HR: 1340h
AN: T13A-1341    [Abstracts]
TI: A search for temporal variations in the scattered wavefield associated with the 1993 Parkfield aseismic transient event: a calibration between borehole and surface instruments
AU: * Cheng, X
EM: xcheng@rice.edu
AF: Department of Earth Science, Rice University, 6100 Main Street, Houston, TX 77005 United States
AU: Niu, F
EM: niu@rice.edu
AF: Department of Earth Science, Rice University, 6100 Main Street, Houston, TX 77005 United States
AU: Silver, P G
EM: silver@dtm.ciw.edu
AF: Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington, DC 20015 United States
AU: Nadeau, R M
EM: nadeau@seismo.berkeley.edu
AF: Berkeley Seismological Lab, 207 McCone Hall Univ. of California, Berkeley, CA 94720 United States
AB: Extracting subsurface deformation information from temporal changes in the seismic properties of the crust has been a long-time pursuit of seismologists. One of the more promising approaches to this problem has been the comparison of the codas of similar events occurring at different times and recorded at the same station. If one forms a cross correlation, {\it C(t)}, within a time window centered on elapsed time {\it t}, the lag time {\it $\tau$} at the maximum of the cross correlation $C_{m}(t)$ and the decorrelation index {\it D(t)} (defined as {\it 1-$C_{m}(t)$}) are both measures of the temporal change in the medium, if the contribution from differential event location is negligible. The variation can be due either to a temporal change in velocity, or a change in the location or amplitude of scatterers that produce the coda. This approach has been successfully applied to similar events that have followed major earthquakes. The change appears to reflect the gradual post-seismic healing of cracks in the shallowest crust. We have previously applied this approach to a repeat-earthquake dataset recorded by the High Resolution Seismic Network (HRSN) along the Parkfield segment of the San Andreas Fault ({\it Niu et al.}, 2003). We found a systematic difference in the S-wave coda between earthquakes that occurred before and after the onset of the 1993 Parkfield aseismic transient, which was observed on a variety of geodetic and seismic instruments in the region. We additionally found that the difference can be explained by the motion of scatterers located very close to the initiation region of the aseismic transient. Given the close correspondence between the change in the medium and the aseismic event, we hypothesized that the observed structural change was due to a change in the stress field induced by the aseismic event. We are presently expanding the scope of this study to include a larger section of the San Andreas fault system. While low-noise borehole networks are particularly well suited to the task of detecting small variations in the scattered wavefield, their spatial distribution is presently limited. We have thus begun to evaluate the utility of the noisier, but far more numerous surface stations that would provide greater density and spatial coverage. To address this issue, we are conducting a calibration experiment at Parkfield between the borehole (HRSN) and surface network (NCSN) stations to examine whether the temporal change observed with the HRSN can be successfully performed with data recorded by surface instruments. We find that in general, the signal-to-noise ratio of the NCSN data is about 2 orders of magnitude lower than that of the HRSN data, even in the case where the NCSN and HRSN stations are very close to each other ($<$ 0.1 km). The resulting decorrelation index between repeat seismograms for the surface stations is $\sim$0.1, which is about a factor of 20-100 higher than those calculated from the borehole stations ($\sim$0.001-0.005). Despite the high noise level of the NCSN data, however, we are nevertheless able to detect the changes between the pre- and post 1993 seismograms. This promising result suggests that our technique can be applied to any region of the San Andreas fault system that possesses both repeat earthquakes and a reasonable distribution of surface stations.
DE: 8168 Stresses--general
DE: 7203 Body wave propagation
DE: 7205 Continental crust (1242)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting