HR: 0830h
AN: S21F-0394    [PDF]
TI: Seismic Images and Subsurface Structure of the San Bernardino Basin Between the San Jacinto and San Andreas Faults, Southern California
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: Catching, 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: 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: We acquired an approximately 20-km-long, combined seismic reflection and refraction profile across the San Bernardino basin that shows the complexity of this tectonically active area. The seismic profile spanned both the San Jacinto and San Andreas faults and had nominal shot and seismograph spacings of 1 km and 50 m, respectively. The refraction profile shows that the deepest part of the basin is centered about 1 km southwest of the San Jacinto fault, where the basin is about 2.0 km deep and about 5 km wide. For about 2 km to the northeast of the deep basin, depth to basement decreases along a series of abrupt, apparently faulted steps to about 800 m. For about 6 km farther northeast, the basin depth remains at about 800 m across most of the seismic profile except within about 4 km of the San Andreas fault, where a sub-basin extends to a depth of about 1.2 to 1.6 km. Structure of the San Bernardino basin is largely related to fault activity, as the most active trace of the San Jacinto fault is located over the central part of the deep basin, and the Rialto-Colton fault, part of the San Jacinto fault zone, appears to be the basin-bounding fault on the southwest side. Seismic reflection images suggest that many of the faults within the San Bernardino basin, including the Rialto-Colton fault, form a flower structure; individual faults dip toward and merge with the San Jacinto fault. The flower structure converges at a depth of about 5 km or less. Dipping reflectors within basement rocks at a depth below 3 km suggest layering of the crystalline basement, or that a decollement extends between the San Jacinto and San Andreas faults. Seismic P-wave velocities range from about 1.8 km/s immediately below the surface to about 6.0 km/s within the upper 4 km. The relatively low velocities within the basin and the proximity of numerous fault strands in the highly populated areas above the basin suggest that even moderate earthquakes on any of these faults may result in amplified shaking.
DE: 7200 SEISMOLOGY
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2003 Fall Meeting