San Andreas fault, great fracture (see fault fault, in geology, fracture in the earth's crust in which the rock on one side of the fracture has measurable movement in relation to the rock on the other side. Faults on other planets and satellites of the solar system also have been recognized.
..... Click the link for more information. ) of the earth's crust in California. It is the principal fault of an intricate network of faults extending more than 600 mi (965 km) from NW California to the Gulf of California. The San Andreas fault, a strike-slip fault, also extends vertically at least 20 mi (30 km) into the earth. It is located on the boundary between two sections of the earth's lithosphere—the North American plate and the Pacific plate (see plate tectonics plate tectonics, theory that unifies many of the features and characteristics of continental drift and seafloor spreading into a coherent model and has revolutionized geologists' understanding of continents, ocean basins, mountains, and earth history.
..... Click the link for more information. )—and separates SW California from the North American continent. The Pacific plate is moving northwest in relation to the North American plate, and it is believed that the total displacement along the fault since its formation more than 30 million years ago has been about 350 mi (560 km). Movement along the fault causes earthquakes; several thousand occur annually, although only a few are of moderate or higher magnitude. The destructive San Francisco earthquake of 1906 was caused by a movement in which land surfaces on either side of the fault were displaced horizontally up to 21 ft (6.4 m).

San Andreas Fault

Zone of transform faults at the boundary between two tectonic plates in the western U.S. The fault runs along the coast of northern California for more than 800 mi (1,300 km) and passes seaward in the vicinity of San Francisco. Movement along the fault is characterized by frequent earthquakes, including the major San Francisco quake of 1906, when parts of the fault line moved as much as 21 ft (6.4 m); the less serious earthquake of 1989; and a destructive quake centred in the Los Angeles suburb of Northridge in 1994.