HR: 17:30h
AN: T24B-07    [Abstracts]
TI: San Andreas Fault Branching at SAFOD From Fault Guided Wave Mapping and P-wave Tomography
AU: * Malin, P E
EM: malin@duke.edu
AF: Earth & Ocean Science, Duke University, Box 90227, Durham, NC 27708-0227 United States
AU: Shalev, E
EM: shalev@duke.edu
AF: Earth & Ocean Science, Duke University, Box 90227, Durham, NC 27708-0227 United States
AB: In 2004, drilling in the San Andreas Fault Observatory at Depth Main Hole (MH) stopped at ~2.5 km underground and ~0.7 m short of the SAF surface trace. A seismograph temporarily placed there recorded fault zone guided waves from SAF earthquakes, but only from events more than ~2 km northwest and ~3 km southeast of the seismograph. P-wave tomography of seismograms recorded in the SAFOD Pilot Hole (PH) show the seismograph was near the bottom of a synclinal low velocity zone, which also extends across the surface trace. Evidently, the seismograph was in a branch of the SAF that bounds a sliver of sedimentary rock. Our observations were take with a 3-component, 4.5 Hz seismograph temporarily locked in near the bottom of the MH. The seismograph's position was on the southwestern edge of a structure previously imaged using scattered microearthquake waves as recorded on the PH array. It is also near a highly fractured lithological contact seen in the drill core recovered from the end of the 2004 drilling. Seismograms from SAF microearthquakes originating several km to the northwest and southeast of the seismograph appear to contain large fault zone guided waves ("Fg" waves). These data imply that the structure seen in the migration image and drill core is a significant fault zone with a low velocity core that can trap seismic waves. They further suggest that the fault at the end of the 2004 drilling maybe connected to the active trace of the SAF to the north and south of the SAFOD site. Since the SAFOD site is situated on the flower structure of the Middle Mountain segment of the SAF, this branch may still connect to the surface trace fault but at a depth greater than the target events. Previous geological mapping show that the SAFOD site sits above the Middle Mountain syncline. Our P-wave velocity tomography also suggests that its structure extends deeper than indicated by the surface geology. Further, both the 2004 branch and surface-trace-fault cut through this structure, possibly trapping a sliver of sediments that related to the Great Valley side of the SAF zone. The presence of the branch fault has implications not only for the geology of the SAFOD site but for the mechanics of this fault zone and the drilling of multilateral cores in the MH in 2007.
DE: 8111 Continental tectonics: strike-slip and transform
DE: 8150 Plate boundary: general (3040)
DE: 8194 Instruments and techniques
SC: Tectonophysics [T]
MN: Fall Meeting 2005