A Study of Recent Earthquakes

by Charles Davison

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.