A crack in the Earth's crust, almost 1000 km long, running from the San Bernardino area north past San Francisco. The famous San Francisco fire of 1906 was caused by a slippage of this fault line, along the SAFZ (San Andreas fault zone).

The San Adreas is a strike-slip fault, meaning the two sides move in opposition - see liha's writeup below for the mechanics. In the great 1906 earthquake the displacement along the fault was 21 feet near Point Reyes.

The fault's major stress point is thought to be Black Mountain near Palo Alto, where the fault line changes direction by about 10 degrees.

The San Andreas Fault is a right-lateral strike-slip fault meaning if a person stands on one side of the fault, the opposite side appears to have moved to the right.

The Pacific Plate is moving towards the northwest, while the North American Plate is moving due west. So, whenever there is a major earthquake along this fault, the west side of the fault is offset to the north. This is hard to imagine, however the evidence is obvious from aerial photographs and topographic maps or just observing the region in person. Rivers, fences and roads that were once straight are now offset by many meters. The 1906 San Francisco earthquake caused an offset of several yards, including the road at the head of Tomales Bay which was offset by 21 feet! A simplified, plan-view example of an offset road is pictured below..

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