Collaborative Research: Developing a Methodology for Imaging Stress Transients at Seismogenic Depth

William Marsh Rice University, Houston TX

Investigators

Abstract

EAR-0352134 Niu We seek to develop a robust methodology for directly measuring temporal variations in subsurface tectonic stress by exploiting the stress dependence of seismic velocity. The dependence of crustal seismic velocities on crack properties and in turn, the dependence of crack properties on stress, means that seismic velocity exhibits stress dependence. This dependence constitutes, in principle, a powerful instrument for studying subsurface transient changes in stress. While these relationships and their scientific potential have been known for several decades, the use of time-dependent seismic imaging, has not yet developed into a reliable means of measuring subsurface seismogenic stress changes. There are two primary reasons for this: 1) lack of sufficient time-delay precision to detect small changes in stress, and 2) the difficulty in establishing a reliable calibration between stress and seismic velocity. These two problems are coupled because the best sources of calibration are the solid-earth tides and barometric pressure, both of which produce weak stress perturbations of order 102-103 Pa. Detecting these sources requires measurement of fractional velocity changes on the order of 10-5-10-6, based on laboratory experiments. We propose to conduct a cross-well experiment in the Parkfield area of the San Andreas Fault, with the goal of successfully measuring delay-time variations induced by tides and barometric pressure, as a means of demonstrating that known stress changes can be reliably measured. By exploiting advances in the characteristics of artificial seismic sources (such as rapid and reliable repeatability), dramatic advances in computational power (allowing massive stacking of seismic waveforms), it will be possible to measure variations in seismic velocity with unprecedented precision that meet the requirement noted above. This will permit a meaningful stress calibration of this method. The proposed experiment consists of two phases: a test phase to take place at two locations at and near Lawrence Berkeley National Laboratory, and an experimental phase at Parkfield on the site of the 2-km-deep SAFOD Pilot Hole, where progressively more realistic geometries will be utilized. Ultimately, we will shoot to receivers in the Pilot Hole, which will allow us to retrieve stress information at seismogenic depths. ***