GENERALIZATIONS CONCERNING EARTHQUAKES 15 



as the geologic evidence is that this country has passed 

 the maximum stage of its seismic activity, and is 

 now on the down curve towards quiescence, as will be 

 shown. 



THE LINES OF GREATEST SEISMICITY 



There are certain great rift lines exposed at the sur- 

 face along which the great deep-seated shocks become 

 manifest. Such earthquake movements are local and 

 do not simultaneously extend all along the rift line as 

 is popularly supposed. The entire length of the line of 

 faulting may represent the summation of many of 

 these movements which have taken place, little by lit- 

 tle, in past times. The entire line along which such 

 movements have occurred may be designated as a line 

 of seismicity. 



The San Andreas Rift is usually considered to repre- 

 sent the surface line of greatest seismicity in Cali- 

 fornia. Its path is supposedly a bar sinister which 

 begins somewhere beyond Alaska and runs diagonally 

 across the fair state of California from north of San 

 Francisco to near Yuma, where it crosses the boundary 

 line and continues indefinitely southeast into Mexico. 

 Its landward extent across the State of California, 

 after it comes ashore in Humboldt County sixty miles 

 west of north of San Francisco, is, first, east of south 

 to a point in the Piru Wilderness, some miles north 

 of Ventura, which portion constitutes a division that 

 we herein call the Northern section. From thence 

 its almost rectilinear direction is southeast through 

 Tejon, Cajon and San Gorgonio Passes into the Colo- 

 rado Desert beyond Banning, and thence on, less 

 definitely, through the Salton Sea towards Yuma, and 

 thence on into Mexico. 



