San Andreas Fault

The San Andreas Fault forms the main strand of the plate boundary, running from the Gulf of California (Baja California, Mexico) north to the region of Cape Mendocino. The fault in the San Francisco Bay region is a largely strike-slip fault running through the Santa Cruz Mountains, the Gulf of the Farallons west of the Golden Gate, through Tomales Bay and Bodega Bay, and north to Fort Ross and Point Arena. Northward of Point Arena, the location and character of the San Andreas Fault is less well known. The fault in this region is locked, exhibiting no creep at the surface and generating very few microearthquakes that are associated with minor slipping at depth. Through the San Francisco Bay Area, the slip rate on the San Andreas Fault is about 20 mm/yr (4/5 inch/year).

The October 17, 1989 Loma Prieta earthquake was the most recent major earthquake associated with the San Andreas Fault. While the earthquake was not produced by the main San Andreas Fault, it occurred on a closely associated blind thrust fault that had formed as a result of a bend in the San Andreas Fault, south of the bay. Although that earthquake struck along a remote segment of the Santa Cruz Mountains, 64 deaths resulted, most from the collapse of the Cypress Freeway in Oakland. About 16,000 homes and apartment units were uninhabitable after the earthquake. The San Francisco-Oakland Bay Bridge was closed for more than a month because of a collapse of a section of its eastern span.

The left bend in the San Andreas Fault in the Santa Cruz Mountains favors thickening of the crust and uplift of the Earth's surface, and is thought to be responsible to the formation of the Santa Cruz Mountains.

The M7.9 April 18, 1906 San Francisco earthquake was the most recent great earthquake on the San Andreas Fault and it ruptured from approximately Cape Mendocino south to San Juan Bautista. The 1906 earthquake was the largest earthquake to strike Northern California in historic times, and is thought to have killed more than 3,000 Bay Area residents. The epicenter of that earthquake is now estimated to be offshore about 2 miles west of San Francisco. The fire following the 1906 earthquake burned 5 square miles of San Francisco and resulted in 225,000 homeless refugees of the earthquake.

A large (magnitude 6.8) earthquake in 1838 is often assumed to have occurred on the Peninsula segment of the San Andreas Fault. To date, however, unambiguous observations placing that earthquake on the San Andreas Fault have not been found.

The 2003 Working Group for California Earthquake Probability assigned a 21% probability that the San Andreas Fault would produce a magnitude 6.7 or larger earthquake in the next 30 years.