File:San Andreas Fault, Carrizo Plain National Monument, California (21394880568).jpg

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.

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. In 1953, geologist Thomas Dibblee astounded the scientific establishment with his conclusion that hundreds of miles of lateral movement could occur along the fault.

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.

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 southwestward motion of the North American Plate towards the Pacific is creating compressional forces along the eastern side of the fault. The effect is expressed as the Coast Ranges. The northwest movement of the Pacific Plate is also creating significant compressional forces which are especially pronounced where the North American Plate has forced the San Andreas to jog westward. This has led to the formation of the Transverse Ranges in Southern California, and to a lesser but still significant extent, the Santa Cruz Mountains (the location of the Loma Prieta earthquake in 1989).

Studies of the relative motions of the Pacific and North American plates have shown that only about 75 percent of the motion can be accounted for in the movements of the San Andreas and its various branch faults. The rest of the motion has been found in an area east of the Sierra Nevada mountains called the Walker Lane or Eastern California Shear Zone. The reason for this is not clear. Several hypotheses have been offered and research is ongoing. One hypothesis - which gained interest following the Landers earthquake in 1992 - suggests the plate boundary may be shifting eastward away from the San Andreas towards Walker Lane.

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.

en.wikipedia.org/wiki/San_Andreas_Fault