HR: 0800h
AN: T21A-0444    [Abstracts]
TI: The Branching Pattern of Low-Velocity Structure on the San Andreas Fault near the SAFOD Site at Parkfield from Fault-Zone Guided Waves
AU: Alvarez, M
T21A-0444 AF: New Mexico Institute of Mining and Technology, PASSCAL Instrument Center, Soocorro, NM 87801 United States
AU: * Li, Y
EM: ygli@usc.edu
AF: University of southern California, Department of Earth Sciences, Los Angeles, CA 90089 United States
AU: Malin, P E
T21A-0444 AF: Duke University, Department of Geology, Durham, NC 27706 United States
AU: Cochran, E S
T21A-0444 AF: University of California at Los Angeles, IGPP, Los Angels, CA 90095 United States
AU: Vidale, J E
T21A-0444 AF: University of California at Los Angeles, IGPP, Los Angels, CA 90095 United States
AB: We use fault-zone guided waves recorded at surface seismic arrays deployed across and along the San Andreas fault for explosions and local earthquakes combined with the guided waves recorded at borehole instrument in the SAFOD main hole to delineate the low-velocity structure of the SAF at seismogenic depths. The data from events occurring within the fault zone at different depths and with the raypath incidence angles to the surface array smaller than 30o from vertical show that the length of fault-zone guided wavetrains after S-arrivals at stations within the fault zone increases progressively from ~1 to ~2 s as the event depth increases from ~3 to ~10 km, with greater increasing at depths shallower than 5 km. In contrast, the data recorded at away-fault stations for the same events and the data recorded at on-fault stations but for events occurring away from the fault zone show shorter length of wavetrains after S-arrives and a much milder depth-dependent trend. We also observed guided waves partitioning on the secondary fault at station located at 875 m southwest of the SAF surface trace. The data from seismometers in the SAFOD main hole show fault-zone guided waves channeling on this secondary fault, which passes the borehole station at the depth of 2.7 km and ~400 m southwest of the nearly vertical SAF, for the events occurring on the SAF at depths deeper than ~3 km (Shalev and Malin, 2005). Observations and 3-D finite-difference simulations of fault-zone guided waves show a branching pattern of the low-velocity structure on the SAF at shallow depth near the SAFOD drilling site while the low-velocity zone likely extends across seismogenic depths with the smaller velocity reduction within the deeper potion of fault zone due to larger confined stress at greater depths. Shear-wave splitting analyses of the data suggest slight depth dependence to the measured delays of slow S waves for source depths between 2 and 7 km.
DE: 7200 SEISMOLOGY
DE: 7209 Earthquake dynamics (1242)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8004 Dynamics and mechanics of faulting (8118)
SC: Tectonophysics [T]
MN: Fall Meeting 2005