HR: 0830h
AN: S21F-0403    [PDF]
TI: Seismic Imaging Evidence for the Extension of the Silver Creek Fault from the Southern Santa Clara Valley into the East Bay
AU: * Steedman, C E
EM: csteedman@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, CA 94025 United States
AU: Catchings, R D
EM: catching@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, CA 94025 United States
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: Rymer, M J
EM: mrymer@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, CA 94025 United States
AB: The San Francisco Bay is bounded by several strike-slip faults of the San Andreas fault system, including the San Andreas fault to the west and the Hayward and Calaveras faults to the east. Other faults in this complex system, however, are less well-mapped. The northwest-southeast trending Silver Creek fault (SCF) has been mapped in the southern Santa Clara Valley and seismically imaged in the northern part of the valley. Two seismic reflection profiles, gravity data, and water-well data, obtained between the Santa Clara Valley and Fremont, suggest that the SCF extends farther northwest of San Jose into the East Bay. To investigate the possible northwestward extension of the SCF into the East Bay, the USGS High Resolution Seismic Imaging Group acquired two high-resolution, combined seismic reflection and refraction profiles in the Fremont area along Coyote Creek at the southernmost San Francisco Bay. Each profile was about 1.5 km long with geophone and shot spacings of 5 m. Seismic sources were generated by a Betsy Seisgun using 8-gauge shotgun blanks in 0.3-m-deep holes. Each shot was recorded for 2 s at a sample rate of 0.5 ms on 180 channels using three Geometrics Strataview RX-60 seismographs. We observe reflections to about 1000 ms. Preliminary analysis of the data indicates shallow velocities range from 1000 to 1600 m/s in the upper 50 m with multiple low- and high-velocity zones. Shot gathers indicate unusual structure in the Coyote Creek Area (in the vicinity of Albrae Slough) along the northward projection of the SCF, suggesting that the structure may be related to the SCF. Farther northward projection of the SCF and the Albrae Slough structure aligns with a fault imaged in the San Leandro area, an INSAR lineament in the Oakland area, and a mapped fault in the Richmond area. If these indicators of faulting are all associated with the SCF, then the SCF can be interpreted to extend the length of the East Bay.
DE: 7200 SEISMOLOGY
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2003 Fall Meeting