HR: 14:10h
AN: T53C-03    [Abstracts]
TI: Small-Scale Structures Derived From Microearthquake Locations Using SAFOD and HRSN Data
AU: * Oye, V
EM: volker@norsar.no
AF: NORSAR, Instituttveien 25, Kjeller, 2027, Norway
AU: Ellsworth, W L
EM: ellsworth@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States
AB: In order to improve the understanding of the small-scale fault within the San Andreas Fault Zone at Parkfield, California, we analyze high frequency seismic data (4000 Hz) recorded at the San Andreas Fault Observatory at Depth (SAFOD). Deep borehole seismic data from the Pilot Hole (PH) and Main Hole (MH) collected from August to mid September 2006 have been processed together with additional data (250 Hz) from 13 stations of the shallow borehole High Resolution Seismic Network (HRSN). The MIMO system (Oye and Roth, 2003) is used for automatic processing of the data. A single channel STA/LTA based detection algorithm provides primary detections. Detections are associated to form a single event with a generalized beam forming algorithm, similar to the approach used in array seismology. A detection on the MH instruments was required, whereas any further detection from PH and HRSN channels was associated with the event when appropriate. Over this 6 week period about 8900 events were identified, of which about 1000 events were only detected with one of the MH geophones. Another 5100 events were detected on one MH geophone and one PH or HRSN station and 2100 events were detected with both MH instruments. Most of these events were associated with tool-slip in the borehole. However, we observed a series of about 20 ultramicroaftershocks (M < 0) following the August 11^th M 1.8 "Hawaii" target event. These events were only observed on the MH. About 720 events were detected on both MH geophones and at least one HRSN station. Subsequently, P- and S-wave onsets as well as azimuth angles are determined automatically using the MIMO monitoring software. To locate the events, we conduct a grid search using look-up tables created by ray tracing in a 3D velocity model that was provided by S. Roecker. We have also established semiautomatic processing routines for shear wave splitting analysis for application to events that locate close to the MH stations. Thus we hope to image shear wave splitting along the San Andreas Fault and map the anisotropy related to the fracture density distribution within the Parkfield section of the San Andreas Fault Zone.
DE: 7200 SEISMOLOGY
DE: 7215 Earthquake source observations (1240)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8100 TECTONOPHYSICS
DE: 8164 Stresses: crust and lithosphere
SC: Tectonophysics [T]
MN: 2007 Fall Meeting