HR: 0830h
AN: S21F-0393    [PDF]
TI: Shallow Geometry and Velocities Along the Rialto-Colton Fault, San Bernardino Basin, California
AU: * Gandhok, G
EM: ggandhok@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: Goldman, M R
EM: goldman@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road MS 977, Menlo Park, CA 94025 United States
AB: The Rialto-Colton fault, part of and located southwest of the San Jacinto fault zone, extends along the southwestern margin of the San Bernardino basin. There, the Rialto-Colton fault is buried beneath Quaternary alluvium. Ground-water studies suggest that the Rialto-Colton fault extends northwestward, structurally bounding the San Bernardino basin and acting as a ground-water barrier. Potential-field studies also show evidence for variations in basement depth along the presumed trace of the fault. To better locate the Rialto-Colton fault and to better assess its potential earthquake hazard, the U.S. Geological Survey acquired five combined, high-resolution seismic reflection and refraction profiles along the expected trace of the fault. Our seismic reflection images show that the Rialto-Colton fault consists of an approximately 1-km-wide zone of small-offset (approximately 20 to 30 m) faults in the upper 1 km depth. No single large-offset fault was imaged along any of the seismic profiles. Reflections extend to depths of at least 800 m along most of the seismic profiles, suggesting that basement is at least 800 m deep. We also used tomographic methods to invert first-arrival refractions for the velocity structure along each of the seismic profiles. P-wave velocities range from about 300 m/s at the surface to about 2500 m/s at depths of about 100 m. Our seismic velocity images show that the ground-water table largely correlates with the 1500 m/s contours, and the velocity images show that the ground-water table is disrupted across the reflection-imaged fault zone at shallow depths. Some of the faults extend to within a few meters of the surface, suggesting that there has been recent activity on the Rialto-Colton fault. Because the densely populated and growing cites of San Bernardino, Rialto, Colton, and Fontana are located along the projected trend of the Rialto-Colton fault, rupture along the fault may present a significant earthquake hazard to the region.
DE: 7200 SEISMOLOGY
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2003 Fall Meeting