HR: 17:15h
AN: T24B-06    [Abstracts]
TI: Characterization of the San Andreas Fault at Parkfield Using a Massive 3D VSP
AU: * Chavarria, J
EM: andres.chavarria@paulsson.com
AF: Paulsson Geophysical Services, 1215 West Lambert Rd., Brea, CA 92821 United States
AU: Goertz, A
EM: alex.goertz@paulsson.com
AF: Paulsson Geophysical Services, 1215 West Lambert Rd., Brea, CA 92821 United States
AU: Karrenbach, M
EM: martin.karrenbach@paulsson.com
AF: Paulsson Geophysical Services, 1215 West Lambert Rd., Brea, CA 92821 United States
AU: Milligan, P
EM: paul.milligan@paulsson.com
AF: Paulsson Geophysical Services, 1215 West Lambert Rd., Brea, CA 92821 United States
AU: Paulsson, B
EM: bjorn.paulsson@paulsson.com
AF: Paulsson Geophysical Services, 1215 West Lambert Rd., Brea, CA 92821 United States
AB: In preparation for the drilling of SAFOD's Phase II we installed an 80 level array of 3C seismometers inside the well. The goal of the array was to refine the existing velocity model to better locate the target events, and to monitor the local seismicity. The array, with sensors laying mostly within the deviated portion of the well, spans depths ranging from 2.7 to 1.5 km with levels every 15 m. It is this dense spacing what makes 3D VSP capable of bridging the gap between drill-hole observations and observations from the surface like 2D seismics. During April and May 2005 we recorded thirteen far offset shots surrounding the SAFOD site and target event area. Data from these shots was simultaneously recorded by the surface networks and used for better location of the target events. In addition to these, a zero offset shot at SAFOD was generated to refine the structure surrounding the well. The 1D velocity model inverted from the zero offset is representative of the current geologic model at SAFOD. The complexity of the velocity model for this segment of the fault can be inferred from deviations between the zero offset model and the shorter wavelength model derived from well logs. In addition to strong changes in velocity, both zero offset and far offset shots show the presence of strong scattered phases associated to the complex geologic structure of the San Andreas Fault Zone. In addition to the active portion of the experiment we monitored the local seismicity (i.e. aftershocks from the Parkfield 2004 event) over a period of 13 days. During this period of time we recorded continuously at high sampling rates (4kHz) a large number of events, some of which were located by the surface networks and felt onsite. The quiet environment in the borehole enabled us to record microearthquakes that were not present in the NCEDC catalog. In some cases these small events were not even recorded along the entire array. Besides its high level of event detection, the high vector fidelity of the 3C geophones allowed for precise particle motion analysis of first arrivals to determine the location of microearthquakes recorded during this effort.
DE: 7203 Body waves
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7250 Transform faults
DE: 7294 Seismic instruments and networks (0935, 3025)
DE: 8111 Continental tectonics: strike-slip and transform
SC: Tectonophysics [T]
MN: Fall Meeting 2005