Detection of Subsurface Stress/Strain Changes with Active Source Monitoring at the Parkfield SAFOD Drill Site

The time-varying stress field at seismogenic depth is arguably the single most important parameter for understanding the earthquake triggering process. Measuring stress changes within seismically active fault zones has consequently been a long-sought goal of seismology. It is well known from laboratory experiments that seismic velocities vary with the level of the applied stress. In principle, this dependence constitutes a stress meter, provided the induced velocity changes can be measured precisely and continuously. We have conducted several continuous active-source cross-well experiments to measure in situ seismic velocity changes along fixed baselines at the Earth's surface and near-seismogenic depth. At the Parkfield SAFOD drill site in 2005-2006, we conducted a continuous active-source cross-well experiment to measure in-situ seismic velocity changes across a ~10 m baseline at ~1 km depth. Over a two-month period, we found a 0.3% change in the average S-wave velocity, which shows a good negative correlation with barometric pressure, corresponding to a stress sensitivity of 2.4x10{-7}Pa-1. We also observed two large excursions in the delay-time measurement, corresponding to 0.55% and 0.15% decreases of seismic velocity, that are coincident with two earthquakes that are among those predicted to produce the largest coseismic stress changes at SAFOD. Interestingly, the two excursions started approximately 10 and 2 hours before the events, respectively, suggesting that they may be related to pre-rupture dilatancy observed in the early laboratory studies, and that cross-well monitoring might provide an effective tool for understanding the stress changes that accompany and perhaps precede seismic activity.