HR: 0830h
AN: S21F-0392    [PDF]
TI: Subsurface Structure of the Eastern Santa Clara Valley From Downtown San Jose to the Calaveras Fault
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
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: Gandhok, G
EM: ggandhok@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road MS 977, Menlo Park, CA 94025 United States
AB: The Santa Clara Valley, located in the southern San Francisco Bay area, is a topographically low-lying region that is bound on the southwest by the Santa Cruz Mountains and the San Andreas fault system and on the northeast by the Diablo Mountains and the Hayward-Calaveras fault systems. In addition to the major valley-bounding faults, other faults have been mapped in bedrock near the southern end of the Santa Clara Valley, and these faults are believed to trend northward beneath the alluvial fill of the valley. Two large basins, the Evergreen and the Cupertino basins, have been identified on the northeastern and southwestern sides, respectively, of the valley. The basins and the subsurface faults are potential sources of strong shaking from local or regional faults. To better understand the basin, velocity, and fault structures of the Santa Clara Valley, the U.S. Geological Survey acquired an approximately 20-km-long, northeast-southwest-trending, combined reflection and refraction seismic imaging profile across the eastern Santa Clara Valley from downtown San Jose to the Calaveras fault in May 2003. The seismic profile was centered on the Evergreen basin, crossing the Hayward and Silver Creek faults and terminating at the Calaveras fault. Seismic sources were generated by 5-to-50-kg buried explosions in 15-to-20-m-deep holes, spaced approximately 1 km apart. We used approximately 400 "Texan" seismographs, spaced at 50-m intervals along the profile to record the data. P-wave velocities were measured from the near surface to maximum depths of about 5.5 km, with velocities ranging from about 1.8 km/s near the surface to greater than 6.3 km/s at 5 km depth. On the basis of the 4.0 km/s velocity contour (basement rocks), the Evergreen basin is an approximately 6-km-wide, 2.5-km-deep (maximum), well-developed basin, with basement highs to the northeast and southwest. Southwest of the Evergreen Basin, the downtown San Jose area is underlain by a basement high with depths to basement less than about 1 km. Northeast of the Evergreen Basin, velocities indicative of basement rocks approach to within a few hundred meters of the surface in the Diablo Mountains. Between the northeastern end of the Evergreen Basin and the Diablo Mountains, basement rocks are underlain by a northeastward-dipping, vertical low-velocity zone. Seismic imaging data from this study and previously acquired data from the western side of the valley, suggest that the Santa Clara Valley is underlain by a series of faults and a complex basin structure derived from tectonics associated with those faults.
DE: 7200 SEISMOLOGY
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2003 Fall Meeting