HR: 0800h
AN: S31A-1026    [Abstracts]
TI: Structure of the San Bernardino Basin Between the San Jacinto and San Andreas Faults
AU: * Rymer, M J
EM: mrymer@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: Goldman, M R
EM: goldman@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: Steedman, C E
EM: steedman@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94025 United States
AB: We acquired high- and medium-resolution seismic reflection and refraction data along two profiles through the San Bernardino Basin. The high-resolution profile (5 m shot and geophone spacing) was acquired along the I-215 freeway from I-10 to State Road 30. The medium-resolution profile (1 km and 50 m shot and geophone spacing, respectively) was acquired northeastward from southern Colton to the San Andreas fault (SAF). Images from both profiles show that the San Bernardino Basin has a maximum depth of 1.2 km northeast of the I-215/I-10 interchange. From southwest to northeast, the top of basement steps down about 800 m between surface traces of the Rialto-Colton and San Jacinto fault zones (RCFZ and SJFZ, respectively); basement rises in steps northeastward along faults between the interstate interchange and the SAF. We interpret three zones of faulting between the RCFZ and the SAF, including, the SJFZ, the Tippecanoe fault zone (TFZ), and the East Highlands fault zone (EHFZ). Double-difference-relocated earthquakes (Hauksson et al., 2003) illuminate all of these fault zones. The RCFZ and the SJFZ form a flower structure that is ~8 km wide in the upper crust and is centered near the I-215/I-10 interchange. The TFZ is ~3 km wide, has dominantly near-vertical fault strands, and is centered near downtown San Bernardino. Over the past 20 years, the TFZ has been the most seismically active fault zone in this part of the San Bernardino Basin. The EHFZ appears to be centered on, and probably is structurally related to, a series of hills exposed above alluvium near the northeast edge of the basin. The EHFZ forms an 8-km-wide flower structure where it crosses our medium-resolution profile. Because our medium-resolution profile extended only a few hundred meters northeast of the SAF, we did not image major strands of the SAF, suggesting that the SAF may dip to the northeast. Sediment P-wave velocities within the San Bernardino Basin range from about 800 m/s at the surface to about 3700 m/s at a depth of 1.2 km. Basement velocities are highly variable, ranging from about 4000 m/s at the sediment-basement contact to about 6400 m/s at 4 km depth. The thickness of the San Bernardino Basin, the relatively low sediment velocities, and the presence of active faults within the basin all suggest that even moderate local earthquakes may result in appreciable losses.
DE: 7218 Lithosphere and upper mantle
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
DE: 6982 Tomography and imaging
DE: 0905 Continental structures (8109, 8110)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting