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

合作研究：开发震源深度应力瞬变成像方法：数据分析和解释

• 批准号: 0453471
• 负责人: Fenglin Niu
• 金额: --
• 依托单位: William Marsh Rice University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2005
• 资助国家: 美国
• 起止时间: 2005-04-01 至 2009-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0453471&HistoricalAwards=false
• 关键词: Collaborative; Research; Developing; Methodology; Imaging

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).

0453471Niu断层系统的时变应力状态可能是控制地震事件的顺序和成核的最重要的属性。作为 Earthscope 的一部分，表面大地测量仪器的部署将通过对随时间变化的表面应变场的综合观测，为我们提供对该应力场的重要约束。作为增强这些限制的一种方法，PI 目前正在开发一种基于地震速度应力依赖性的主动源方法，以测量地下应力瞬态。几十年来的大量实验室研究表明，地壳岩石的弹性特性（地震速度、衰减、各向异性）明显表现出应力依赖性。 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微裂纹的打开/闭合。 因此，原则上可以通过利用发震地壳弹性特性的应力敏感性来检测应力变化。几十年来，人们一直在努力利用这种压力依赖性，尽管迄今为止这个目标还难以实现。造成这种情况的主要原因有两个：1) 缺乏足够的时延精度来检测应力的微小变化，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地球潮汐和气压，两者都会产生 102-103 Pa 量级的微弱应力扰动。检测这些源需要根据实验室实验测量 10-5-10-6 量级的分数速度变化。 PI 一直在不同尺度上进行一系列跨孔主动源实验：在劳伦斯伯克利国家实验室 (LBNL) 设施中进行 3 m 间距，在 LBNL 里士满现场站 (RFS) 中进行 30 m 间距，以及在 2 km 深度处进行 300 m 间距，从 Parkfield 的 SAFOD 先导孔向 SAFOD 主孔的传感器进行射击。 到目前为止，他们已经完成了第一个站点的工作，在 RFS 进行了多次测量，并正在完成 RFS 的最终测试。对两个测试数据集的初步分析表明，可以达到所需的10-6阶延迟时间精度，并且可以观测到气压和潮汐引起的走时变化。利用 RFS 和 Parkfield 的最终数据集，PI 计划结合数值建模进行以下分析： (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波的幅度测量； (4) 横波分裂测量； (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，以及2）计算这种介质的相应的地震特性。然后，最有希望的特性将用于开发定量应力校准，通过选择适当的多孔弹性介质来建立，该介质考虑了观察到的应力敏感性和地震特性（包括散射）。