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SOUTHERN CALIFORNIA GEOLOGY 



SUB-RECENT PARALLEL FAULTS 

 San Andreas Rift, near Pettiway 



lines. These vibrations are often noticeable over great 

 areas. 



The movement along the fault line is not the ultimate 

 cause of the earthquake, however. This is something 

 deeper and less explicable. It is most probably some 

 great mass-movement within the plastic sub-crust — a 

 jarring, vertical settling or horizontal movement of a 

 segment of the rocks of the outer zone of fracture, like 

 that which takes place when one piece of floating ice 

 is broken from another. 



Owing to this relationship between the earthquakes 

 and fault structures it becomes very necessary for one 

 who wishes to understand the subject of earthquakes 

 in relation to Southern California also to appreciate 

 the origin, occurrence and history of its extensive sys- 

 tems of faulting. 



