HR: 1320h
AN: S43A-0976    [Abstracts]
TI: Shallow Seismic Reflection Survey at Garner Valley Digital Array
AU: * Lawrence, Z S
EM: zlawrenc@memphis.edu
AF: University of Memphis CERI, 3876 Central Ave, Suite 1, Memphis, TN 38152 United States
AU: Brackman, T B
EM: tbrackmn@memphis.edu
AF: University of Memphis CERI, 3876 Central Ave, Suite 1, Memphis, TN 38152 United States
AU: Bodin, P
EM: pbodin@memphis.edu
AF: University of Memphis CERI, 3876 Central Ave, Suite 1, Memphis, TN 38152 United States
AU: Stephenson, W J
EM: wstephens@usgs.gov
AF: U.S. Geological Survey, Box 25046, MS 966, Denver, CO 80225 United States
AU: Steidl, J H
EM: steidl@crustal.ucsb.edu
AF: UC Santa Barbara, Institute for Crustal Studies, Santa Barbara, CA 93106 United States
AU: Gomberg, J
EM: gomberg@usgs.gov
AF: U.S. Geological Survey, 3876 Central Ave, Suite 2, Memphis, TN 38152 United States
AB: The Garner Valley Digital Array (GVDA) site is a NEES-sponsored facility in a small, sediment-filled, intermountain valley in Southern California, established for the purpose of investigating ground motion site response and soil-structure interaction, in situ. The site has been well-characterized geotechnically, and is thoroughly instrumented with both surface and downhole instrumentation of various types. Nevertheless, a borehole recently drilled into lake bed sediments and deeply weathered granitic rocks that comprise the valley fill at GVDA encountered hard, unweathered bedrock at an unexpected depth, suggesting an apparent 38 meter offset in the unweathered bedrock between two wells 40 meters apart. The apparent offset can be most easily explained either by faulting, or as a buried erosional surface. The Hot Springs fault, a strand of the San Jacinto fault zone, runs through Garner Valley, although its inferred location is several hundred meters east of GVDA. To better characterize the subsurface strata, particularly the existence and configuration of faulting that may disturb them; we conducted a 120-meter long, 12-fold shallow seismic reflection common midpoint (CMP) survey at GVDA using a 24-channel seismograph, vertical 4.5 Hz geophones at 2-meter intervals and a sledgehammer seismic source. Preliminary processing reveals strong refractors and surface waves that may mask reflections, although reflections are visible in some raw shot records. Semi-continuous reflections seen in the CMP section from a shallow reflector may coincide with the water table. There are also deeper, discontinuous reflectors obscured by bands of coherent noise. We plan to present a fully migrated and interpreted CMP record section.
DE: 7299 General or miscellaneous
DE: 7200 SEISMOLOGY
DE: 7203 Body wave propagation
SC: Seismology [S]
MN: 2004 AGU Fall Meeting