HR: 1340h
AN: T13A-1340    [Abstracts]
TI: Scattered Wavefield Within the San Andreas Fault System, California
AU: * Taira, T
EM: taka@dtm.ciw.edu
AF: DTM, Carnegie Institution of Washington, 5241 Broad Branch Rd., Washington, DC 20015 United States
AU: Silver, P G
EM: silver@dtm.ciw.edu
AF: DTM, Carnegie Institution of Washington, 5241 Broad Branch Rd., Washington, DC 20015 United States
AU: Niu, F
EM: niu@rice.edu
AF: Department of Earth Science, Rice University, 6100 Main St., Houston, TX 77005 United States
AU: Nadeau, R M
EM: nadeau@seismo.berkeley.edu
AF: UC Berkeley, Berkeley Seismological Lab., 215 McCone Hall, Berkeley, CA 94720 United States
AB: Since transient aseismic deformation at seismogenic depth is one of the key phenomena related to earthquake occurrence, it is important to estimate the physical characteristics of such stress/strain transients within the deeper structure of the fault zone. Analysis of coda (scattered) waves has the potential for identifying such transients because the scattered wavefield is attributed mainly to small-scale heterogeneity that is likely formed by these transient events. In addition, the sampling area of scattered waves is concentrated within the fault zone, compared to the area sampled by direct waves. We have begun a program to map the spatial distribution of fault-zone scatterers and their time dependence within two regions of San Andreas Fault system: the Hayward Fault and the Parkfield segment of the San Andreas Fault. In order to most reliably evaluate the scattered wavefields, we limited our analysis to records from borehole seismographs recorded by the Hayward Fault Network (HFN) and the High-Resolution Seismic Network (HRSN) in each area. For the Hayward fault, we mapped the spatial distribution of scatterers by analysis of the S-wave coda amplification factor (CAF) in a manner similar to Taira and Yomogida (2003). CAF is defined as the amplitude ratio of coda waves among different stations after corrections for source, station, and overall propagation effects (e.g., coda Q). This parameter allows for a statistical characterization of the distribution of scatterers. The station effect for each station and the coda Q averaged over all the seismograms in this area were estimated by the coda-normalization and maximum likelihood methods, respectively, using five regional earthquakes (epicentral distance $>$ 50 km). We evaluated the CAF value of each source-station pair for the transverse-component, using 294 seismograms for 39 local earthquakes (epicentral distance $<$ 50 km) recorded by 14 stations of the HFN. A map of CAF values for stations near the Hayward Fault reveals that they are relatively large ($>$ 2.1 times) along the north-western part of the fault. This result suggests that heterogeneities are concentrated in the north-western segment of this fault, manifested locally as a large degree of scattering. This spatial variation of the scattered wavefield along the Hayward Fault may be related to variations in the stress/strain field. In the case of the Parkfield segment, the high density of borehole seismographs permits the identification of individual scatterers. We applied the imaging method of Taira and Yomogida (2004) incorporating the source array technique presented by Spudich and Bostwick (1987). We selected seismograms from a cluster of 23 aftershocks of the December 20, 1994, M=4.7 Parkfield earthquake, recorded at 10 stations of the HRSN. First, we relocated these aftershocks, using the earthquake catalog data (location and phase) produced by the Northern California Seismic Network and the double-difference earthquake relocation algorithm introduced by Waldhauser and Ellsworth (2000). We then analyzed the coda of observed seismograms in the frequency range 2 - 4 Hz where the signal level was highest, and found what we interpret as a strong S-to-S scatterer located in the south-eastern part of this area at a depth of between 6 - 12 km. We test the hypothesis that this scatterer is related to the 1993 Parkfield aseismic transient event (Gao et al., 2000) by assessing possible time dependence in scatterer properties, through the use of a larger suite of events that spans the time period of the transient.
DE: 8010 Fractures and faults
DE: 8015 Local crustal structure
DE: 7200 SEISMOLOGY
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting