HR: 0800h
AN: T51C-0681    [Abstracts]
TI: Rapid Temporal Changes of Fault Zone Site Response Associated With Strong Ground Motion
AU: * Wu, C
EM: chunquanwu@gatech.edu
AF: School of Earth and Atmospheric Sciences, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332,
AU: Peng, Z
EM: zpeng.seismo@gmail.com
AF: School of Earth and Atmospheric Sciences, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332,
AU: Ben-Zion, Y
EM: benzion@usc.edu
AF: Department of Earth Sciences, University of Southern California, Los Angeles, 3651 Trousdale Pky, Los Angeles, CA 90089-0740,
AB: We systematically analyze temporal changes in fault zone (FZ) site response along the Karadere-Düzce branch of the North Anatolian fault that ruptured during the 1999 \.{I}zmit and Düzce earthquake sequences. The study involves primarily comparisons of strong motion seismic data recorded by station VO inside the Karadere fault and station FP ~300 m away from the fault starting 8 days before and ending 72 days after the D¨ązce mainshock. The spectral ratio between stations VO and FP is computed from the averaged spectra for the two horizontal components, and is used as a measure for FZ site response. The peak spectral ratio increases 80-150% and the peak frequency drops 20-40% at the time of the Düzce mainshock, and is followed by near-complete recovery with time scale of ~1 day. The observed temporal changes of FZ site response can be explained as reduction of seismic velocities by opening of pre-existing cracks inside the FZ due to strong ground motion, followed by logarithmic recovery. Our observations suggest nonlinear behavior of the fault zone material under strong ground motion of nearby major earthquakes. We also apply this method to the weak motion records generated by the 36 repeating earthquake clusters identified by Peng and Ben-Zion (2006) during the same period, but no clear temporal changes of peak spectral ratio or peak frequency is observed. This is likely because the first post-Düzce events in the repeating clusters occurred at least a few hours after the Düzce mainshock, when most of the rapid coseismic changes have been recovered.
DE: 7200 SEISMOLOGY
DE: 7205 Continental crust (1219)
DE: 7250 Transform faults
DE: 8100 TECTONOPHYSICS
DE: 8111 Continental tectonics: strike-slip and transform
SC: Tectonophysics [T]
MN: 2007 Fall Meeting