HR: 17:30h
AN: S22F-07    [PDF]
TI: Fine-Scale Structure of the San Andreas Fault and Location of the SAFOD Target Earthquakes
AU: * Thurber, C
EM: thurber@geology.wisc.edu
AF: UW-Madison, 1215 W. Dayton St., Madison, WI 53706 United States
AU: Zhang, H
EM: hjzhang@geology.wisc.edu
AF: UW-Madison, 1215 W. Dayton St., Madison, WI 53706 United States
AU: Roecker, S
EM: roecks@rpi.edu
AF: RPI, 110 8th Street, Troy, NY 12180 United States
AU: Ellsworth, W
EM: ellsworth@usgs.gov
AF: US Geological Survey, 345 Middlefield Rod, Menlo Park, CA 94025 United States
AU: Malin, P
EM: malin@duke.edu
AF: Duke University, Box 90235, Durham, NC 27708 United States
AB: Arrival-time data from about 900 local earthquakes and several dozen shots are inverted for earthquake locations and three-dimensional Vp and Vp/Vs structure of the San Andreas fault (SAF) zone at Parkfield, CA, using several different tomography methods. Data are included from a temporary array of surface seismic stations installed around the SAFOD site, a vertical array of geophones installed in the 2-km-deep SAFOD Pilot Hole drilled in summer 2002, and permanent network stations. The inversion methods applied are conventional ray-tracing tomography, conventional finite-difference tomography, and double-difference tomography. For double-difference tomography, we use both a regular grid and an adaptive grid based on tetrahedra and Voronoi diagrams. The primary features of the different velocity models are generally consistent with each other. A low-Vp zone (about 25% slower) adjacent to the fault trace penetrates to a depth of as much as 10 km. Nearly all the earthquakes occur almost directly beneath the surface trace of the SAF, on the edge of a zone of high horizontal Vp gradient. Regions of anomalously high Vp/Vs ($>$ 2.0) are found at shallow depths along and northeast of the SAF. These high-Vp/Vs anomalies can be associated with features in an existing resistivity model that are interpreted to represent fluid-saturated zones. We locate several magnitude 1 to 2 earthquakes at depths $<$ 3 km and locations that are on the order of 100 meters southwest of the surface fault trace that are potential target events for drilling of the main SAFOD hole. We discuss a range of tests of our absolute and relative location capability for events in the target region, and identify additional steps needed to reduce location uncertainties to $<$ 100 m and ultimately $<$ 10 m.
DE: 7203 Body wave propagation
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2003 Fall Meeting