HR: 0800h
AN: T21A-0445    [Abstracts]
TI: A Simultaneous Imaging Method of Multiple Scattering Modes for Detecting a Fault-Zone Heterogeneous Structure of the San Andreas Fault, Parkfield, California
AU: * Taira, T
EM: taka@dtm.ciw.edu
AF: DTM, Carnegie Institution of Washington, 5241 Broad Branch Rd. NW, Washington, DC 20015 United States
AU: Silver, P G
EM: silver@dtm.ciw.edu
AF: DTM, Carnegie Institution of Washington, 5241 Broad Branch Rd. NW, 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 Laboratory, 215 McCone Hall, Berkeley, CA 94720 United States
AB: One approach to understanding the generation process of earthquakes is to image fault-zone heterogeneity through the use of single-point scatterers. We present an imaging methodology for imaging multiple scattering modes (P-P, P-S, S-P, and S-S) to assess the relative amplitude of heterogeneity in the bulk and shear modulus in the fault zone. This method is designed for a network of three-component seismic stations and a source array produced from an aftershock sequence. Scattering modes and scatterer locations are determined by the following procedure. For each station, the wave-type and slowness (i.e., propagation) vector for the source-to-scatterer part of the path are estimated by performing a semblance analysis. For the scatterer-to-station segment of the path, the wave-type is constrained by comparing the observed polarization vector, inferred from particle motion, with the predicted propagation vectors from candidate scatterer locations, assuming a half-space velocity model (V_p and V_s are 6.40 km/s and 3.45 km/s, respectively in this study). Candidate scatterer locations and allowable scattering modes are then evaluated by comparison of the observed slowness and polarization vectors with a probability density function based on the 95 per cent confidence levels for these two parameters, in addition to the travel time residual between the observed and predicted travel times. We apply the method to borehole seismograms from 10 relocated aftershocks of the October 20, 1992, M=4.7 Parkfield earthquake recorded by eight stations of the High Resolution Seismic Network. To examine the spatial resolution of the image sections and the ability of our data set to distinguishing among scattering modes, we perform a simple numerical experiment with synthetic seismograms in the frequency range of 8-16 Hz where the signal level was highest, adding 20 per cent of Gaussian random noise to the average signal level at each station. We place three scatterers of each scattering mode at various locations along the San Andreas Fault. We find that the scatterers are generally well recovered so that we expect to resolve each of the scattering modes. From the Parkfield data, we obtain image sections for P-P, P-S, S-P, and S-S scattering modes in the frequency band used in the synthetic test. We find a well constrained region for S-S scattering mode that is located about 10 km south-southeast of the epicenter of the 1966 M=6 Parkfield earthquake at 5 km depth. The size is estimated to be 300 m. We observe no other scattering modes with this data set in this region. The strength of the S-S scattering, combined with the absence of P-P, P-S, and S-P scattering modes implies that structural heterogeneity in the region is dominated by variations in the shear modulus. As such, we hypothesize that the S-S scatterer is associated with fluid-filled cracks or fractures (O'Connell and Budiansky, 1974).
DE: 7200 SEISMOLOGY
DE: 8010 Fractures and faults
DE: 8015 Local crustal structure
SC: Tectonophysics [T]
MN: Fall Meeting 2005