HR: 0830h
AN: S41A-09    [Abstracts]
TI: Seismic Reflection/Refraction-Imaging at the San Andreas Fault
AU: * Bleibinhaus, F
EM: bleibi@vt.edu
AF: Virginia Tech, Department of Geosciences 4044 Derring Hall, Blacksburg, VA 24061 United States
AU: Hole, J A
EM: hole@vt.edu
AF: Virginia Tech, Department of Geosciences 4044 Derring Hall, Blacksburg, VA 24061 United States
AU: Ryberg, T
EM: trond@gfz-potsdam.de
AF: GeoForschungszentrum Potsdam, Telegrafenberg E322, Potsdam, 14473 Germany
AB: A refraction/reflection seismic survey at the SAFOD drill site near Parkfield carried out in 2003 provides a detailed characterization of the subsurface structures. Three-component stations at a spacing of 50m were deployed on a 46 km long line perpendicular to the surface trace of the SAF. From the first arrival times of 62 explosive shots, a refraction p-wave velocity model was obtained. The most prominent feature of the model is a strong lateral contrast near SAF between Salinian granitic rocks and the Franciscan sedimentary melange. This model was used to perform a prestack migration of the reflected wavefield focusing on steep dips. The resulting images show a bright reflector down to at least 2 km depth at the position of the Coast Range Fault (CRF), which separates the Great Valley sequence from the Franciscan. Its near-vertical dip suggests a strike slip rather than a thrust contact. The reflection is interpreted to arise from fault-zone structure rather than from an impedance contrast across the fault. Despite a subtle p-velocity gradient at that position, neither p- nor s-wave velocities indicate a significant contrast. The high reflection strength may indicate the presence of fluids. The SAF, which was previously imaged at the same location by a shallower survey at a few hundred meters depth, is not a strong feature in the prestack migration result. This finding may be related to some extent to image quality, which is affected by large shot spacing, rugged topography and statics, and a complex direct arrival wavefield over shallow basement. We are currently working on a full-wavefield inversion to improve the velocity model, and thereby the migration image.
DE: 7200 SEISMOLOGY
DE: 7205 Continental crust (1242)
DE: 7260 Theory and modeling
DE: 8105 Continental margins and sedimentary basins
DE: 8180 Tomography
SC: Seismology [S]
MN: 2005 Joint Assembly