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Southern California Geology


Major Faults of Southern California

San Andreas Fault System in the Inland Empire and Salton Trough

The Inland Empire and Salton Trough regions of San Bernardino and Riverside Counties have numerous faults that at first glance appear to have no coherent patterns or relations to each other. However, geologic research over the last 40 years has shown that the distribution and behavior of these faults can be explained if they are viewed as components of a larger San Andreas Fault System.
San Andreas fault system in S. CA.
Click to enarge image.

By fault system we mean a network of faults and folds whose orientation, movement style, and long-term geologic history result from the interactive nature of the entire network. Each element in the network plays a role as the earth's crust in southern California responds to stresses generated within the Pacific-North American plate boundary. The San Andreas Fault itself is just one component of this fault network. This master fault is a right-lateral strike-slip fault that is flanked by other right-slip, left-slip, contractional, and extensional fault zones and associated fold belts.
The Inland Empire-Salton Trough region is geologically and seismologically the most complex part of the San Andreas Fault system in southern California. Here, several strands of the master San Andreas Fault have succeeded each other through the last 15 million years, each strand in turn continuing the inexorable northwestward movement of coastal California relative to the desert interior. Complexities in this right-lateral-shear history have caused subsidiary fault zones to develop—some sharing the right-lateral strike-slip role of the San Andreas itself, others developing where the earth's crust has been squeezed (contractional faults and fold belts) or stretched (extensional faults).

Click here to start your tour of the San Andreas Fault System in the Inland Empire and Salton Trough regions of southern California.


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