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.
Fig. 78.—Diagram of Thrust-planes.[ToList]
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 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.