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

我们试图开发一种强大的方法，通过利用地震速度的应力依赖性来直接测量地下构造应力的时间变化。地壳地震速度对裂缝性质的依赖，反过来，裂缝性质对应力的依赖，意味着地震速度表现出应力依赖性。这种依赖性在原则上构成了研究地下应力瞬态变化的有力工具。虽然这些关系及其科学潜力在几十年前就已为人所知，但时间相关地震成像的使用尚未发展成为测量地下发震应力变化的可靠手段。造成这种情况的主要原因有两个：1)缺乏足够的时间延迟精度来检测应力的微小变化；2)难以在应力和地震速度之间建立可靠的校准。这两个问题是耦合的，因为校准的最佳来源是固体潮汐和大气压，两者都产生102-103 Pa数量级的弱应力扰动。探测这些源需要在实验室实验的基础上测量10-5-10-6量级的分数速度变化。我们建议在圣安德烈亚斯断层的帕克菲尔德地区进行一次井间实验，目的是成功测量潮汐和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（如快速可靠的可重复性），计算能力的巨大进步（允许大量叠加地震波形），将有可能以前所未有的精度测量地震速度的变化，从而满足上述要求。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：一个测试阶段将在劳伦斯伯克利国家实验室及其附近的两个地点进行，另一个实验阶段将在帕克菲尔德的2公里深的SAFOD先导孔现场进行，在那里将逐步使用更真实的几何形状。最终，我们将发射到先导孔中的接收器，这将使我们能够在发震深度检索应力信息

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.***