National Geographic : 1906 May
THE PROBABLE CAUSE OF THE SAN FRAN CISCO EARTHQUAKE* BY FREDERICK LESLIE RANSOME GEOLOGIST, U. S . GEOLOGICAL SURVEY AS these words are written, three days only have passed since San Francisco was shaken by the most destructive earthquake in her history, and the subsequent unpar alleled ruin wrought by fire is not yet ended. In such a stunning dis aster, when communication with the out side world is interrupted, when to the heart-shaking terrors of heaving ground and toppling buildings is added a form of devastation even more appalling, and when the human aspect of the tragedy so overwhelms all other considerations, it is impossible to obtain at once and at a distance from the scene the data neces sary for a satisfactory explanation of the initial catastrophe. The few facts that can be gathered for this purpose from the moving story of destruction, heroism, and fortitude are meager indeed, and the sciolist alone would pretend to find in them adequate material for deducing the real cause of the earthquake. Neverthe less a brief account of the geological history and structure of the region ad jacent to San Francisco Bay may be of interest to those who are not professional geologists or who have never had an op portunity to study for themselves this part of the coast of California. Such an account will show that the present dis aster was not altogether unexpected, and that the rocky structure of the peninsula upon which the city stood-in fact, of the whole Coast Ranges-suggests the prob ability of serious seismic disturbances in the future. It will serve, moreover, as regards this particular catastrophe, to eliminate improbable guesses as to cause and to supply a basis of fact that will aid in the intelligent interpretation of infor- mation which will gradually become more detailed and accurate as excitement sub sides and communications are restored. That the following hastily prepared sketch, involving consideration of so complex a subject as the geology of the Coast Ranges, must in many respects be unsatisfactory and imperfect is of course freely admitted and, under the circum stances, seems hardly to require apology. CAUSES OF EARTHQUAKES Most authorities on earthquakes dis tinguish two main classes-(I) volcanic quakes and (2) tectonic, or dislocation, quakes. The former originate in districts of active vulcanism and at comparatively shallow depth. According to Major C. E. Dutton, the greater number of such shocks are initiated at depths less than 2 miles. They are characterized by a fairly definite centrum, a relatively short radius of influence, and the absence of subordi nate after-quakes. They are phenomena that could probably be closely imitated by the explosion of a large quantity of dyna mite at the bottom of a deep mine. Tec tonic quakes, on the other hand, may originate at greater depth; they usually have indefinite or elongated centra; they are characterized by a greater radius of activity, and the main shock is usually followed by after-quakes. Most of the great destructive earthquakes recorded in history belong to this class. Such, for example, was the Mino-Owari earth quake in Japan, which in 1891 killed over 7,000 people, wounded over 17,000 more, and destroyed more than 200,000 houses. This quake was plainly caused by move ment along a fissure which appeared at the surface as a fault about 70 miles long, *For a comprehensive description of earthquakes the reader is referred to that interesting book, " Earthquakes," by Major Clarence E Dutton, published by G. P. Putnam's Sons.