San Francisco Bay Region Geology and Geologic Hazards
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Hayward - Creeping Fault
The easiest faults to find (aside from one that just ruptured in a large earthquake) are those that creep. Fault creep occurs when some or all of the fault plane is not locked by friction, and rocks on either side of the fault are able to slide along slowly in response to the forces driving the fault. Although the rate of creep is measured in millimeters per year, after a few years the sliding can leave a noticeable offset in structures built across the fault, especially those originally built straight such as curbs and walls.
Some creeping faults are totally unlocked, and so do not build up the stress required to generate a large earthquake. However others, like the Hayward Fault, are only partially unlocked. These creep at a fraction of the rate that the Earth's crust on either side is moving (this rate is now carefully measured using Global Positioning), and so geologists believe they are locked at depth, and building up towards future earthquakes. The Hayward Fault generated a major earthquake in 1868.
Learn more about the Hayward Fault at the following sites: