San Francisco Bay Region Geology and Geologic Hazards

About Quaternary Faults > What is a Fault?

What is a Fault?

A fault is a break in the rocks that make up the Earth’s crust, along which rocks on either side have moved past each other.

Not every crack in the ground is a fault. What defines a fault is the movement of the rock on either side. When that movement is sudden, the released energy causes an earthquake. Some faults are tiny, but others are part of great fault systems along which rocks have slid past each other for hundreds of miles. These fault systems are the boundaries of the huge plates that make up the Earth's crust. In the San Francisco Bay region, the Quaternary-active faults are part of the boundary between the Pacific and North American plates.

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This photo shows a small fault in San Mateo County. Matching layers across the fault shows that it has offset the sandstone (lighter layers) and shale (darker layers) about ten feet.

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Some major faults can even be seen from space, as shown by this LANDSAT image of the San Andreas Fault in southern California.


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Page Last Modified: 8/18/2006