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Seismological Studies at Parkfield VIII: Modeling the Observed Travel-Time Changes

Center for Computational Seismology
Lawrence Berkeley National Laboratory
(V. A. K., T. V. M., E. D. K.)

Berkeley Seismological Laboratory
University of California, Berkeley, California, 94720
(T. V. M.)

Enterprise Oil Exploration Limited
Maroussi 151 25 Athens
Greece
(E. D. K.)

For 10 years, as one element of the Parkfield, California, Prediction Experiment, the borehole seismographic network was illuminated routinely by a large shear-wave Vibroseis from several source points to investigate the stability of wave propagation in the fault zone and the possibility of nucleation-related premonitory phenomena. Clear and progressive travel-time changes of up to 50 msec were detected during the study, most prominent in the S-wave coda, and localized to propagation paths through the shallow fault zone (above about 500 m depth) southeast of Middle Mountain, the section of the fault where previous M 6 earthquakes have initiated. We model the observations successfully as interaction (reflection and transmission) of the shallow wavefield with a 200-meter-wide low-velocity fault zone in which the velocity increases by 6%, due, we hypothesize, to hydrological changes accompanying a significant pulse in fault slip rate and seismicity.

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