Pacific Coastal & Marine Science Center
Coastal and Marine Earthquake Studies
|Central California/San Francisco Bay Earthquake Hazards Project, 1998
Controlled Source Imaging
A compressed air seismic source is towed behind a research vessel. The return signals (reflections from geologic structures) from the air bursts are recorded on the bottom of the bay, using sensitive recording arrays.
Deep seismic studies of the San Francisco Bay area are focused on the relationship between the San Andreas and Hayward faults. It has been suggested that a flat fault may connect these two vertical strike slip faults at depth as diagramed below:
These cross-section diagrams show the key problems addressed by deep seismic studies. Our upcoming field work in San Francisco Bay (September 1997) will address the depth that the Hayward and San Andreas faults extend into the Earth. In scenario (A) above, reflections from our acoustic source are recorded on both sides of the fault, implying a more shallow Hayward fault. In scenario (B) above, reflections are truncated by the Hayward fault, implying a deeper fault. The rupture depth of an earthquake can play an important role in the amount of shaking experienced at the surface. The depth that the vertical faults extend can tell us whether or not they are connected by a horizontal fault.
See full-size image (37 kb).
Example of reflections from beneath San Francisco Bay.
From Brocher, T.M., McCarthy, J., Hart, P.E., Holbrook, W.S., Furlong, K.P., McEvilly, T. V., Hole, J.A., and BASIX Working Group, 1994, Seismic Evidence for a Possible Lower-Crustal Decollement Beneath San Francisco Bay, Science, v. 265, p. 1436-1439.
See more about Earthquake Tomography.