A Study of Recent Earthquakes

by Charles Davison

In the more complete records, and especially in those given by the Italian apparatus, Mr. Oldham distinguishes three phases of motion. The first consists of rapid and nearly horizontal movements of the ground. In Italy, it begins at about 11.17 A.M.—that is, about 12½ minutes after the commencement of the shock at the epicentre (Fig. 71, a). Without any break in the movement, and after a further interval of about 8½ minutes, the second phase begins; the vibrations are similar to the preceding, but they are larger and more open, and are accompanied 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 have been aptly compared by Professor Milne to the movements caused by an ocean-swell. As they travelled across Europe, the surface of the ground was thrown into a series of flat waves, 34 miles in length, and 20 inches in maximum height, the complete period of each wave being 22 seconds. This phase is by far the longest of the three; in the more sensitive instruments, two or three hours elapsed before their traces ceased to show any sign of movement.

Fig. 72.—Seismographic Record of Indian Earthquake at Edinburgh. (Heath.)[ToList]

Knowing the distances of the different observatories from the epicentre, and the times taken by each phase to reach them, we can form some idea of the rates at which they travelled. If the early tremors moved in straight lines, their mean velocity for the first phase was 9.0, and for the second 5.3, kilometres per second; but, if they moved along curved paths through the body of the earth, their mean velocities must have exceeded these amounts. For the first undulations of the third phase, the velocity would be 2.9 kilometres per second if they travelled along straight lines, and 3.0 kilometres per second if they were confined to the surface of the earth.

The existence of the second phase was noticed for the first time by Mr. Oldham in the records of the Indian earthquake, but he has since detected it in those of other shocks. He believes, in common with most seismologists, that the first phase corresponds to waves of elastic compression, or longitudinal waves, travelling through the body of the earth; and the second phase he attributes to waves of elastic distortion, or transversal waves, travelling in the same way, in which the particles move at right angles to the direction in which the wave travels, thus causing a slight tilting of the surface. It is probable that the waves of both phases move along curved, rather than straight, lines through the earth, that the curves are concave towards the surface, and that the velocity of the waves increases with the depth of their path below the surface.

On the other hand, the surface-velocity of the first undulations of the third phase is practically constant for all distances from the epicentre, and, in the case of the Indian earthquake, it agrees almost exactly with that obtained for the velocity within the disturbed area, and as far as Bombay. It is therefore difficult to resist the conclusion that the third phase consists of undulations which travel along the surface of the earth. Diverging in two dimensions only, they fade away much more slowly than the vibrations of the other two phases.

We may thus imagine these surface-undulations speeding outwards from the epicentre in ever-widening circles until they have passed over a quarter-circumference of the earth, when they should begin to converge towards the antipodes. Here they should cross each other, and again spread out as circular waves, once more in their course passing the same observatories where they were first recorded, but in the opposite order. It has been reserved for the most violent earthquake of modern times to verify this interesting conclusion. Faint, but decided, are the traces of the second crossing. At Edinburgh, they occur at 2.6 P.M., at about the same time at Shide, at Leghorn 2.10, Catania 2.12¾, while at Ischia there are several movements between 2 and 3 P.M. At Rocca di Papa, near Rome, the time is slightly earlier, but the undulations, like those at the first crossing, have a complete period of about 20 seconds. The distances traversed by the waves are more than 20,000, instead of less than 5000 miles; but the mean velocity with which they travelled is almost exactly the same as at first—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 alluvial plains. Not only were they remarkably abundant, more so than in any other known earthquake, but they occurred over an unusually wide area. Wherever the necessary conditions prevailed, they were found to be numerous over a district bounded approximately by the isoseismal 1 (Fig. 68), and measuring about 400 miles from east to west, and about 300 miles from north to south; and they were present, though in smaller numbers, over an area nearly 600 miles long in an east-north-east and west-south-west direction. They were naturally more frequent near river-channels and reservoirs, on account of the absence of lateral support, and as a rule were parallel to the edge of the bank, a few hundred yards in length, and in width varying from some inches to four or five feet.