Based on ABAG “On Shaky Ground” Reports
The fact that a devastating earthquake occurred in 1906 — the San Francisco earthquake — is common knowledge. Larger earthquakes generally affect larger areas; the San Francisco earthquake caused extensive damage in Oakland, San Jose and Santa Rosa. More recently, the 1989 Loma Prieta earthquake caused extensive damage in the Santa Cruz Mountains, as well as in Oakland and San Francisco tens of miles away. But many moderate to great earthquakes (over magnitude 6.0) have affected the Bay Area; 22 such events have occurred in the last 160 years — for an average of one every seven years.
Graphic Source: U.S. Geological Survey
Note that the level of earthquake activity in the last 20 years is closer to the period prior to the 1906 San Francisco earthquake, while the 1911 to 1979 period, when most of the Bay Area developed, is exceptionally quiet.
Earthquakes occur in the Bay Area when forces underground cause the faults beneath us to rupture and suddenly slip. If the rupture extends to the surface, we see movement on a fault (surface rupture). But strong earthquakes can occur when the fault rupture does not extend to the surface. The fault rupture of the ground generates vibrations or waves in the rock which we feel as ground shaking. Because faults are weaknesses in the rock, earthquakes tend to occur over and over on these same faults. Almost all of the major faults in the Bay Area are strike-slip faults where the rupture extends almost vertically into the ground and the ground on one side moves past the ground on the other side of the fault.
Thrust faults, where ground moves over adjacent ground, are much more common in the Los Angeles area than the Bay Area because the San Andreas fault makes a large bend to the west there before heading northwest. Thrust faults in southern California are caused by this bending.
To view a map of active faults in the Bay Area, click here. (Adobe Acrobat PDF)