HR: 0830h
AN: S21F-0395    [PDF]
TI: Subsurface Structure of San Leandro From the San Francisco Bay to the Hayward Fault
AU: * Goldman, M R
EM: goldman@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road MS 977, Menlo Park, CA 94025 United States
AU: Catching, R D
EM: catching@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road MS 977, Menlo Park, CA 94025 United States
AU: Rymer, M J
EM: mrymer@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road MS 977, Menlo Park, CA 94025 United States
AU: Gandhok, G
EM: ggandhok@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road MS 977, Menlo Park, CA 94025 United States
AU: Steedman, C E
EM: csteedman@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road MS 977, Menlo Park, CA 94025 United States
AB: The city of San Leandro, California is located in the eastern San Francisco Bay area between the Bay and the East Bay hills (Diablo Mountains). The major known tectonic structures in the immediate San Leandro area are the Hayward fault to the east and the San Leandro basin, a deep sedimentary basin, beneath the western side of the city. To better understand the San Leandro basin, its subsurface fault structures, and the effect of these structures on ground water and earthquake hazards, the U.S. Geological Survey acquired an approximately 10-km-long, high-resolution, combined reflection and refraction seismic imaging profile across the city in June 2002. The seismic profile originated within the waters of the San Francisco Bay and ended at the Hayward fault. Seismic sources were generated by a combination of 400-grain, Betsy-Seisgun blanks in 0.3-m-deep holes and 0.25-to-0.5-kg, buried explosions in 1.5-m-deep holes. The combined spacing of seismic sources was 5 m. The seismic data were recorded on an array of four 60-channel Geometrics Strataview seismographs, with 40-Hz single-element, vertical sensors spaced at 5 m. P-wave velocities range from about 800 m/s at the surface to greater than 2000 m/s at about 100 m depth. Prominent lateral low-velocity areas are evident at several locations along the profile. Reflection images show that the low-velocity areas are largely related to zones of faulting. Near-surface faults are observed on the reflection images, including southwest-dipping faults at the edge of the bay and near-vertical faults within the Hayward fault zone, but the most prominent fault occurs approximately 1 km east of the bay, where it bounds the ~1-km-deep San Leandro basin. Because the near-surface faults are observed within a few meters of the surface and because epicenters of small-magnitude earthquakes correlate with these faults, it is likely that they are active and represent potential hazards. The reflection and velocity images show that some of the near-surface faults vertically offset the upper ground-water aquifer, suggesting that lateral flow of ground water in the East Bay region is affected by the faults.
DE: 7200 SEISMOLOGY
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2003 Fall Meeting