It is a strain on the imagination to try and picture the displacement of so huge a mass. We may think, if we will, of a slice of rock three or four miles in thickness and large enough to reach from Dover to Exeter in one direction and from London to Brighton in the other; not slipping intermittently in different places, but giving way almost instantaneously throughout its whole extent; crushing all before it, both solid rock and earthy ground alike; and, whether by the sudden spring of the entire mass or by the jar of its hurtling fragments, shattering the strongest work of human hands as easily as the frailest. Such a thrust might well be sensible over half a continent, and give rise to undulations which, unseen and unfelt, might wend their 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. At Livorno, the first movements were registered by seismographs at 11.17 A.M. (G.M.T.), and tremors were noticed by some persons at rest at about 11.15 A.M. At Spinea, a sensible undulatory shock from south-east to north-west, and lasting about four seconds, was felt at the moment when all the seismographs were set in motion by the Indian earthquake. In spite of the great distance, the perception of the earthquake in Italy is not impossible, but the records seem to me to refer to local tremors rather than to the very slow evanescent oscillations 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 the next section it will be found convenient to use the latter standard.

[71] It may be useful to give references to works in English in which the principal instruments for registering distant earthquakes are described. 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's horizontal pendulum, Seismology, pp. 58-61; Rebeur-Paschwitz's horizontal pendulum, Brit. Assoc. Rep., 1893, pp. 303-308.

[72] The beginnings of the second and third phases are shown more clearly 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 results at which we have arrived from the study of recent earthquakes, and this can, I think, be done best by describing what may be regarded as an average or typical earthquake, though it may be convenient occasionally to depart slightly from such a course. Few shocks have contributed more to our knowledge than the majority of those described in this volume; but, on certain points, we gain additional information from the investigation of other earthquakes, and these are referred to when necessary for the purpose in view.

FORE-SHOCKS.

At the outset, we are met by a question of some interest and great practical importance—namely, whether there are any constant signs of the coming of great earthquakes by means of which their occurrence might be predicted and their disastrous effects mitigated.

Excluding the Ischian earthquakes, which belong to a special class, it is evident that there is generally some slight preparation for a great earthquake. For a few hours or days beforehand, weak shocks and tremors are felt or rumbling noises heard within the future meizoseismal area. But, unfortunately, it has not yet been found possible to distinguish these disturbances from others of apparently the same character which occur alone, so that for the present they fail to serve as warnings.

In Japan, where the organisation of earthquake-studies is more complete than elsewhere, it is possible that a vague forecast might be made, if the distribution of the fore-shocks of the earthquake of 1891 should prove to be a general feature of all great earthquakes. It was at first supposed that this earthquake occurred without preparation of any kind; but a closer analysis of the records shows that during the previous two years there was a very decided increase in the seismic activity of the district, and also that the distribution of the epicentres marked out the future fault-scarp, and at the same time exhibited a tendency to comparative uniformity over the whole fault-region.

For the present, then, the only warning available is that given by the preliminary sound, which may precede the strongest vibrations by as much as five or ten or even more seconds. Though two or three seconds may elapse before its character is recognised, the fore-sound thus allows time for many persons to escape from their falling houses. Some races, however, are less capable of hearing the sound than others, and this may be one reason why Japanese earthquakes are so destructive of human life.

DISTURBED AREA.

It is usual with some investigators to measure the intensity of an earthquake roughly by the extent of its disturbed area. The depth of the seismic focus must of course have some influence on the size of this area, and this condition is only neglected because we have no precise knowledge of the depth in any case. Thus, Mr. Oldham regards the Indian earthquake of 1897 as rivalling the Lisbon earthquake of 1755, which is generally considered to hold the first place, because its disturbed area was not certainly exceeded by that of the latter.

That disturbed area is, however, an untrustworthy measure of intensity will be evident from the following table, in which the earthquakes described in this volume (omitting those of Ischia) are arranged as nearly as may be in order of intensity, beginning with the strongest:—

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 a greater area than the Indian earthquake, while the Neapolitan earthquake was inferior to that of Hereford in this respect. The explanation of course is that the boundaries of the disturbed areas are isoseismal lines corresponding to different degrees of intensity, the inhabitants of Great Britain and the United States being evidently more sensitive to weak tremors, or more observant, than those of Italy, Spain, or Central Asia. The only disturbed areas that are bounded by isoseismals of the same intensity are the two last. Very roughly, then, we may say that the intensity of the Hereford earthquake was three times as great as that of the Inverness earthquake.

POSITION OF THE EPICENTRE.

One of the first objects in the investigation of an earthquake is to determine the position and form of the epicentre. In a few rare cases, as in the Japanese and Indian earthquakes, when the fault-scarp is left protruding at the surface, only careful mapping is required to ascertain both data. But, in the great majority of earthquakes, the fault-slip dies out before reaching the surface and the position of the epicentre is then inferred by methods depending chiefly on the time of occurrence or on the direction or intensity of the shock.

At first sight, methods that involve the time of occurrence at different places seem to be of considerable promise. No scientific instruments are so widely diffused as clocks and watches; but, on the other hand, few are so carelessly adjusted. It is the exception, rather than the rule, to find a time-record accurate to the nearest minute; and, as small errors in the time may be of consequence, methods depending on this element of the earthquake are seldom employed. If, however, the number of observations is large for the size of the disturbed area, the construction of coseismal lines may define approximately the position of the epicentre. In the Hereford earthquake 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 of two or more lines of direction of the shock was first suggested by Michell in 1760,[79] and has been employed by Mallet in investigating the Neapolitan earthquake, by Professors Taramelli and Mercalli in their studies of the Andalusian and Riviera earthquakes, as well as by other seismologists. The diversity of apparent directions at one and the same place caused its temporary neglect, until Professor Omori showed in 1894 that the mean of a large number of measurements gives a trustworthy result (p. 19). His interesting observations should reinstate the method to its former place among the more valuable instruments at the disposal of the seismologist.

No observations, however, are at present so valuable for the purpose in 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 the area of greatest damage to buildings; and, when the area is small, the assumption cannot be much in error. It is of course merely a rough way of obtaining a result that is generally given more accurately by means of isoseismal lines; but there are exceptional cases, such as the Neapolitan and Ischian earthquakes, when the destruction wrought by the earthquake furnishes evidence of the greater value.

A single isoseismal accurately drawn not only gives the position of the epicentre with some approach to exactness, but also by the direction of its longer axis determines that of the originating fault. When two or three such lines can be traced, the relative position supplies in addition the hade of the fault (p. 219). The successful application of the method requires, it is true, a large number of observations, and these cannot as a rule be obtained except in districts that are somewhat thickly and uniformly populated, such as those surrounding the cities of Hereford and Inverness. In the Charleston earthquake, also, the position and form of the epicentres were deduced from the trend of isoseismal lines based on the damage to railway-lines and various structures within a sparsely inhabited meizoseismal area.

In a few cases, of which the Indian earthquake may be regarded as typical, a fourth method has recently been found of service. The numerous after-shocks which follow a great earthquake originate for the most part within the seismic focus of the latter; and, as they usually disturb a very small area, it is not difficult to ascertain approximately the positions of their epicentres. Some, as in the Inverness after-shocks of 1901, result from slips in the very margin of the principal focus; but, as a rule, the seat of their activity tends to contract towards a central region of the focus. Bearing in mind, then, that some of the succeeding shocks originate at and beyond the confines of the focus, and that others may be sympathetic shocks precipitated by the sudden change of stress, it follows that the shifting epicentres of the true after-shocks map out, in part at any rate, the epicentral area of the principal earthquake.

DEPTH OF THE SEISMIC FOCUS.

It is much to be regretted that we have no satisfactory method of determining so interesting an element as the depth of the seismic focus. That it amounts to but a few miles at the most is certain from the limited areas within which slight shocks are felt or disastrous ones exhibit their maximum effects. Nor can we suppose that the rocks at very great depths are capable of offering the prolonged resistance and sudden collapse under stress that are necessary for the production of an earthquake.

The problem is evidently beyond our present powers of solution, and its interest is therefore mainly historical. All the known methods are vitiated by our ignorance of the refractive powers of the rocks traversed by the earth-waves. But, even if this ignorance could be replaced by knowledge, most of the methods suggested are open to objection. Falb's method, depending on the time-interval between the initial epochs of the sound and shock, is of more than doubtful value. Dutton's, based on the rate of change of surface-intensity, is difficult to apply, and in any case gives only an inferior limit to the depth. Time-observations have been employed, especially in New Zealand; but the uncertainty in selecting throughout the same phase of the movement, and the large errors in the estimated depth resulting from small errors in the time-records, are at present most serious objections. There remains the method devised by Mallet, and, though he claimed for it an exaggerated accuracy, it still, in my opinion, holds the field against all its successors. When carefully applied, as it has been by Mallet himself, by Johnston-Lavis and Mercalli, we probably obtain at least some conception of the depth of the seismic focus.

Professor Omori and Mr. K. Hirata have recently[80] lessened the chief difficulty in the application of Mallet's method. They have deduced the angle of emergence from the vertical and horizontal components of the motion as registered by seismographs, instead of from the inclination of fissures in damaged walls. In two recent earthquakes recorded at Miyako in Japan, they find the angle of emergence to be 7.2 and 9 respectively, the corresponding depths of the foci being 5.6 and 9.3 miles. These are probably the most accurate estimates that we possess, and it will be noted that they differ little from the mean values obtained for the Neapolitan, Andalusian, and Riviera earthquakes—namely, 6.6, 7.6, and 10.8 miles.

NATURE OF THE SHOCK

In one respect, the earthquakes described above fail to represent the progress of modern seismology. They furnish no diagrams made by accurately constructed seismographs within their disturbed areas. The curve reproduced in Fig. 36, as already pointed out, is no exception to this statement. For another reason, the records that were obtained in Japan of the earthquake of 1891 are trustworthy for little more than the short-period initial vibrations; for, owing to the passage of the surface-waves, visible in and near the meizoseismal area, the Japanese seismographs registered the tilting of the ground rather than the elastic vibrations that traversed the earth's crust.

Notwithstanding this defect, personal impressions of an earthquake-shock give a fairly accurate, if incomplete, idea of its nature. Nearly all observers placed under favourable conditions agree that an earthquake begins with a deep rumbling sound, accompanied, after the first second or two, by a faint tremor which gradually, and sometimes rapidly, increases in strength until it merges into the shock proper, which consists of several or many vibrations of larger amplitude and longer period, and during which the attendant sound is generally at its loudest; the earthquake dying away, as it began, with tremors and a low rumbling sound.



The vibrations that produce the sensible shock are by no means all that are present during an earthquake. The Indian earthquake, for instance, seemed to last about three or four minutes at Midnapur; but the movements of the bubble of a level showed that the ground continued to oscillate for at least five minutes longer (p. 280). Many of these unfelt waves are rendered manifest by seismographs, although there are still others that elude registration either from the extreme shortness or the great length of their periods.

In Fig. 79 is shown the principal part of a diagram obtained at Tokio during the Japanese earthquake of June 20th, 1894 (p. 18), the curve representing the N.E.-S.W. component of the horizontal motion during the first 25 seconds of the record. The instrument employed is one specially designed for registering strong earthquakes, and is unaffected by very minute tremors. Those which formed the commencement of this earthquake lasted for about 10 seconds, as shown by ordinary seismographs, and the vibrations had attained a range of a few millimetres before they affected the instrument in question. For the first 2-1/2 seconds, they occurred at the rate of four or five a second. The motion then suddenly became violent, and the ground was displaced 37 mm. in one direction, followed by a return movement of 73 mm., and this again by one of 42 mm., the complete period of the oscillation being 1.8 seconds. The succeeding vibrations were of smaller amplitude and generally of shorter period for a minute and a half, then dying out during the last three minutes as almost imperceptible waves with a period of two or more seconds.[81]

Though incomplete in some respects, this diagram illustrates clearly the division of the earthquake-motion into three stages—namely, the preliminary tremors, the principal portion or most active part of an earthquake, and the end-portion or gradually evanescent slow undulations. In all three stages, however, both tremors and slow undulations may be present; and, as the latter, owing to their long period, are more or less insensible to human beings, the ripples of the final stage give the impression of a tremulous termination as described above. The duration of each stage varies considerably in different earthquakes. Thus, in a valuable study of 27 earthquakes recorded at Miyako, in Japan, during the years 1896-98, Messrs. Omori and Hirata show[82] that the duration of the preliminary stage varies from 0 to 26 seconds, with an average of about 10 seconds; that of the principal portion from 0.7 to 26 seconds, also with an average of about 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 two minutes; while, at Tokio, a horizontal pendulum designed by Professor Omori oscillated for at least two hours. The periods of both ripples and slow undulations, again, vary from one earthquake to another; but it is worthy of notice that the average period of the undulations is almost constant in all three stages of the motion, being 1.1, 1.3, and 1.3 seconds, respectively, for the east-west component of the horizontal motion, and 1.0 second throughout for the north-south component. For the ripples, the average period is .08 second in the preliminary stage, .10 second in the principal portion, and .08 second again in the end portion; those of the principal portion being slightly larger in amplitude, as well as longer in period, than the ripples of the first and third stages.

SOUND-PHENOMENA.

Besides the ripples already mentioned, there are others of still smaller amplitude and shorter period that are sensible, but as a rule only just sensible, to us as sounds. All the known evidence points to the extraordinary lowness of the earthquake-sound. According to some observers, it seems as if close to their lower limit of audibility; while others, however intently they may listen, are unable to hear the slightest noise. In other words, the most rapid vibrations present in an earthquake do not recur at a rate of much more than about 30 to 50 per second; or, if they do, they are not strong enough to impress the human ear.

To most observers, the sound seems to increase and decrease in intensity with the shock, and so gradually and smoothly does this change take place that the sound is frequently mistaken for that of an underground train approaching the observer's house, passing beneath it, and receding in the opposite direction. Some persons, especially if situated within the meizoseismal area, hear also loud crashes in the midst of the rumbling sound and simultaneously with the strongest vibrations. At a moderate distance, say from 30 to 40 miles, the sound becomes more harsh and grating while the shock is felt; and, at a greater distance, even this change disappears, and nothing is heard but an almost monotonous sound like the low roll of distant thunder. The explanation of this is that the sound-vibrations are of different periods and varying amplitude, and the limiting vibrations tend to become inaudible with increasing distance, the lower on account of their long period, the higher owing to their small amplitude.

The magnitude of the sound-area depends, even more than that of the disturbed area, on the personal equation of the observers. The lower limit of audibility varies not only in different individuals, but also in different races. In Great Britain, it is doubtful whether an earthquake ever occurs unaccompanied by sound; and in the meizoseismal area the noise is heard by nearly all observers. With Italians, the average lower limit of audibility is higher than with the Anglo-Saxon race; slight shocks frequently occur without noticeable sound, but with strong ones, the larger number of observers is sure to include one or more capable of hearing the rumbling noise. The Japanese are, however, seldom affected by the most rapid earthquake-vibrations, and the strongest shocks may be unattended by any recorded sound. The result is manifest in the size of the sound-area in different countries. In the Hereford earthquake, the sound-area contained 70,000 square miles; in the Neapolitan earthquake, about 3,300 square miles; while, in Japanese earthquakes, the sound is rarely heard more than a few miles from the epicentre.

Another effect of this personal equation of the observers is that the sound-vibrations apparently outrace those of longer period. The Italians, for instance, generally hear the sound that precedes the shock, and more rarely the weaker sound that follows it. In Japan, only the earlier sound-vibrations, if any, seem to be audible. In Great Britain, on the contrary, the fore-sound is perceptible to four, and the after-sound to three, out of every five observers; and these proportions are maintained roughly to considerable distances from the epicentre. It follows, therefore, that the sound-vibrations and those which constitute the shock must travel with nearly, if not quite, the same velocity; and that the greater duration of the sound is due either to the prolongation of the initial movement or to the overlapping of the principal focus by the sound-focus. Neither alternative can be regarded as improbable, but observations made on British earthquakes point to the latter explanation as the true one.

It will be sufficient to refer to two phenomena in support of this statement. In the first place, the percentage of observers who hear the fore-sound varies with the direction from the epicentre. Thus, during the Inverness earthquake of 1901, the majority of observers in Aberdeenshire regarded the sound as beginning and ending with the shock; while, in counties lying more nearly along the course of the great fault, the sound was generally heard both before and after the shock (p. 253). In this case, then, the initial and concluding sound vibrations must have come chiefly from the margins of the seismic focus; and those from the margin nearest to an observer would be more sensible than those from the farther margin. Again, in slight earthquakes, such as the Cornwall earthquake of April 1, 1898,[83] the curves of equal sound intensity, while their axes are parallel to those of the isoseismal lines, are displaced laterally with respect to these curves, owing to the arrival of the strongest sound-vibrations from the upper margin of an inclined seismic focus.

When a fault-slip occurs, the displacement is obviously greatest in the central region, and dies out gradually towards the margins of the focus. The phenomena described above show that the evanescent displacement within these margins generate sound-vibrations only; and that the greater slip within the central region produces also the more important vibrations that compose the shock. As the former are perceptible over a limited district, while the latter may be felt through half a continent, it is clear that the sound-area should bear no fixed relation in point of size to the disturbed area, but should be 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 how great 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 the Charleston earthquake, and, at the other end of the scale, a value of 0.9 km. per sec. for the Hereford earthquake. Between them, and equally trustworthy, lie the estimates of 3.0 km. per sec. for the Indian earthquake, and 2.1 kms. per sec. for the Japanese earthquake and its immediate successors.

It is difficult to account entirely for such discordance. Errors of observation 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 consequence when the distances in question are not very great. In the Japanese and Hereford earthquakes, all three may have combined to produce the divergent results, the distance in these cases being only 275 and 142 kms. respectively.

In the Indian and Charleston earthquakes, the distances are much greater (1944 and 1487 kms.), and the variety of rocks traversed must tend to give a truer average. In the former, the result obtained (3.0 kms. per sec.) agrees so closely with the velocity of the long-period undulations of distant earthquakes as to suggest that it was these waves that were timed at the stations west of Calcutta and disturbed the magnetographs at Bombay.[84]

Omitting, then, the Indian estimate, we find that, for the Japanese and Charleston earthquakes, the velocity increases with the distance as measured along the surface. To a certain extent, such a result might have been expected, had we assumed the earthquake-waves to travel along the chords joining the focus to very distant places of observation.

The wave-paths that penetrate the earth are straight lines, however, only when the conditions that determine the velocity are uniform throughout, and such uniformity we have no reason to expect. From what we know of the earth's interior, there can, indeed, be little doubt that the velocity of earthquake-waves increases with the depth below the surface, and that the wave-paths in consequence are curved lines with their convexity downwards. It would be out of place to state more than the principal result of the recent investigations by Dr. A. Schmidt[85] and Prof. P. Rudzki[86] on this subject. These are based on the assumptions that the velocity increases with the depth below the surface, and that it is always the same at the same depth. From the focus of the earthquake, wave-paths diverge in all directions. Those which start horizontally curve upwards, and intersect the surface of the earth in a circle dividing the whole surface into two areas of very unequal size. Within the small area, the surface-velocity is infinite at the epicentre, and decreases outwards until it is least on the boundary-circle. In the larger region beyond, the surface-velocity increases with the distance from the epicentre, until, at the antipodes of that point, it is again infinite. But, as the depth of the focus is always slight compared with the radius of the earth, the small circular area surrounding the epicentre is practically negligible, and we may regard the surface-velocity of the waves that traverse the body of the earth as a quantity that continually increases with the distance from the epicentre.

How fully this interesting theoretical result has been confirmed is well shown in Mr. Oldham's recent and very valuable investigation on the propagation of earthquake-motion to great distances.[87] A study of the records of the Indian earthquake revealed the existence of three series of waves, the first two consisting in all probability of longitudinal and transversal waves travelling through the body of the earth, and the third of undulations spreading over its surface (pp. 282-285). Extending his inquiries to ten other earthquakes originating in six different centres, Mr. Oldham distinguishes the same three phases in their movements; the third phase being the most constantly recorded, the second less so, while the first phase is the most frequently absent. With the exception of a few very divergent records, the initial times of these phases and the maximum epoch of the third phase are plotted on the accompanying diagram (Fig. 80), in which distances from the epicentre in degrees of arc are represented along the horizontal line and the time-interval in minutes along the perpendicular line. The dots near the two lower curves refer to the records of the heavily weighted Italian instruments, and the crosses to those of the light horizontal pendulums, which respond somewhat irregularly to the motion of the first two phases (p. 282). In the third phase, there is less divergence between the indications of the two classes of instruments, and dots are used in each case for the initial, and crosses for the maximum epoch.



Of the smoothed curves drawn between these series of points, those marked A, B, and C represent the time-curves of the beginnings of the first, second, and third phases respectively, while D is the time-curve for the maximum of the third phase.

The concavity of the two lower lines towards the horizontal base-line shows that the surface-velocity of the corresponding waves increases rapidly with the distance, far more so than would be possible with rectilinear motion. The rates at which these waves travel through the earth therefore increase with the depth, and the wave-paths must in consequence 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 the initial velocities of the corresponding waves, which prove to be about 5 and 3 kms. per sec. respectively. Now, according to recent experiments made by Mr. H. Nagaoka on the elastic constants of rocks,[88] the mean velocity of seven archaean rocks is 5.1 kms. per sec. for the longitudinal waves, and 2.8 kms. per sec. for the transversal waves—values which agree so closely with those obtained for the first two series of earthquake-waves as to leave little doubt with regard to their character.

The other time-curves, C and D, corresponding to the initial and maximum 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 define the commencement of this phase with precision. At any rate, the observations show no distinct sign of an increase in the surface-velocity of these waves with the distance from the origin. It may therefore be concluded that they travel along the surface with velocities which are practically constant for each individual earthquake, 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 great earthquakes, though many of the earlier observations and measurements left much to be desired in accuracy and completeness. The Japanese earthquake of 1891, however, placed the reality of such movements beyond doubt, and revealed the existence of a fault-scarp, with a height in one place of 18 or 20 feet, and a length of at least 40, if not of 70, miles. In the Indian earthquake of 1897, the fault-scarps were shorter, though more pronounced in character, the largest known (the Chedrang fault) being about 12 miles long, and having a maximum throw at the surface of 35 feet. In some other recent earthquakes, also, remarkable fault-scarps have been developed. After the great shocks felt in Eastern Greece on April 20th and 27th, 1894, a fissure was traced for a distance of about 34 miles, running in an east-south-east and west-north-west direction through the epicentral district, and varying in width from an inch or two to more than three yards. That it was a fault, and not an ordinary fissure, was evident from its great length, its uniform direction, and its independence of geological structure. The throw was generally small, in no place exceeding five feet.[89] Again, in British Baluchistan, after the severe earthquake of December 20th, 1892, a fresh crack was observed in the ground running for several miles in a straight line parallel to the axis of the Khojak range. It coincided almost exactly with a line of springs, and was clearly produced by a fresh slip along an old line of fault, for before the earthquake it had the appearance of an old road, and the natives assert that the ground has always cracked along this line with every severe shock. In 1892, the change in relative height of the two sides of the fault was small, in one place where it was measured being only two inches.[90]

But other changes, besides those in a vertical direction, occasionally take place; though, owing to their recent discovery, comparatively few examples are as yet known. While the throw of the Japanese fault varied greatly in amount, and once even in direction, there was also a constant shift towards the northwest of the ground on the north-east side of the fault, the displacement at one spot being as much as 13 feet. In the fault-scarp formed in 1894 in Eastern Greece, a similar shift took place, though to what extent is unknown. There is, moreover, evidence of actual compression of the earth's crust at right angles to the fault-line. The Neo valley, traversed by the Japanese fault, was apparently narrower after the earthquake than it was before, and plots of ground were reduced from 48 to 30 feet in length—i.e., by nearly 40 per cent. In British Baluchistan, the formation of the fissure referred to above was accompanied both by compression perpendicular, and by shifting parallel, to the fault. The actual displacement in each direction is unknown, but the resultant was not less than 27 inches.

There can be no doubt that a fault-scarp is formed in the first place with great rapidity. So abrupt, indeed, were the structural displacements in the epicentral area of the Indian earthquake, that they contributed very materially to the intensity of the shock, giving rise to the excessive velocities observed at Rambrai and elsewhere (p. 273). The growth of the scarp does not, however, always cease with the first 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 few succeeding days "by some hundred minor ones (though of no inconsiderable violence), which seemed to come from the same quarter from which the first had proceeded; whilst, on the other hand, the level of the ground was certainly not raised by them; but, on the contrary, after an interval of some weeks, it stood rather lower than it did immediately after the great convulsion."[91]

AFTER-SHOCKS.

A series of after-shocks, more or less long, is a constant attendant on every great tectonic earthquake, and few are the earthquakes of any degree of strength that can be regarded as completely isolated. Even in those which visit this country, after-shocks are seldom absent. For instance, confining ourselves to the last few years, the Pembroke earthquake of 1892 was followed by 8 shocks, the Inverness earthquake of 1890 by at least 10, and possibly by 19 shocks, and that of the same district in 1901 by 15 well-defined after-shocks in addition to many others recorded by one observer. Of 300 Italian earthquakes strong enough to cause some damage to buildings, Dr. Cancani finds that every one was either preceded or followed, and chiefly followed, by its own train of minor shocks.

For some hours, and even for days, after a great earthquake, the shocks are so numerous that it is often impossible to keep count of them. Many local centres spring into activity in different parts of the epicentral area; and, though only the strongest shocks can be identified elsewhere, it is clear that as a rule the shocks felt at any one station are quite distinct from those observed at another.

The enormous number of after-shocks that follow some earthquakes can only be realised when they are subjected to continuous seismographic registration; and, even then, countless earth-sounds and the slightest tremors must escape detection. The shocks may, indeed, succeed one another so rapidly that one begins before another ends, and the result is an almost incessant tremulous motion rendered manifest by the quivering of water-surfaces or the swinging of chandeliers. Of the total number of after-shocks, we may form some idea from recent records in Japan. After the Mino-Owari earthquake of 1891, 3,365 shocks were recorded within little more than two years at Gifu, and 1,298 at Nagoya, but neither of these figures includes the shocks felt within the first few hours. Of the Kumamoto earthquake of July 28th, 1889, the after-shocks recorded at Kumamoto until the end of 1893 amount to 922; and those of the Kagoshima earthquake of September 7th, 1893, recorded at Chiran until the end of January 1894, to 480. During the first 30 days, the numbers recorded were 1,746 at Gifu, 340 at Kumamoto, and 278 at Chiran; showing, as Professor Omori remarks, that the after-shocks diminish in frequency with the size of the disturbed areas,[92]—i.e., roughly with the initial intensity of the shocks.

Next to absolute number, the rapid decline in general frequency is the most marked characteristic of after-shocks. Professor Omori has shown that, excluding minor oscillations, it follows the law represented geographically by the curves in Fig. 51, and algebraically by the equation 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 of this formula, it is possible to estimate roughly the interval of time that must elapse before the seismic activity of the central district resumes its normal value. For the Mino-Owari earthquake, this proves to be about forty years, for the Kumamoto earthquake about seven or eight years, and for the Kagoshima earthquake about three or four years.

In a recent memoir on Italian after-shocks,[93] Dr. Cancani has urged that other factors besides initial intensity determine the duration of a seismic period, and prominently among these he places the depth of the seismic focus. When the depth is very small, the duration of the period is short, not much more than ten days; when the depth is moderate, the duration may extend to three months; and, when great, it may amount to several years.

The principal law that governs the distribution of after-shocks in time may be regarded as well-established. It is otherwise with regard to their distribution in space. This has been examined only in the cases of the Japanese earthquake of 1891 and the Inverness earthquake of 1901. So far as we can judge from the evidence which they furnish, after-shocks appear to be most numerous within and near the central portion of the seismic focus; though the area of maximum activity is subject to continual oscillation. In this region, also, there is evidence of a gradual decrease in the depths of the after-shock foci; while, near the extremities of the epicentral area, there occur districts of slightly greater frequency than elsewhere. With the lapse of time, there seems therefore to be a constant extension, both upwards and longitudinally, of the area over which the principal fault-slip took place.

ORIGIN OF EARTHQUAKES.

In the introductory chapter, a brief sketch is given of the different causes to which earthquakes are assigned. With those due to rock-falls in subterranean channels, we need have little to do. The shocks are invariably slight, and the part they play in the shaping of the earth's crust is insignificant. Volcanic earthquakes possess a higher degree of interest. They represent, no doubt, incipient or unsuccessful attempts to produce an eruption. They may be the forerunners of a great catastrophe.

Of far higher importance in the history of our globe is the third class of earthquakes, including all those connected with the manifold changes which the crust has undergone. In the slow annealing process, to which it has been subjected from the earliest times, the crust has been crumpled and fractured, elevated into the loftiest mountain ranges or depressed below the level of the sea. Every sudden yielding under stress is the cause of an earthquake. It is chiefly, perhaps almost entirely, in the formation of faults that this yielding is manifested. The initial fracturing may be the cause of one or many shocks, but infinitely the larger number must be referred to the slow growth of the fault, the intermittent slips, now in one part, now in another, which, after the lapse of ages, culminate in a great displacement. Of the length of time occupied in the formation of a single fault, we can make no estimate in years. The anticlinal fault of Charnwood Forest dates from a pre-carboniferous period. In 1893 it had not ceased to grow.[94]

Still less can we conceive, however faintly, the number of elemental slips that constitute the history of a single fault. We may think, if we please, of the 143 tremors and earth-sounds noted at Comrie in Perthshire during the last three months of 1839, of the 306 earthquakes felt in the Island of Zante during the year 1896, or the 1,746 shocks recorded at Gifu during thirty days in 1891; but we shall be as far as ever from realising the vast number of steps involved in the growth of a fault, let alone a mountain-chain.

Yet, all over the land-surface of the globe, the crust is intersected by numberless faults, and hardly any portion is there in which some or many of these faults are not growing. One country, indeed, such as Great Britain, may have reached a condition of comparative stagnancy; the fault-slips are few and slight, and earthquakes in consequence are rare and generally inconspicuous. In another, like Eastern Japan and the adjoining ocean-bed, the movements are frequent, occasionally almost incessant, and few years pass without some great convulsion by which cities are wrecked and hundreds of human lives are lost. At such times, we magnify the rle of earthquakes, and are in some danger of forgetting that, in the formation of a mountain-chain or continent, they serve no higher purpose than the creaking of a wheel in the complex 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 Indian earthquake should not have produced a sensible shock even as far as Italy. Taking their amplitude in that country at 508 mm. and their period at 22 sec. (p. 283), the maximum acceleration would be about 40 mm. per sec., corresponding to the intensity 2 of the Rossi-Forel scale. (Amer. Journ. Sci., vol. xxxv., 1888, p. 429.)

[85] Nature, vol. lii., 1895, pp. 631-633.

[86] Gerland's Beitrge 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

Bron, 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

Daubre, 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

Fouqu, 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

Lvy, 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

Nogus, 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

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