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

EAR-0352119Silver我们寻求开发一种稳健的方法，通过利用地震速度的应力相关性来直接测量地下构造应力的时间变化。地壳地震速度对裂隙性质的依赖，进而裂隙性质对应力的依赖，意味着地震速度表现出应力相关性。原则上，这种相关性构成了研究地下应力瞬变的有力工具。虽然这些关系及其科学潜力几十年来就已为人所知，但依赖时间的地震成像的使用尚未发展成为测量地下孕震应力变化的可靠手段。这主要有两个原因：1)缺乏足够的时延精度来检测应力的微小变化，2)在应力和地震速度之间建立可靠的校准是困难的。这两个问题是耦合在一起的，因为最好的校准源是固体固体潮和大气压，这两者都会产生102-103pA的弱应力扰动。探测这些震源需要在实验室实验的基础上测量10-5-10-6量级的速度变化分数。我们建议在圣安德烈亚斯断层的帕克菲尔德地区进行井间实验，目的是成功地测量潮汐和大气压引起的延迟时间变化，作为证明已知应力变化可以可靠测量的一种手段。通过利用人工震源特性的进步(如快速可靠的重复性)、计算能力的显著进步(允许大规模叠加地震波形)，将有可能以前所未有的精度测量地震速度的变化，以满足上述要求。这将允许对该方法进行有意义的应力校准。拟议的实验包括两个阶段：一个测试阶段在劳伦斯伯克利国家实验室及其附近的两个地点进行，另一个实验阶段在帕克菲尔德的2公里深的SAFOD试验孔现场进行，在那里将使用越来越逼真的几何形状。最终，我们将发射到先导孔的接收器，这将使我们能够检索地震孕育深度的应力信息。*

EAR-0352119SilverWe 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.***