HR: 0804h
AN: NS11E-0840    [Abstracts]
TI: High-Resolution Seismic Reflection and Refraction Imaging of the Hayward Fault in Fremont, Alameda County, California
AU: * Everson, E D
EM: ever0149@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota – Twin Cities, 210 Pillsbury Drive, Minneapolis, MN 55455, United States
AU: Rymer, M J
EM: mrymer@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States
AU: Goldman, M R
EM: goldman@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States
AU: Catchings, R D
EM: catching@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States
AB: In July 2007, the U.S. Geological Survey acquired a 60-m-long seismic reflection and refraction profile across the main trace of the Hayward fault in Fremont Central Park, Fremont, California. The profile was designed to determine the geometry, seismic velocities, and possible structural complexities of the fault. The study was along a part of the surface rupture of the 1868 M 7.0 Hayward earthquake. We used single-element, 40-Hz vertical geophones placed at 1-m intervals along the profile with 0.5-m lateral offset from the shot points, also with 1-m intervals. Seismic sources were generated by multiple sledgehammer blows at each shot point. Data were recorded unfiltered in the field on a Geometrics Strataview RX-60 seismograph at a sampling rate of 0.5 ms for 2 s. Geophone locations were measured in 3D using differential GPS. We developed a velocity model using the Hole (1992) code to invert P-wave first arrivals of the refraction data. Seismic P-wave velocities range from about 200 m/s near the surface to approximately 800 m/s at a depth of 13 to 16 m. The velocity model was then applied to the reflection data to develop an unmigrated common depth point (CDP) stack. The reflection data indicate the presence of at least three fault strands in an approximately 20-m-wide zone. We believe the three strands define an upwardly flaring ‘flower structure', with the central strand being the main strand of the Hayward fault. The three strands project to merge at a depth of about 150 m; the overall dip of the fault zone in the upper 100 m is to the northeast, at about 88 degrees.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7250 Transform faults
DE: 8107 Continental neotectonics (8002)
DE: 8175 Tectonics and landscape evolution
SC: Near-Surface Geophysics [NS]
MN: 2007 Fall Meeting