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Generalized Geologic Map of the Conterminous United States
North America Shaded Relief
 

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  The North American Tapestry of Time and Terrain

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San Andreas Fault

  Map of San Andreas Fault

The topographic texture of western California is controlled by the San Andreas fault system, the tectonic expression of the Pacific plate sliding northwestward along the western margin of the North American plate. Hundreds of miles long and up to a mile wide, the San Andreas Fault Zone has been active since its original development in the Tertiary.

About 10 percent of the present plate motion is compressional, which means horizontal forces are shortening and wrinkling the crust along the fault zone. This movement has created the parallel coastal northwest-southeast mountain ranges such as the Coastal Ranges along California's central coast. Comparatively quiet during the period between the 1906 San Francisco earthquake and the 1989 Loma Prieta event, the fault is again showing activity.