Developing a Methodology for Measuring Stress Transients at Seismogenic Depth

The dependence of crack properties on stress means that crustal seismic velocity exhibits stress dependence. This dependence constitutes, in principle, a powerful means of studying transient changes in stress at seismogenic depth. While its scientific potential has been known for decades, time-dependent seismic imaging has not (yet) become a reliable means of measuring subsurface stress changes. This is because of 1) insufficient delay-time precision necessary to detect small changes in stress, and 2) the difficulty in establishing a reliable in-situ calibration between stress and seismic velocity. These two problems are coupled because the best sources of calibration, solid-earth tides and barometric pressure, produce weak stress perturbations of order 10{^{2}-10{^3}} Pa that require precision in the measurement of the fractional velocity change dlnv of order 10^-6, based on laboratory experiments. We have initiated a series of three experiments to demonstrate the detectability of these stress-calibration signals in progressively more tectonically relevant settings. Initial tests have been completed on the smallest scale, with two boreholes 17 m deep and 3 meters apart. We have used a piezoelectric source (0.1ms source pulse repeated every 100ms) and a string of 24 hydrophones to record P waves with a dominant frequency of 10KHz. Recording was conducted for 160 hours. The massive stacking of ~36,000 high-SNR traces/hr leads to delay-time precision of 6ns (hour sampling) corresponding to dlnv precision of 3 x 10{^-6}. We find that barometric pressure fluctuations are easily observed in the delay time data with a SNR of 1000. Also, while lower in amplitude, diurnal and semidiurnal solid-earth-tidal components are also observed. We have also conducted preliminary tests at the Richmond Field Facility, which permits cross-borehole recordings at a distance of 30 m, and depths to 70 m, using the same equipment. The dominant frequency in this case was 1KHz. While only very short time segments have thus far been analyzed, the preliminary data show that we are able to attain the same high precision (dlnv of order 10{^-6}) as in the first experiment. A third experiment is planned using the 7-level 3-compoment geophone string in the SAFOD Pilot hole at Parkfield,CA, that spans the depth range 850m - 1100 m. Making use of a specially designed 18-element piezoelectric source deployed in a relatively shallow source hole (100m) on the same pad as the Pilot Hole, we plan to shoot along a near-vertical trajectory to these geophones to again detect the presence of tidal and barometric fluctuations in the seismic wavefield. We expect to obtain stress-induced temporal changes in interval properties (between the shallowest and deepest levels) which, if confirmed, would demonstrate the ability to measure KPa-level stress variations at near-seismogenic depth.