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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