What Is the Role of Thrust Faulting in Bay Area Earthquake Hazards?
Based on ABAG "On Shaky Ground" Reports

Most of the major faults in the Bay Area are strike-slip faults, where the rupture plane extends almost vertically into the ground and the ground on one side slips horizontally past the ground on the other side of the fault. There are, however, several thrust or reverse faults in the Bay Area, where ground moves upward and over adjacent ground. (These faults are more common in southern California than the Bay Area because the San Andreas fault makes a large bend to the west there before heading northwest. Many thrust faults in southern California are caused by this bending.)

In the Bay Area, thrust faults are less well understood than strike-slip faults. However, the U.S. Geological Survey is actively conducting studies of several of these faults or is funding studies by other researchers.

One of the most dangerous Bay Area thrust faults, because of its location near an urban area, is the Monte Vista fault on the western side of the Santa Clara Valley. However, this fault has a long recurrence interval for large earthquakes - on the order of several thousand years. As with other thrust faults, we know generally where the fault is located, but it is difficult to identify the actual surface trace.

We estimate that an earthquake on the Monte Vista fault might generate 15,000 uninhabitable housing units, almost as many as the Loma Prieta earthquake. Notably, 13,500 would be in Santa Clara County, making it as damaging in the county as a magnitude 7.3 on the entire Hayward fault.
The most active thrust fault in the Bay Area is the Mt. Diablo thrust fault. This fault has made Mt. Diablo the fastest growing mountain in the Bay Area.

Each of the seven Great Valley faults identified along the western side of the Central Valley are also thrust faults. The location and size of earthquakes generated by the Great Valley faults are less well understood than for the Monte Vista. The recurrence intervals for earthquakes on segments of this fault system may be as short as approximately 500-600 years, but this estimate is uncertain.

Because the Northridge earthquake was caused by a reverse fault and there was a small thrust component in the Loma Prieta earthquake, it is possible to test various ways to model shaking caused by movement of thrust faults. ABAG is testing such a model in cooperation with researchers at USGS. Interim maps using that model are used to create shaking intensity maps for the Mt. Diablo and Monte Vista thrust faults, as shown below.

ABAG, the Association of Bay Area Governments, is the regional planning and services agency for the nine-county San Francisco Bay Area.
Source - 2003 "On Shaky Ground" documentation prepared by ABAG.


jbp 10/15/03