GENERALIZATIONS CONCERNING EARTHQUAKES 33 



of which almost all are marked as "probable" live 

 faults. These lines all belong to one directional fault 

 group, instead of to five or six v^idely separated ones 

 as do the faults of Southern California. The system 

 to v^'hich the northern faults belong is that of the 

 Second Northwest Group and specifically the San 

 Andreas zone, as described in the later chapter of this 

 paper, although individual names are there given to its 

 several members. This narrow zone of faults close to 

 the sea constitutes the line of maximum seismicity in 

 Northern California. (See Plate III.) 



The several faults of Northern California are in gen- 

 eral parallel to the seashore, instead of being widely 

 separated and distributed throughout a cross-section 

 of three hundred miles as in Southern California. 

 Furthermore, the zone or belt in Northern California 

 lies closely adjacent to the seashore and not far from 

 the abyssmal scarp, in positions where the earth 

 movements are more liable to occur than if they were 

 inland, while the City of Los Angeles and most of the 

 fault lines of Southern California lie at considerable 

 distance from the sea. 



Owing to the narrowness of the northern zone, its 

 proximity to the sea and the nature of the geologic 

 materials, the stresses are apt to be concentrated along 

 it, and the relief shocks which accompany them are 

 relatively more severe. 



In Southern California, on the other hand, as will be 

 shown in the next part of this work, the fault frac- 

 tures belong to, at least, four distinct ages and direc- 

 tional systems, which are widely distributed through- 

 out a width of over two hundred miles. They also 

 cross one another so that the fractures are checker- 



