55; after-shocks, 56; origin of, 70; bibliography, 73 Ischian earthquake of 1883, investigation of, 56; preparation for, 57; isoseismal lines and disturbed area, 58; position of epicentre, 60; depth of focus, 61; nature of shock, 64; landslips, 64; after-shocks, 65; origin of, 70; bibliography, 73 Ischian earthquakes, characteristics of, 66; origin of, 70 Isoseismal lines, definition of, 3; their use in determining position of epicentre, 219, 249, 325; of Neapolitan earthquake, 9; of Ischian earthquakes, 51, 58; of Andalusian earthquake, 78; of Charleston earthquake, 104; of Riviera earthquake, 143; of Japanese earthquake, 178, 182; of Hereford earthquake, 216; of Inverness earthquake, 247; of Indian earthquake, 264 Issel, A. , 139, 163, 164, 166, 175 Japanese earthquake of 1887, 18 Japanese earthquake of 1891, investigation of, 177; structure of meizoseismal area, 179; damage caused by, 181; isoseismal lines and disturbed area, 182; nature of shock, 184; the great fault-scarp, 189; minor shocks, 197; distribution of after-shocks in time, 198; preparation for, 201; distribution of after-shocks in space, 203; sound-phenomena of after-shocks, 207; sympathetic earthquakes, 209; origin, of, 211; bibliography, 213 Japanese earthquake of 1894, 18, 329 Johnston-Lavis, H. J. , 50-72, 327 Kilian, W. , 76 Koto, B. , 177, 180, 181, 184, 190-196, 209, 212, 213 Lakes formed by bending of river-bed during Indian earthquake, 310 Lakes formed by fault-scarp of Japanese earthquake, 194; of Indian earthquake, 305 Landslips caused by Ischian earthquake, 64; by Andalusian earthquake, 97; by Indian earthquake, 291 Lévy, M. , 76 Lisbon earthquake of 1755, 75, 82 McGee, W. J. , 134 Macpherson, J. , 101 Magnetographs, earthquakes recorded by, 82, 157, 160, 189, 277, 282 Mallet, R. , 7-44, 85, 102, 124, 150, 294-296, 325 Mallet's method of determining depth of focus, 25, 327 Masato, H. , 178, 213 Mascart, E. , 159, 160 May Hill anticlinal and Hereford earthquake, connection between, 242 Meizoseismal area, definition of, 3; of Andalusian earthquake, 99; of Japanese earthquake, 179 Mercalli, G. , 11, 57, 58, 60, 61, 63, 67, 70-73, 76, 80, 84, 85, 88, 90, 101, 138-175, 325, 327 Michell, J. , 325 Milne, J. , 35, 177, 181, 182, 186, 189, 200, 213, 281, 283 Minor shocks of Neapolitan earthquake, 40; of Japanese earthquake, 197 Mountain ranges, effect of, on intensity of shock, 95, 106 Moureaux, T. , 161 Nagaoka, H. , 177, 214, 339 Nagoya, records of Japanese after-shocks at, 183, 197 Nature of shock, Neapolitan earthquake, 30; Ischian earthquakes, 55, 64; Andalusian earthquake, 87; Charleston earthquake, 108; Riviera earthquake, 150; Japanese earthquake, 184; Hereford earthquake, 220; Inverness earthquake, 250; Indian earthquake, 266 Nausea, feeling of, caused by Charleston earthquake, 132 Neapolitan earthquake, investigation of, 7, 12; isoseismal lines and disturbed area, 9; damage caused by, 10; position of epicentre, 14; depth of focus, 25; nature of shock, 30; sound-phenomena, 37; velocity of earth-waves, 39; minor shocks, 40; origin, 41; bibliography, 44 Ness, Loch, connection between Inverness earthquakes and formation of, 255, 257, 261 Nogués, A. F. , 101 Oddone, E. , 175 Offret, A. , 76, 158, 159, 175 Oglialoro, A. , 73 Oldham, R. D. , 262-320, 337, 340 Omori, F. , 19, 20, 177, 183-186, 188, 197-199, 207, 214, 262, 325, 327, 329, 331 Origin of earthquakes, 2, 5, 345; of Neapolitan earthquake, 41; of Ischian earthquakes, 70; of Andalusian earthquake, 101; of Charleston earthquake, 134; of Riviera earthquakes, 174; of Japanese earthquake, 211; of Hereford earthquake, 240; of Inverness earthquake, 255; of Indian earthquake, 317 Overturned bodies, maximum acceleration deduced from, 184, 272 Palmieri, L. , 57, 72, 73 Periodicity of Japanese after-shocks, 199 Perrey, A. , 7 Potenza, evidence of damaged church at, 15, 26 Prediction of earthquakes, possible, 322 Preparation for earthquakes, 40, 57, 76, 107, 142, 201, 238, 246, 321 Rails, flexure of, by Charleston earthquake, 112; by Japanese earthquake, 182; by Indian earthquake, 288 Railway-tunnels, observations of Riviera earthquake in, 166 Rebeur-Paschwitz, E. Von, 281 River-beds, rise of, caused by Indian earthquake, 290 Riviera earthquake, investigation, 138; damage caused by, 139; preparation for, 142; isoseismal lines and disturbed area, 143; position of epicentre, 146; depth of principal focus, 149; nature of shock, 150; sound-phenomena, 156; the unfelt earthquake, 157; effects of earthquake at sea, 162; destruction of fishes, 162; seismic sea-waves, 163; connection between geological structure and intensity of shock, 164; observations in railway-tunnels, 166; after-shocks, 167; recent movements in the Riviera, 170; seismic history of the Riviera, 171; origin of, 171; bibliography, 175 Rocca di Papa, record of Indian earthquake at, 281, 282, 285 Rossi, M. S. De, 57, 74, 82, 101, 175 Rossi-Forel scale of seismic intensity, 104, 216, 247 Rotation of pillars, caused by Hereford earthquake, 294; by Indian earthquake, 293; explanation of, 295 Rudzki, P. , 336 Rumi, Prof. , 169 Samin, fault-scarp at, 308 Sand-craters caused by Charleston earthquake, 130; by Indian earthquake, 288 Schmidt, A. , 336 Seismic sea-waves of Riviera earthquake, 142, 163 Seismic vertical, 12, 29, 62 Seismographic records of Riviera earthquake, 154; of Japanese earthquake of 1894, 329 Sekiya, S. , 18, 19 Serpieri, A. , 74 Shillong, nature of Indian earthquake at, 266 Sloan, E. , 103, 117-119, 134, 135 Sound-area, definition of, 3; of Neapolitan earthquake, 38; of Andalusian earthquake, 92; of Hereford earthquake, 234; of Inverness earthquake, 252; of Indian earthquake, 275 Sound-phenomena, nature of sound, 38, 229, 252, 332; inaudibility to some observers, 231, 274; its cause, 233; isacoustic lines, 234-236; variations in nature of sound throughout sound-area, 237; time-relation of sound and shock, 238, 253; origin of earthquake-sounds, 334; sound-phenomena of Neapolitan earthquake, 37; of Andalusian earthquake, 91; of Charleston earthquake, 133; of Riviera earthquake, 156; of Japanese after-shocks, 207; of Hereford earthquake, 229; of Inverness earthquake, 252; of Indian earthquake, 274 Structural changes, distribution of, in Indian earthquake, 315 Subsultory shock, 5 Sympathetic earthquakes of Japanese earthquake, 209; of Inverness earthquake, 259 Tanakadate, A. , 177, 214 Taramelli, T. , 76, 84, 85, 88, 90, 101, 138, 150, 165, 175, 325 Tectonic earthquakes, 5 Thrust-plane, Indian earthquake due to movement along, 318 Time-curve of Indian earthquake, 278; of principal epochs of earthquake-waves of distant origin, 338 Time-records, general inaccuracy of, 324 Time-relations of sound and shock in Hereford earthquake, 238; in Inverness earthquake, 253 Trigonometrical survey, revised, of Khasi hills after Indian earthquake, 312; interpretation of results, 314 Twin earthquakes, origin of, 32, 89, 120, 153, 174, 223; Neapolitan earthquake, 31; Andalusian earthquake, 87; Charleston earthquake, 108; Riviera earthquake, 149, 150; Hereford earthquake, 221 Undulatory shock, 5 Unfelt earth-waves, Andalusian earthquake, 82; Riviera earthquake, 157; Indian earthquake, 280 Uzielli, G. , 143, 176 Velocity, maximum, of wave-motion, in Neapolitan earthquake, 35; in Indian earthquake, 272 Velocity of earth-waves, methods of determining, 39, 93, 127, 229; variation with depth, 336; form of wave-paths, 336; velocity of different phases, 339; of Neapolitan earthquake, 39; of Andalusian earthquake, 92; of Charleston earthquake, 126; of Japanese earthquakes, 188; of Hereford earthquake, 229; of Indian earthquake, 275, 279, 284 Visible earth-waves in Charleston earthquake, 110; in Japanese earthquake, 186; in Indian earthquake, 268 Volcanic earthquakes, 5, 70 Vorticose shock, 5 Water, effect of Andalusian earthquake on underground, 97 Waterfalls caused by fault-scarps of Indian earthquake, 305 Wave-path, 13 West, C. D. , 272 Woolhope anticlinal and Hereford earthquake, connection between, 241 * * * * * +-----------------------------------------------------------+ | Typographical errors corrected in text: | | | | Page 54: Casamenello replaced with Casamenella | | Page 117: 'Captain Dutton' replaced with 'Major Dutton' | | Page 119: 'Capt. Dutton' replaced with 'Major Dutton' | | Page 315: Rangsonobo replaced with Rangsanobo | | Page 336: 'per sec. Per sec. ' replaced with 'per sec. ' | | Page 337: negligeable replaced with negligible | | | +-----------------------------------------------------------+ * * * * *