The spots situated just over the crack, of which we spoke on page 25, suffered the most serious damage. Next to them, devastation befell especially localities which, like Santa Rosa, San José, and Palo Alto with Leland Stanford Junior University, are situated on the loose soil of the valley, whose deepest portions are covered by the bay of San Francisco. The splendidly endowed California University, in Berkeley, and the famous Lick Observatory, both erected on rocky ground, fortunately escaped without any notable damage.
The map sketch (Fig. 15) by Suess represents the earthquake lines of Sicily and Calabria. These districts have, as mentioned before, been devastated by severe earthquakes, of which the most terrible occurred in the year 1783, and again in 1905 and 1907. They have also been the scene of many smaller concussions.
Fig. 15.—Earthquake lines in the Tyrrhenian depression
The bottom of the Tyrrhenian Sea—between Italy, Sicily, and Sardinia—has been lowered in rather recent ages and is still sinking. We notice on the map five dotted lines, corresponding to cracks in the crust of the earth. These lines would intersect in the volcanic district of the Lipari Islands. We further see a dotted circular arc corresponding to a fissure which is regarded as the source of the Calabrian earthquakes of 1783, 1905, and 1907. The earth-crust behaved somewhat after the manner of a windowpane which was burst by a heavy impact from a point corresponding to the Island of Lipari. From this point radiate lies of fracture, and fragments have been broken off from the earth-crust by arc-shaped cracks. The volcano Etna is situated on the intersection of the radial and circular fissures.
Fig. 16.—Seismogram recorded at Shide, Isle of Wight, on August 31, 1898
In recognition of the high practical importance of earthquake observations, seismological stations have in recent days been erected in many localities. At these observatories the earthquakes are recorded by pendulums whose styles draw lines on tapes of paper moved by clock-work. As long as the earth is quiet the drawn line is straight. When earthquakes set in, the line passes into a wavy curve. As long as the movement of the paper is slow, the curve merely looks like a widened straight line. The subjoined illustration (Fig. 16) represents a seismogram taken at the station of Shide, on the Isle of Wight, on August 31, 1898. The earthquake recorded originated in the Centre G, in the Indian Ocean. The origin has been deduced from the moments of arrival of the different waves at different stations. We notice on the seismogram a faint widening of the straight line at 20 hrs. 5 min. 2 sec. (8 hrs. 5 min. 2 sec. P.M.). The amplitude of the oscillations then began to widen, and the heaviest concussions were noticed at 20 hrs. 36 min. 25 sec., and 20 hrs. 42 min. 49 sec., after which the amplitudes slowly decreased with smaller shocks. The first shock of 20 hrs. 5 min. 2 sec. is called the preliminary tremor. This tremor passes through the interior of the earth at a velocity of propagation of 9.2 km. (5-3/4 miles) per second. It would require twenty-three minutes to pass through the earth along a diameter. The tremor is very feeble, which is ascribed to the extraordinarily great friction characteristic of the strongly heated gases which are confined in the interior of the earth. The principal violent shock at 20 hrs. 36 min. 25 sec. was caused by a wave travelling through the solid crust of the earth. The intensity of this shock is much less impaired than that of the just-mentioned tremor, and it travels with the smaller velocity of about 3.4 km. (2.1 miles) along the earth’s surface.
The velocity of propagation of concussion pulses has been calculated for a mountain of quartz, in which it would be 3.6 km. (2.2 miles) per second, very nearly the same as the last-mentioned figure. We should expect this, since the firm crust of the earth consists essentially of solid silicates—i.e., compounds of quartz endowed with similar properties.
Measured at small distances from the origin, the velocity of propagation of the wave appears smaller, and the first preliminary tremor is frequently not observed. The velocity may be diminished to 2 km. (1-1/4 miles) per second. The reason is that the pulse partly describes a curve in the more solid portions of the crust, and partly passes through looser strata, through which the wave travels at a much slower rate than in firm ground; for instance, at 1.2 km. through loose sandstones, at 1.4 km. through the water of the ocean, and at 0.3 km. through loose sand. We recognize that it should be possible to calculate the distance between the point of observation and the origin of the earthquake from the data relating to the arrivals of the first preliminary tremor and of the principal shock of maximum amplitude. The violent shock is sometimes repeated after a certain time, though with decreased intensity. It has often been observed that this secondary, less violent, shock seems to have travelled all round the earth via the longest road between the origin and the point of observation, just like one portion of the aerial waves in the eruption of Krakatoa (compare page 27). The velocity of propagation of this secondary shock is the same as that of the principal shock.