SAN ANDREAS FAULT, CALIFORNIA

Partial Boundary Between the Pacific and North America Plates

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The low cliff at the mission at San Bautista, California, was formed by movement of the San Andreas Fault. This was the southern limit of ground breakage during the 1906 earthquake.

The San Andreas Fault runs for eight hundred miles from the redwood forest of Cape Mendocino southward to to the Sonoran Desert and ends south of the Salton Sea. Along the way it passes through dozens of communites and close to two of the largest cities in the United States, San Francisco and Los Angeles. It is most readily apparent as an unusual alignment of river systems and valleys. In Northern California, the Gualala and Garcia rivers lie along it. In central California, the fault runs through narrow Bear Valley east of Point Reyes and through the fault's namesake, the San Andreas Valley. In the southern part of the state, Cuddy Valley and Leona Valley and arid Lone Pine Canyon mark its trend. It is responsible for the I-5 corridor through Tejon Pass, the I-15 corridor through Cajon Pass and the I-10 corridor through  San Gorgonio Pass, as well as for several desert oases near Palm Springs.

Lost Lake in Cajon Pass is a sag pond that has formed along the San Andreas Fault in Cajon Pass north of San Bernadino.

This mud puddle just east of Lost Lake in Cajon Pass marks a trace of the San Andreas Fault, where the fault has disrupted the flow of groundwater and forced it to the surface.

The San Andreas Fault is the main strand of a system of faults that runs across California. Other strands include the Hollywood Fault, the Calaveras Fault and the Hayward Fault.

The sudden rise along Vine Street north of the Capitol Records Building in Hollywood was formed by and marks the trace of the Hollywood Fault.

Offset in a sidewalk in Hollister, California, was caused by slow continuous movement along the Calaveras Fault (a movement known as fault creep).

Fault creep along the Hayward Fault is evident as the sigmoidal patterns in the brickyard at Contra Costa College.

The San Andreas Fault is often erroneously described as being the boundary between the Pacific and the North America plates. Measurements show that about 75 percent of the relative motion between the Pacific and the North America plates occurs along the San Andreas system of faults. The rest occurs farther east.

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About 20 percent of the relative motion occurs along the Walker Lane Seismic Zone that runs close to the California-Nevada border from Lake Tahoe through Death Valley and into the Mojave Desert. The remainder of the motion occurs along the Intermountain Seismic Belt that includes the Wasatch Fault east of the Great Salt Lake and may extend south into the Rio Grande Rift that bisects, from north-to-south, New Mexico.

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And so the boundary between the Pacific and North America plates is actually the western third of the United States.