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 he?ving ground 

 and toppling buildings is ;: Aided 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. 



1 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 189 1 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. 



