Collaborative Research: Developing a Methodology for Imaging Stress Transients at Seismogenic Depth: Data Analysis and Interpretation

断层系统随时间变化的应力状态也许是控制地震事件的顺序和成核的唯一最重要的属性。作为“地球镜”的一部分，地面大地测量仪器的部署将通过对随时间变化的地面应变场的全面观测，为我们提供对这一应力场的重要制约。作为增强这些约束的一种方式，PI目前正在开发一种基于地震速度的应力依赖性的有源方法，以测量地下应力瞬变。几十年来的大量实验室研究表明，地壳岩石的弹性性质（地震速度、衰减、各向异性）明显地表现出应力依赖性。 这种依赖性归因于由于垂直于裂纹表面的应力的变化而导致的微裂纹的打开/闭合。 因此，原则上可以通过利用孕震地壳弹性特性的应力敏感性来探测应力变化。几十年来，人们一直在努力利用这种压力依赖性，尽管这一目标迄今为止还难以实现。这主要有两个原因：1）缺乏足够的时间延迟精度来检测应力的微小变化，以及2）难以在应力和介质的地震特性之间建立可靠的校准。 这两个问题是耦合的，因为最好的校准源是固体潮汐和大气压，这两者都产生102-103 Pa量级的弱应力扰动。检测这些源需要基于实验室实验测量10-5-10-6量级的分数速度变化。 研究所一直在进行一系列不同规模的跨孔有源实验：在劳伦斯伯克利国家实验室（LBNL）设施的3米间距，在LBNL里士满野外站（RFS）的30米间距，以及在2公里深处的300米间距，从帕克菲尔德的SAFOD先导孔射击到SAFOD主孔的传感器。 到目前为止，他们已经完成了第一个站点的工作，在RFS进行了几次测量，并正在RFS完成最后的测试。两个测试数据集的初步分析表明，它是可能的，以达到所需的延迟时间精度的顺序10-6，和大气压力和潮汐引起的旅行时间的变化可以观察到。利用RFS和Parkfield的最终数据集，PI计划进行以下分析以及数值建模：（1）P波及其尾波的延迟时间估计;（2）S波及其尾波的延迟时间估计;（3）P波和S波的振幅测量;（4）S波分裂测量;（5）使用P波和S波尾波的散射场成像。数值建模包括：1）确定相应的多孔弹性介质的特性及其对已知应力的响应，和2）计算这种介质的相应地震特性。最有前途的属性，然后被用来开发一个定量的应力校准，建立通过选择一个适当的孔隙弹性介质，占所观察到的应力敏感性和地震属性（包括散射）。

0453471NiuThe time-varying stress state of fault systems is perhaps the single most important property controlling the sequencing and nucleation of seismic events. The deployment of surface geodetic instrumentation, as part of Earthscope, will provide us with important constraints on this stress field, through comprehensive observations of the time-varying surface strain field. As a way of augmenting these constraints, the PIs are presently developing an active-source methodology based on the stress dependence of seismic velocity to measure subsurface stress transients. Numerous laboratory studies over several decades have shown that the elastic properties (seismic velocity, attenuation, anisotropy) of crustal rocks clearly exhibit stress dependence. Such dependence is attributed to the opening/closing of microcracks due to changes in the stress normal to the crack surface. Thus stress changes can, in principle, be detected by exploiting the stress sensitivity of the elastic properties of the seismogenic crust. For decades there have been efforts to exploit this stress dependence, although this goal has thus far been elusive. There are two primary reasons for this: 1) lack of sufficient time-delay precision necessary to detect small changes in stress, and 2) the difficulty in establishing a reliable calibration between stress and the seismic properties of the medium. 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. The PIs have been conducting a series of cross-hole active-source experiments at different scales: 3 m spacing at the Lawrence Berkeley National Laboratory (LBNL) facility, 30 m spacing at the LBNL Richmond Field Station (RFS), and, 300 m spacing at 2 km depth, shooting from SAFOD Pilot Hole at Parkfield to sensors at the SAFOD main hole. Thus far they have completed work at the first site, have made several measurements at the RFS, and are completing the final test at RFS. Preliminary analyses of the two test datasets suggest that it is possible to reach the required delay time precision of order 10-6, and that barometric pressure and tidally-induced changes in travel time can be observed. With the final datasets from RFS and Parkfield, the PIs are planning to conduct the following analysis together with numerical modeling: (1) delay time estimates of the P wave and its coda; (2) delay time estimates of the S wave and its coda; (3) amplitude measurements for both P and S wave; (4) S-wave splitting measurements; (5) scattered-field imaging using P- and S-wave coda. Numerical modeling includes: 1) determining the characteristics of the corresponding poroelastic medium and its response to known stresses, and 2) calculating the corresponding seismic properties of such a medium. The most promising properties are then to be used to develop a quantitative stress calibration, established through the choice of an appropriate poroelastic medium that accounts for both the observed stress sensitivity and seismic properties (including scattering).