Seismo Blog

Where earthquakes occur in the San Francisco Bay Area

October 7, 2008

Bay Area faults

Faults in the Bay Area (USGS and PG&E)

The slipping and sliding between the Pacific Plate and its continental counterpart, the North American Plate, not only generate most of the seismicity in California (see blog October 3, 2008), they are also the cause for the earthquakes in the Bay Area. While in most of the State the boundary between the two plates is defined solely by the San Andreas Fault, the situation is different in our region. Here the movement of the two plates is not confined to one fault alone. For reasons that are not yet fully understood by Earth scientists, the tectonic slip in the Bay Area is spread over several fault lines which run roughly parallel to each other in a 50 mile wide corridor of seismic danger.

The main strand is of course the San Andreas Fault itself, running under the coastal hills west of San Jose and along the spine of the Peninsula. In Daly City it dips into the ocean, only to appear again at Stinson Beach, carving out the elongated cigar shaped valley of Tomales Bay further to the north. Since the conquest of "Alta California" by Spain in the late 18th century, most of the strong quakes in our region have occured along the San Andreas Fault, such as the Great San Francisco earthquake of 1906 and the Loma Prieta Earthquake of 1989.

The Calaveras Fault branches off from the San Andreas just south of Hollister and then turns north, with the potential to wreak havoc among the towns and cities along the 680 freeway all the way to Walnut Creek and Concord. The Hayward Fault is sandwiched between the two other fault lines. It runs for approximately 50 miles along the foothills of the East Bay. Other significant faults are the Greenville Fault in the Livermore Valley, the offshore San Gregorio Fault which runs through Half Moon Bay, and the Rodgers Creek Fault between San Pablo Bay and Healdsburg.

This widening of the boundary zone between the two plates not only spreads the seismicity over a wide area. It is also responsible for the unique landscape in the Bay Area. Because the three major fault lines take up the plate movement and essentially split the tectonic sliding almost equally amongst themselves, most topographical features in the Bay Area follow the northwesterly trend defined by the plate boundary. Road builders, for instance, conveniently used the valleys formed by the three faults to lay out part of the highway network, like the 280 Freeway on the Peninsula, Highway 13 in Piedmont and Montclair and the 680 corridor in the East Bay. Not only the valleys but also the crestlines of the various hills in the Bay Area follow this northwesterly trend. (hra008)