A Study of Recent Earthquakes Part 23

For a similar reason, the slight general upheaval of the hills indicated by the repeated calculations, must be regarded as doubtful, for it depends on the a.s.sumed fixity of the station of Rangsan.o.bo, whereas it is more probable that it was the height of Taramun Tila that remained unchanged. Reducing the calculated heights of all the other stations by six feet (the a.s.sumed rise of the latter), it follows that, on the whole, the height of the Khasi hills underwent but little change, except at Mautherrican and Landau Modo, and the secondary stations of Mairang and Kollong Rock, near Maonoi. The apparent elevations of 24, 17, 11, and 15 feet at these places exceed the probable error of the observations; and it is worthy of notice that all four stations lie close to the edge of fault-scarps, while Landau Modo is not far from two of the pools formed by distortion of the surface unaccompanied by faulting.

If, then, the revised triangulation of the Khasi hills has failed to provide absolute measures of the displacements in the epicentral area, it has, nevertheless, proved that important movements, both horizontal and vertical, have taken place.

_Distribution of the Structural Changes._--The boundary of the epicentral area, as drawn in Figs. 68 and 75, lays no claim to great accuracy; but its departure from the true line is probably in no place considerable. It must evidently include all the districts where marked structural changes occurred, and must therefore extend east of Maophlang and west of Tura. Towards the north, these changes have been traced to the foot of the Garo hills, and there is some, though not very certain, evidence of alterations of level along the course of the Brahmaputra. The very large number of after-shocks recorded at Borpeta and Bijni also points to an extension of the epicentral area beyond these places. To the east, the course of the boundary becomes doubtful, but it must pa.s.s close to Gauhati and east of Shillong, and probably ends a short distance beyond Jaintiapur. The southern boundary must coincide nearly with the north edge of the alluvial plains of Sylhet, for there is no evidence of its intrusion into the plains. On the west side, the epicentral area includes the Garo hills and part of the alluvial plain to the west; and, from the large number of after-shocks felt at Rangpur and Kaunia, and the great violence of the shock at the former, we may infer that both places lie within the boundary-line. If, then, there is no great error in the mapping of this line, it follows that the epicentre was about 200 miles long from east to west, not less than 50, and possibly as much as 100, miles in maximum width, and contained an area of at least 6000 square miles.

Near the boundary, the permanent displacements must have been comparatively small; but they were certainly marked in the northern part of the a.s.sam hills for a distance of 100 miles from east to west.

At the limits of the latter area, as Mr. Oldham remarks, "the evidence points to the changes being of the nature of long, low rolls, the change of slope being insufficient to cause any appreciable change in the drainage channels. Then comes a zone in which the surface changes are more abrupt, the slopes of the stream beds have been altered so as to cause conspicuous changes in the nature of the streams, but any fracture or faulting which may have taken place has died out before the surface was reached. And north of this, close to the edge of the hills, the rocks have been fractured and faulted right up to the surface."

ORIGIN OF THE EARTHQUAKE.

Almost every feature of the great earthquake points to an origin very different from that of the others described in this volume. The suddenness with which the shock began, its unusual duration, and the occurrence of many maxima of intensity, are inconsistent with a simple fault-displacement. Again, the excessive velocities of projection at Rambrai and elsewhere, the existence of isolated fault-scarps and fractures, the local changes of level, the compression indicated by the revised trigonometrical survey, the wide area over which these structural changes took place, and the numerous distinct centres of subsequent activity, all these phenomena demonstrate the intense and complex character of the initial disturbances, as well as the widespread bodily displacement of the earth's crust within the epicentral area. There may, it is conceivable, have been a number of foci, nearly or quite detached from one another, and giving rise to a group of nearly concurrent shocks. Or--and this is a far more probable supposition--there may have been one vast deep-seated centre, from which off-shoots ran up towards the surface, each partaking to a greater or less degree in the movement within the parent focus.

As Mr. Oldham points out, we have recently become acquainted with a structure exactly corresponding to that which is here inferred. The great thrust-planes, so typically developed in the Scottish Highlands, are only reversed faults which are nearly horizontal instead of being highly inclined; and they are accompanied by a number of ordinary reversed faults running upwards to the surface. In Fig. 78, the main features of a section drawn by the Geological Survey of Scotland are reproduced; T, T, representing thrust planes, and _t_, _t_, minor thrusts or faults. A great movement along one of the main thrust-planes would carry with it dependent slips along many of the secondary planes. Direct effects of the former might be invisible at the surface, except in the horizontal displacements that would be rendered manifest by a renewed trigonometrical survey; whereas the latter might or might not reach the surface, giving rise in the one case to fissures and fault-scarps, in the other to local changes of level, and in both to regions of instability resulting in numerous after-shocks.

[Ill.u.s.tration: FIG. 78.--Diagram of Thrust-planes.]

The enormous dimensions of the parent focus will be obvious from the phenomena that have been described above. Mr. Oldham has traced the probable form of the epicentre. It may in reality be neither so simple nor so symmetrical as is represented in Fig. 75, but there are good reasons for thinking that it does not differ sensibly either in size or form from that laid down. The part of the thrust-plane over which movement took place must therefore have been about 200 miles long, not less than 50 miles wide, and between 6000 and 7000 square miles in area. With regard to its depth, we have no decisive knowledge. It may have been about five miles or less; it can hardly have been much greater.

It is a strain on the imagination to try and picture the displacement of so huge a ma.s.s. 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 ma.s.s 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.-x.x.x., 1-379, with 44 plates and 3 maps.

8. ---- "List of After-shocks of the Great Earthquake of 12th June 1897." _Ibid._, vol. x.x.x., pt. i., 1900, pp. 1-102.

9. ---- "On Tidal Periodicity in the Earthquakes of a.s.sam." _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 princ.i.p.al instruments for registering distant earthquakes are described. For Cancani's vertical pendulum, see _Brit. a.s.soc. Rep._, 1896, pp. 46-47; Darwin's bifilar pendulum, _Brit. a.s.soc. 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. a.s.soc. 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] _j.a.pan 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 cla.s.s, 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 j.a.pan, 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 a.n.a.lysis 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 j.a.panese 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:--