HR: 0800h
AN: S31A-1021    [Abstracts]
TI: Newly Recognized Fault Structures and Earthquake Hazards in the Western Santa Clara Valley, San Francisco Bay Area, California
AU: * Catchings, R D
EM: catching@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94587 United States
AU: Goldman, M R
EM: goldman@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94587 United States
AU: Gandhok, G
AF: U.S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94587 United States
AU: Steedman, C E
AF: U.S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94587 United States
AU: Rymer, M J
AF: U.S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94587 United States
AB: A combined reflection and refraction imaging investigation from the San Andreas Fault to the central Santa Clara Valley, California shows the complex structures and tectonics of the Santa Cruz Mountains and the western Santa Clara Valley. Our seismic images show numerous faults within the Santa Cruz Mountains, consistent with surface geologic mapping. Because of the high velocities (> 6.4 km/s) in the upper 5 km, we interpret steeply northeast-dipping reflectors within basement in the Santa Cruz Mountains to be part of the Coast Ranges Ophiolitic rocks. Within the Santa Clara Valley, offset reflectors and folded Pre-Pliocene strata suggest multiple faults beneath the alluvium of the western Santa Clara Valley, and seismicity recorded over the past 30 years show that these faults are active. Some of these faults extend to the shallowest reflectors imaged (~20 m), including the base of the ground-water aquifer system, suggesting at least post-Pliocene movement on faults at shallow depths. The seismic image suggests that the principal fault zone beneath the western Santa Clara Valley is about 6 km wide in the uppermost crust but tapers to about 4 km in width at 5 km depth, similar to known flower structures elsewhere. Basement velocities are low (4 to 5 km/s) beneath this western Santa Clara fault zone relative to those velocities (~6 km/s) along the remainder of the seismic profile, an observation seen in a nearby seismic profile across the San Andreas fault zone. The high-population density within the valley and the proximity of these buried faults makes the earthquake hazard in the western Santa Clara Valley potentially high. This potential hazard is compounded by the thick sequence (~1.6 km) of low-velocity (< 4 km/s) sediments within the Cupertino Basin that can amplify seismic waves generated by movement on regional and/or local faults.
DE: 7218 Lithosphere and upper mantle
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
DE: 6982 Tomography and imaging
DE: 0905 Continental structures (8109, 8110)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting