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: 0453638
• 负责人: Sean Solomon
• 金额: --
• 依托单位: Carnegie Institution of Washington
• 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 2010-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0453638&HistoricalAwards=false
• 关键词: Collaborative; Research; Developing; Methodology; Imaging

0453638SilverThe 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).

0453638SILVER，断层系统的时变应激状态可能是控制地震事件测序和成核的最重要属性。作为Earthscope的一部分，表面大地测量仪的部署将通过全面观察到时间变化的表面应变场来为我们提供此应力场的重要限制。作为增强这些约束的一种方式，PIS目前正在基于地震速度的应力依赖性来测量地下应力瞬变的应力依赖性。数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）显然表现出应力依赖性。 这种依赖性归因于由于压力正常的裂纹表面变化而导致的微裂纹开放/关闭。 因此，原则上可以通过利用地震构成外壳的弹性特性的应力敏感性来检测应力变化。几十年来，尽管这一目标迄今难以捉摸，但一直努力利用这种压力依赖性。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压力的微小变化，以及2）难以在应力和培养基的地震特性之间建立可靠的校准。 这两个问题是耦合的，因为校准的最佳来源是固体地球潮汐和气压压力，这两者都会产生较弱的应力扰动，命令102-103 PA。检测这些来源需要根据实验室的实验来测量10-5-10-6的分数速度变化。 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. 到目前为止，他们已经在第一个站点完成了工作，在RFS上进行了多次测量，并正在完成RFS的最终测试。对两个测试数据集进行的初步分析表明，可以观察到可以观察到的延迟时间10-6，并且可以观察到气压压力和潮汐诱发的旅行时间变化。随着RFS和Parkfield的最终数据集，PIS计划与数值建模一起进行以下分析：（1）P WAVE及其尾声的延迟时间估计； （2）S波及其尾声的延迟时间估计； （3）P和S波的振幅测量； （4）S波分裂测量； （5）使用p-和s波尾巴散射的场成像。数值建模包括：1）确定相应的毛弹性培养基的特征及其对已知应力的响应，以及2）计算这种培养基的相应地震特性。然后，最有希望的特性用于开发定量应力校准，该特性是通过选择适当的孔弹性介质来确定的，该培养基既说明了观察到的应力敏感性和地震特性（包括散射）。