HR: 17:00h
AN: S34A-05    [Abstracts]
TI: Seismic Velocity Structure and Seismotectonics of the Hayward Fault System, East San Francisco Bay, California
AU: * Hardebeck, J L
EM: jhardebeck@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd MS 977, Menlo Park, CA 94025 United States
AU: Michael, A J
AF: US Geological Survey, 345 Middlefield Rd MS 977, Menlo Park, CA 94025 United States
AU: Brocher, T M
AF: US Geological Survey, 345 Middlefield Rd MS 977, Menlo Park, CA 94025 United States
AB: The Hayward Fault is considered the most likely fault in the San Francisco Bay Area, California, to have a major earthquake in the next 30 years, posing a serious earthquake risk to more than 2 million people. In order to accurately evaluate various earthquake scenarios for this fault, it is important to understand its structure, kinematics, and physical properties. We present a new seismological study of the Hayward Fault system, including a new 3D seismic velocity model for the East San Francisco Bay, relocated earthquake hypocenters, and improved focal mechanisms. We use these new constraints on structure and seismicity to study the geometry and kinematics of the Hayward Fault. The new East Bay 3D tomography model, based on travel times from earthquakes and controlled-source experiments, reveals a clear velocity contrast across the Hayward Fault. In the upper 10 km, the P-wave velocity in the Franciscan rocks to the west are up to 0.8 km/s faster than in the Great Valley sequence rocks to the east. Below 10 km, where Franciscan rocks are thought to be present on both sides of the fault, there is negligible contrast. The observed P-wave velocities are comparable with velocities observed in deep boreholes in the East Bay. Anomalously low S-wave velocities are observed east of the Hayward Fault, near the Livermore Basin. We relocated more than 20,000 East Bay earthquakes, 1967-2004, with the 3D model. The events illuminate the Hayward Fault at depth, shifting from near-vertical in the north to steeply east-dipping in the south. New focal mechanisms were also computed, using take-off angles from ray tracing in the 3D seismic velocity model. Previous authors found heterogeneous focal mechanisms along the Hayward Fault near San Leandro, interpreted it as a zone of complex fracturing, and speculated that San Leandro marks a probable boundary for major Hayward Fault earthquakes. We find, however, that our high-quality focal mechanisms for events all along the Hayward Fault are consistent with the large-scale orientation and sense of slip of the fault, including those near San Leandro.
DE: 8180 Tomography
DE: 7230 Seismicity and seismotectonics
DE: 7205 Continental crust (1242)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting