. A history of the earthquake and fire in San Francisco; an account of the disaster of April 18, 1906 and its immediate results. es for hundreds ofmiles. These cracks are known as faults, and occurmost frequently between rocks of different charac-ter. The rock masses are thus similar, in a way, tothe stones in an arch. In the earths formative period whole mountainranges were made by the uplifting of the rock onone side of such faults above that on the other. Butnow such movements are trivial ones—a few feetmore or less—and are felt as earthquakes. What theultimate cause of such movements is ca
. A history of the earthquake and fire in San Francisco; an account of the disaster of April 18, 1906 and its immediate results. es for hundreds ofmiles. These cracks are known as faults, and occurmost frequently between rocks of different charac-ter. The rock masses are thus similar, in a way, tothe stones in an arch. In the earths formative period whole mountainranges were made by the uplifting of the rock onone side of such faults above that on the other. Butnow such movements are trivial ones—a few feetmore or less—and are felt as earthquakes. What theultimate cause of such movements is can only be amatter of speculation. It has been suggested thatthe seeping of water through the faults, either fromthe surface or from the oceans, to the molten massesof the earths interior, causes explosions; but theview generally accepted is that these slight shiftingsare the result of changes brought about by the slowcontraction of the earth in its gradual cooling; thatas the earth cools and its crust shrinks, strains areproduced and the weak places slip, just as, when ahouse is shaken by a storm, the plaster cracks in. Modeled by Prof. A. C. Lawson Relief Map of the San Francisco Peninsula 34 THE FAULT LINE the old cracks that have been patched. These faultsare the weak places—the old cracks. Volcanoes aresometimes called safety valves; the faults, similarly,may be considered slip-joints where the earthsminor adjustments are made. The Pacific Coast, from Alaska to SouthAmerica, is, geologically speaking, a weak place ofthis sort; as a result earthquakes are frequent. Verysevere shocks have been experienced in Alaska,Mexico, Central America, Colombia, and Ecuadorsince 1899. The California coast has had no severeshock between 1868 and 1906. Professor F. Omori, head of the Department ofSeismology of the University of Tokio, and inventorof the Omori seismograph, is regarded as the great-est living authority on earthquakes. He spent twomonths in California, studying the fault
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