HR: 08:30h
AN: T11F-03 INVITED     [Abstracts]
TI: SAFOD Site Characterization using the Pilot Hole Seismic Array
AU: * Malin, P
EM: malin@duke.edu
AF: Duke University - Earth and Ocean Sciences, 103 Old Chemistry Bldg. B90229, Durham, NC 27707 United States
AU: Chavarria, J
EM: jac4@duke.edu
AF: Duke University - Earth and Ocean Sciences, 103 Old Chemistry Bldg. B90229, Durham, NC 27707 United States
AU: Shalev, E
EM: shalev@duke.edu
AF: Duke University - Earth and Ocean Sciences, 103 Old Chemistry Bldg. B90229, Durham, NC 27707 United States
AU: Walter, L
EM: lwalter@geo-eng-res.com
AF: Geo Space, 7007 Pinemont, Houston, TX 77040 United States
AB: Since we installed it in July 2002 and its demise in July 2004, we used a 32 level array of 3-component, 15 Hz seismographs in the Pilot Hole to study the structure and properties of the SAFOD site. The array levels were spaced at 40 m intervals, with the deepest level at 2096 m. A catalogue of several hundred earthquakes and explosions was recorded with the array, mostly with a sampling rate of 2 kHz. We found that the seismograms from these events contain significant information about the structure and properties of the SAFOD site and San Andreas Fault. These data show source and receiver position-sensitive travel time variations in the P- and S-waves. They also commonly contain several secondary phases between the direct waves. By analyzing the data with a variety of travel time ratio, tomographic, and wave-field migration techniques, we have been able to map changes in Vp/Vs along the SAF and along the PH itself. Along the SAF, a northwest-to-southeast reduction in Vp/Vs correlates well with a reduction in fault creep. Between the SAF and PH, P-wave tomography indicates that a large body of rocks with slow velocities is present. By migrating the secondary phases with Kirchhoff methods, we have been able to image SAF-paralleling structures, both within the low velocity body, and on its edges. As one possible interpretation, we have suggested that these features represent a faulted and fault bounded sedimentary section. So far this interpretation has fit the results of the SAFOD 2004 drilling. The presence of these faults and the unexpected volume of sedimentary rocks, which also appear fluid saturated, have important implication how local stresses and strains distribute and build during the earthquake cycle.
DE: 7230 Seismicity and seismotectonics
DE: 7294 Instruments and techniques
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting