HR: 0800h
AN: S41B-0983    [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 and Direct P Wave Arrivals
AU: * Lewis, M A
EM: malewis@usc.du
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, Woods Hole Road, Woods Hole, MA 02543 United States
AB: Understanding the structure of large faults is an important step towards the understanding of earthquake processes on those faults. The short length scales that are important for earthquake physics can not be resolved at depth by conventional geophysical methods. The utilization of seismic energy trapped within low velocity fault zone layers can yield detailed images of the fault structure. However, recent studies at a number of locations have indicated that trapped waves are typically generated only by approximately the top 3km of the fault zones, above the seismogenic portion of the structures. Since major faults typically juxtapose rocks with different elastic properties, the contrast in materials can lead to the generation of fault zone head waves that spend the majority of their propagation paths refracting along the fault interface. The incorporation of fault zone head wave in imaging studies can thus resolve important small scale elements of the fault zone structure at seismogenic depths. In this study we perform a joint direct P and head wave travel time inversion to produce a separate 1D velocity model for each side of the San Andreas fault in the Bear Valley region. The data comes from a dense temporary array of seismometers deployed by Thurber et al. (1997) and the permanent northern California seismic network stations in the area. We have picked arrival times from 450 events at up to 54 stations, resulting in over 9800 direct and over 2700 head, P wave arrival times. One set of inversions is preformed upon the whole data set, and 5 inversion sets are done on various data subsets to try to understand details of the velocity structure. The results imply a strong velocity contrast of ~50% in the near surface that reduces rapidly to 10-20% below 3 km. The presence of a shallow damage zone around the fault is detected by inversions using subsets of the data made up of only stations close to the fault. The faster (southwest) side of the fault shows the development of a low velocity layer at the surface as instruments closer to the fault (<5km and <2km) are used; such a feature is not present on inversions using only stations at greater distances from the fault. On the slower (northeast) side of the fault the presence of low velocity shallow layer is only detected in the inversions using the stations within 2km of the fault. The asymmetry of the shallow low velocity layer may reflect a preferred propagation direction of earthquake ruptures. Using events from different portions of the fault, the head wave inversions also resolve small scale features of the fault visible in the surface geology and relocated seismicity.
DE: 7203 Body waves
DE: 7270 Tomography (6982, 8180)
SC: Seismology [S]
MN: Fall Meeting 2005