Central California/San Francisco Bay Earthquake Hazards Project, 1998

Fence in Bolinas offset by the San Andreas Fault during the 1906 earthquake.

The San Francisco Bay area is located in a broad zone of active crustal deformation driven by the motions of the North American and Pacific plates. Each year, about 5-6 cm (~2 inches) of relative plate motion must be accommodated in the Bay Area. During most years, at a given location only a small fraction of this motion occurs, and the rest is saved up, causing stress build-up. Geologic, geodetic, and geophysical studies show that plate motion is eventually released as slip along the San Andreas fault system, a complex group of sub-parallel faults (e.g., the San Gregorio-Hosgri, San Andreas, Hayward, Rodgers Creek, Concord, Green Valley, Calaveras, and numerous unnamed faults). Periodic seismic slip occurs when the stress build-up exceeds frictional resistance on these faults. Such slip can be dangerous and damaging as evidenced by the 1906 San Francisco and 1989 Loma Prieta earthquakes. The central California earthquake hazards project applies remote sensing techniques to study the subsurface and underwater geology of the San Francisco Bay area, where conventional geologic methods are impossible to apply.

Damage caused by the 1989 Loma Prieta Earthquake in San Francisco

Key to determining future earthquake hazards in the San Francisco region is understanding the physical processes that control the evolution of the San Andreas fault system. We are studying the structure of the Earth's crust in the San Francisco Bay area to:

1. identify and characterize new faults,
2. learn more about how faults interact with each other at depth (helps us assess the influence that slip on one fault has on another), and
3. learn how seismic waves pass through the different types of rocks found at the surface and at depth to establish how much shaking is expected in different parts of the Bay Area.

This information is intended to aid earthquake preparedness in the San Francisco Bay area

New 3-D image of the crust beneath the San Francisco Bay area developed from monitoring the paths that earthquake waves pass through it. Colors correspond with different chunks of the Earth's crust that have been pushed together along the San Andreas and Hayward faults. Earthquakes are shown as yellow dots (see task 1 below for more description about how this model was developed).

From: Parsons, T., and Zoback, M. L., Three-dimensional upper crustal velocity structure beneath San Francisco Peninsula, California, J. Geophys. Res. v. 102, p. 5473-5490.

We are conducting four types of field studies:

Earthquakes in the San Francisco Bay area

1. Deep seismic investigations in the San Francisco Bay area:
   By using an acoustic source at the surface, we can image the Earth in much the same way that a medical ultrasound can see inside the body.
2. Geotechnical investigations: The cores from drill-holes along the bridges across San Francisco Bay are being extensively tested to learn how these soft rocks will respond during an earthquake. This information is especially important along the eastern span of the Bay Bridge, where new construction is planned.
3. Shallow seismic imaging of Bay Area faults:
   Information about the locations, recency of motion, and direction of motion of faults can be gathered from shallow, high-resolution seismic imaging.
4. Airborne geophysical surveys:
   The magnetic signature from different types of rocks can be measured from the air. This technique allows us to identify fault structures in the offshore region where they are difficult to identify. The 1906 San Francisco earthquake is thought to have originated along the offshore part of the San Andreas fault, west of the Golden Gate.

Here is a list of related products.

Project Coordinator: Tom Parsons