THE SAX FRANCISCO EARTHQUAKE 101 



study of natural phenomena, was at first individual only, but after- 

 ward was aided by organization. Committees were appointed by 

 various professional societies, national and local, and were charged 

 with the investigation of specific structural questions, and the results 

 of their labors will find place not only in the transactions of the 

 societies, but in revised building - regulations and in important modifica- 

 tions of municipal plants for lighting and water supply. Various 

 bureaus of the national government have also taken part in the struc- 

 tural studies, sending experts to San Francisco and other localities of 

 exceptional earthquake violence. 



The Japanese government promptly sent to California a committee 

 of investigation headed by Dr. Omori, professor of seismology in the 

 University of Tokyo, and composed otherwise of architects and engi- 

 neers. The first conference of these visitors with the state commission 

 warranted the suggestion that we may find it as profitable to follow 

 Japanese initiative in the matter of earthquake-resisting construction 

 as in that of army hygiene. 



The following sketch of the physical features of the earthquake is 

 based chiefly on the body of data gathered by the State Commission: 



An earthquake is a jar occasioned by some violent rupture. Some- 

 times the rupture results from an explosion, but more commonly from 

 the sudden breaking of rock under strain. The strain may be caused 

 by the rising of lava in a volcano or by the forces that make mountain 

 ranges and continents. The San Francisco earthquake of April 18 had 

 its origin in a rupture associated with mountain-making forces. A 

 rupture of this sort may lie a mere pulling apart of the rocks so as to 

 make a crack, but examples of that simple type are comparatively rare. 

 The great majority of instances include not only the making of a crack 

 but the relative movement or sliding of the rock masses on the two 

 sides of the crack ; that is to say, instead of a mere fracture there is a 

 geologic fault. After a fault has been made its walls slowly become 

 cemented or welded together, but for a long time it remains a plane 

 of weakness, so that subsequent strains are apt to be relieved by renewed 

 slipping on the same plane of rupture, and hundreds of earthquakes 

 may thus originate in the same place. From the point of view of the 

 geologist the displacements of rock masses are the primary and impor- 

 tant phenomena: the faults are incidental phenomena, of great value as 

 indices of the displacements ; and the earthquakes are of the nature of 

 symptoms, serving to direct attention to the fact that the great earth 

 forces have not ceased to act. 



A faulting may occur far beneath the surface and be known only 

 by the resulting earthquake; but some of the quake-causing ruptures 

 extend to the surface and thus become visible. The Xew Madrid and 

 Charleston earthquakes are examples of those having deep-seated 



