Stress-induced traveltime variations at SAFOD revealed by continuous cross-well active source monitoring

The time-varying stress/strain field at seismogenic depths is arguable the single most important property controlling the sequencing and nucleation of seismic events. The measurement of stress, however, is notoriously difficult, particularly at seismogenic depths. Seismic imaging, in principle, has the capability to provide this critical depth component. Numerous laboratory studies over the last few decades have shown that the elastic properties of crustal rocks clearly exhibit stress dependence. Such dependence is attributed to the opening/closing of fluid-filled cracks in response to changes in the stress normal to the crack surface. Temporal changes in stress are thus, in principle, measurable through seismic imaging of changes in elastic properties, such as seismic velocity field. We have been conducting continuous cross-well active source experiments utilizing the SAFOD (San Andreas Fault Observatory at Depth) pilot and main holes to develop a seismic stress meter to monitor the subsurface stress field by exploring the velocity-stress sensitivity. In a two-month period in 2005-2006, we found a 0.3% change in the average S-wave velocity, which shows a good correlation with barometric pressure, corresponding to a stress sensitivity of 2.4x10-7Pa-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. The two excursions started approximately 10 and 2 hours before the events, respectively, suggesting that they may be related to pre-rupture stress induced changes in crack properties, as observed in early laboratory studies. We repeated the experiment in early 2010 with a slightly different experiment configuration, and collected 40-days data. The new data confirmed the negative correlation between traveltime and barometric pressure. The estimated stress sensitivity is 2.5 x10-7Pa-1. No tectonic stress changes were observed during this period, which was consistent with the calculated coseismic stress changes of the local seismicity. Both experiments suggest that active seismic monitoring is an effective way for understanding stress changes that accompany and perhaps precede seismic activity.