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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----------------------------------------fault
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