Fig. 13.—Earthquake lines in lower Austria
The smaller earthquakes, of which not less than 30,000 are counted in the course of a year, do not stand in any closer relation to volcanic eruptions. This is also the case for a number of large earthquakes, among which we have to count the San Francisco earthquake.
It is assured with good reason that earthquakes are often produced at the bottom of the sea, where there is a strong slope, by slips of sedimentary strata which have been washed down from the land into the sea in the course of centuries. Milne believes that the seaquake of Kamaïshi of June 15, 1896, was of this character. Concussions may even be promoted by the different loading of the earth resulting from the fluctuations in the pressure of the air above it.
Smaller, though occasionally rather violent, earthquakes are not infrequent in the neighborhood of Vienna. On the map (Fig. 13) we see three lines. The line A B is called the thermal line, because along it a number of hot springs, the thermæ of Meidling, Baden, Vöslau, etc., are located, which are highly valued; the other line B C is called the Kamp line, because it is traversed by the river Kamp; and the third B F is called the Mürz line, after the river Mürz. The main railway-track between Vienna and Bruck follows the valleys of A B and E F.
These lines, which probably correspond to large fissures in the earth-crust, are known as sources of numerous earthquakes. The district about Wiener Neustadt, where the three lines intersect, is often shaken by violent earthquakes; some of their dates have been marked on the map.
The curve which is indicated by the letters X X on the map marks the outlines of an earthquake which started on January 3, 1873, from both sides of the Kamp line. It is striking to see how the earthquake spread in the loose ground of the plain between St. Pölten and Tulln, while the masses of rock situated to the northwest and southeast formed obstacles to the propagation of the earthquake waves.
Fig. 14.—Library building of Leland Stanford Junior University, in California, after the earthquake of 1906. The photograph shows the great strength of iron structures in comparison to the strength of brickwork. The effect of the earthquake on wooden structures can be seen in Fig. 11
Similar conclusions have been deduced from the study of the spreading of the waves which destroyed Charleston, South Carolina, in 1886. Twenty-seven lives were destroyed by this shock. It was the most terrible earthquake that ever visited the United States before the year 1906. In the Charleston concussion the Alleghany Mountains proved a powerful bar against the further propagation of the shocks, which all the more easily travelled in the loose soil of the Mississippi Valley. In San Francisco, likewise, the worst devastation fell upon those parts of the town which had been built upon the loose, partly made ground in the neighborhood of the harbor, while the buildings erected on the famous mountain ridges of San Francisco suffered comparatively little damage, in so far as they were not reached by the destructive fires. As regards the destructive effects of the earthquake in San Francisco, the building-ground of that city has been divided into four classes (the first is the safest, the last the most unsafe)—namely: 1. Rocky soil. 2. Valleys situated between rocks and filled up by nature in the course of time. 3. Sand-dunes. 4. Soil created by artificial filling up. This latter soil "behaved like a semiliquid jelly in a dish," according to the report of the Earthquake Commission.
For similar reasons the sky-scrapers, constructed of steel on deep foundations, stood firmest. After them came brick houses, with well-joined and cemented walls on deep foundations. The weakness of wooden houses proved mainly due to the poor connection of the beams, a defect which might easily be remedied. The superiority of the steel structure will be apparent from the illustrations (Figs. 11 and 14).