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 role 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 _Beitrage 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.