The San Andrea Fault is a line running across California, crossing from the Pacific onto land north of San Francisco and extending south to Los Angeles. It makes the line along which two tectonic plates -- THe Pacific and the North American. Its total length, including the portion under the Pacific, is around 810 miles.
Earthquakes are common along the fault and in many cases, the affect of the slippage, which causes the land on one side to slip horizontally relative to the other side, to be visible on the surface.