ABSTRACT

In situ stress measurement at seismogenic depth is critically important for deciphering fault zone processes. In this study, we conducted a second active‐source crosswell field experiment at the Parkfield San Andreas Fault Observatory at Depth (SAFOD) drill site to investigate the detectability of stress‐induced seismic velocity changes at the top part of the seismogenic zone. We employed the same configuration of our previous experiments, which deployed a piezoelectric source and a three‐component (3C) accelerometer at 1 km deep inside the pilot and main holes, respectively. We also added a hydrophone, which is attached to the source, to monitor the repeatability of the source waveforms. Over a 40‐day recording period, we confirmed an 0.04% travel‐time variation in S wave and coda that roughly follows the fluctuation of barometric pressure. We attributed this correlation to stress sensitivity of seismic velocity and the stress sensitivity is estimated to be 2.0×107  Pa1, which is approximately two orders of magnitude higher than those measured in laboratory with dry rock samples, but is consistent with our previous results. Our results confirm the hypothesis that substantial cracks and/or pore spaces exist at seismogenic depths and thus may be used to monitor the subsurface stress field with active‐source crosswell seismic.

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