HR: 1340h
AN: S23B-0317    [Abstracts]
TI: High-resolution imaging of the deep structure of the Bear Valley section of the San Andreas Fault with joint analysis of fault-zone head waves and direct P arrivals
AU: * Lewis, M A
EM: malewis@usc.edu
AF: University of Southern California, 3651 University Avenue, ZHS, Los Angeles, CA 90089 United States
AU: Ben-Zion, Y
EM: benzion@terra.usc.edu
AF: University of Southern California, 3651 University Avenue, ZHS, Los Angeles, CA 90089 United States
AU: McGuire, J
EM: jmcguire@whoi.edu
AF: Woods Hole Oceanographic Institution, 266 Woods Hole Road, Woods Hole, MA 02543 United States
AB: Understanding the structure of large strike slip faults can be an important step towards the understanding of dynamic earthquake processes. Geologic measurements are limited to selected sites, and surface or shallow fault zones that have undergone the process of exhumation (and thus may not reflect the in situ state). The short length scales and near vertical dip of large strike slip faults also makes conventional geophysical methods ineffective. The utilization of seismic energy trapped within low velocity fault zone (FZ) layers can yield detailed images of the structure. However, recent studies at a number of locations indicate that trapped waves are typically generated only by the top ~3km of the fault zones, above the seismogenic portion of the structures. Major faults that have accumulated significant amounts of slip may not only have damage zone but also juxtapose rocks with different elastic properties. In such structures, fault zone head waves (FZHW) can propagate along material interfaces and arrive at near-fault stations on the slower side before the direct P wave. The FZHW spend the majority of their propagation paths along the fault zone and their incorporation in imaging studies can provide high resolution results on the deep structure of the fault. In this study we perform a joint direct P and head wave travel time inversion of data from a dense temporary array of 49 seismometers deployed by Thurber et al. (1997) in the Bear Valley region of the San Andreas Fault. Within the operational period, ~1200 events were recorded by the array and located. Currently travel time picked were done for 200+ events, yielding ~6304 direct P and ~2017 head wave arrival times. Analysis of the moveout between the direct P and FZHW allows estimates of the velocity contrast and can show variation along strike and with dip. Waveform fits are also produced to further constrain FZ properties. As the FZHW are examined the arrival time picks and picking method are refined with a view to increasing the ray path coverage and performing a joint direct P and head wave travel time inversion. The results obtained so far indicate that the San Andreas Fault in our study area has material interfaces that extend to the bottom of the seismogenic zone and are continuous along strike for several 10s of km. The strength of the material interfaces change with position along strike and depth.
DE: 8180 Tomography
DE: 7205 Continental crust (1242)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting