San Andreas fault, great fracture (see fault) 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)—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).