Bay Area Faults and Earthquakes

The San Francisco Bay Area is part of the complex plate boundary system between the Pacific and Northern American Plates. Although the San Andreas Fault is the dominate expression of the collision between these plates in California, the picture is somewhat complex in the Bay Area.

Near Hollister, the Calaveras fault branches off from the San Andreas fault toward a more northerly direction, and further north, the Hayward fault branches off from the Calaveras toward the northwest. At a finer scale, numerous thrust faults parallel and cross the San Andreas fault in the greater San Francisco Bay area.

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Faults	Recent Seismicity	Historical Earthquakes

These figures illustrate the location of major faults in the Bay Area. The fault segments which have ruptured in historic time are denoted in red. Holocene ruptures are drawn in orange and Quaternery ruptures are drawn in black. The faults are drawn with solid lines where known and dashed and dotted lines where inferred. The location of some major cities as well as Bay Area bridges are also indicated.

Although most of the present-day seismicity in Bay Area generally follows the major faults (San Andreas, Hayward-Mission Creek, Concord-Calaveras, and Antioch faults), a significant fraction occurs in the large (8 km) right-step-over region between the Calaveras and Concord faults and along the Mt. Lewis seismic zone. In addition, numerous earthquakes locate in the regions between the major faults and seismic zones and appear to lie on either the minor faults or on unmapped faults.

The San Francisco Bay Area has experienced a number of large magnitude earthquakes over the last 150 years, including the well-known 1906 San Francisco earthquake. The following table of historical seismicity is taken from the catalog compiled by Dr. William Ellsworth in the USGS Professional Paper 1515 - "The San Andreas Fault System, California". It includes an earthquake in 1836 which has recently been called into question. Recent work [Toppozada & Borchardt, Re-evaluation of the 1836 "Hayward" and 1838 San Andreas Fault Earthquakes, Bulletin of the Seismological Society of American, 1998] evaluating historical records (such as diaries and damage to adobe buildings) indicates that this earthquake did not occur on the Hayward fault and that the event was located near Monterey. These results are supported by recent trenching studies of the northern Hayward fault which indicate that this segment has not ruptured since the mid 18th-century.

For additional information on California earthquake history, visit the California Earthquakes 1769-1994 site hosted by the Museum of the City of San Francisco.