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Aerial photo of the San Andreas Fault in the Carrizo Plain


The San Andreas Fault is a continental transform fault that extends roughly 1300 km (810 miles) through California.

It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizontal).

The fault divides into three segments, each with different characteristics and a different degree of earthquake risk, the most significant being the southern segment, which passes within about 35 miles of Los Angeles.
Vasquez Rocks in Agua Dulce, California are evidence
of the San Andreas Fault line and part of the 2,650 mile Pacific Crest Trail.

The fault was first identified in 1895 by professor Andrew Lawson from UC Berkeley who discovered the northern zone. It is named after San Andreas Lake, a small body of water that was formed in a valley between the two plates. Following the 1906 San Francisco earthquake, Lawson concluded that the fault extended all the way into southern California.

The Pacific Plate, to the west of the fault, is moving in a northwest direction while the North American Plate to the east is moving toward the southwest, but relatively southeast under the influence of plate tectonics. The rate of slippage averages about 33 to 37 millimeters (1.3 to 1.5 in) a year across California.
The San Andreas (reds and orange) and its
major "sister" faults within the San Francisco Bay Area.

Assuming the plate boundary does not change as hypothesized, projected motion indicates that the landmass west of the San Andreas Fault, including Los Angeles, will eventually slide past San Francisco, then continue northwestward toward the Aleutian Trench, over a period of perhaps twenty million years.

The San Andreas began to form in the mid Cenozoic about 30 Mya (million years ago), while The main southern section of the San Andreas Fault proper has only existed for about 5 million years.
The Mormon Rocks within Cajon Pass show the
physical movement of the San Andreas fault in southern California.

The Southern segment of the Fault, which stretches from Parkfield in Monterey County all the way down to the Salton Sea, is capable of an 8.1 magnitude earthquake. At its closest, this fault passes about 35 miles to the northeast of Los Angeles. Such a large earthquake on this Southern segment would kill thousands of people in Los Angeles, San Bernandino, Riverside, and surrounding areas, and cause hundreds of billions of dollars in damage.
History of the San Andreas Fault, Gaspar de Portola,
Andrew Lawson, Alfred Wegener and Harry Hess

Currently, it is believed that the modern San Andreas will eventually transfer its motion toward a fault within the Eastern California Shear Zone. 

A project called the San Andreas Fault Observatory at Depth (SAFOD) near Parkfield, Monterey County, is drilling into the fault to improve prediction and recording of future earthquakes.

wikipedia

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