HR: 0800h
AN: S31A-1023    [Abstracts]
TI: San Francisco Bay Area Velocity Structure From Controlled-Source Seismic Refraction Imaging
AU: * Goldman, M R
EM: goldman@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94025 United States
AU: Catchings, R D
EM: catching@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94025 United States
AU: Steedman, C E
EM: steedman@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94025 United States
AU: Gandhok, G
EM: gandhok@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94025 United States
AU: Boatwright, J
EM: boat@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94025 United States
AU: Rymer, M J
EM: mrymer@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94025 United States
AB: To better understand the velocities and structures of the crust and upper mantle in the San Francisco Bay area, we developed 2-D tomographic velocity models along four seismic refraction profiles acquired along and across the bay area in the early 1990's. The four profiles extended from (1) Hollister to Inverness along the San Francisco and Marin Peninsulas (~200 km long), (2) Hollister to Santa Rosa along the East Bay (~220 km long), (3) the Pacific Ocean to Livermore crossing the bay (~100 km long), and (4) the Pacific Ocean to the western Santa Clara Valley (~25 km long), centered on the epicenter of the1989 M. 6.9 Loma Prieta earthquake. Velocity models were not previously developed for three of the seismic profiles, and the previously developed model for the fourth profile (Catchings and Kohler, 1996) did not include some of the currently available seismic data. The profiles along the bay image structures from the near surface to about 25 km depth, and they show velocity anomalies associated with the major faults (San Andreas, Hayward, Rodgers Creek, Calaveras) and basins along the profile. Velocities range from about 2 km/s in the basins to about 7 km/s at the Moho, which dips southward along both sides of the bay. The cross bay profile shows velocity anomalies associated with six fault zones between the Pacific Ocean and the Livermore Valley and higher upper-crustal velocities (~6.2 km/s) between the San Andreas and Hayward faults than to the southwest (~5 km/s) or northeast (~4 km/s) of those faults. The Loma Prieta profile shows velocities ranging from 2 km/s to 6 km/s in the upper 5 km, with the highest velocities in the epicentral region of the 1989 Loma Prieta earthquake. A pronounced, northeast-dipping, low-velocity zone is located beneath the surface expression of the San Andreas fault zone, but other fault zones along the profile show high-velocity anomalies beneath their surface expressions. Collectively, the velocity images show the complexity of the crustal structure and provide constraints for 3-D tomographic models developed for the San Francisco Bay area.
DE: 7218 Lithosphere and upper mantle
DE: 7230 Seismicity and seismotectonics
DE: 6982 Tomography and imaging
DE: 0905 Continental structures (8109, 8110)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting