Collaborative Research: Imaging Stress Transients and Fault Zone Processes with Continuous Cross-Well Active Source Seismic Measurements at SAFOD

合作研究：通过 SAFOD 连续井间主动源地震测量对应力瞬变和断层带过程进行成像

• 批准号: 1251667
• 负责人: Fenglin Niu
• 金额: $ 46.33万
• 依托单位: William Marsh Rice University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-09-01 至 2021-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1251667&HistoricalAwards=false
• 关键词: Collaborative; Research; Imaging; Stress; Transients

Non-technical SummaryEarthquakes are caused by the sudden release of stresses along faults. Plate tectonics describes the process of long-term strain accumulation, but the specifics of stress release -- what ultimately leads to fault failure and whether an earthquake (small or large), tremor, or slow slip results -- are still not well understood. The time-varying stress/strain field at the depths where earthquakes begin is one of the most important properties controlling the sequencing and nucleation of these seismic/aseismic events. The importance of deformation transients has been illustrated by the success of predicted stress changes in accounting for the spatial distribution of aftershocks, as well as providing one explanation for the clustering of large seismic events, such as the nearby 1992 Landers and 1999 Hector Mine earthquakes. The measurement of stress, however, is notoriously difficult, particularly at depth. Geodesy provides important constraints on the surface deformation field, which can be related to stress through an assumed rheology. Yet, the constraints on the depth distribution of stress and strain from geodesy are limited. These surface constraints need to be combined with other techniques that, while not as directly related to stress and strain, have superior depth resolution. The most promising techniques at present appear to be seismic. For example, patterns of seismicity have long been used to make inferences about the stress state before and after seismic events. Yet, a basic requirement for this approach to work is that changes in stress over time generate seismic waves that can be observed at the surface, which means that aseismic changes in stress and/or strain cannot be observed in this manner. One way of assessing this fully aseismic component is through observations of temporal changes in the elastic properties of the crust. Indeed, such earthquake-related changes have long been predicted and sometimes observed, due to stress-induced changes in the characteristics and or distribution of fluid-filled cracks. Yet, it has been difficult to conclusively observe temporal variations in the medium, due both to the small signal level, and to the problem of accounting for other systematic effects that may produce apparent changes, such as variations in source location or shallow environmental influences. Technical DescriptionWe are conducting a continuous cross-well active-source seismic experiment utilizing the SAFOD (San Andreas Fault Observatory at Depth) pilot and main holes. The broad, long term goal of this experiment is to develop a tool to monitor the time-varying stress field associated with earthquakes and other stress-dependent earth processes, such as aseismic slips and non-volcanic tremors through the detection of temporal changes in the crustal velocity structure at seismogenic depths. This technique would be a type of "stress meter". Such a monitoring system would perhaps be the single most important means of understanding the triggering processes of seismic and aseismic events. The fundamental physics behind a seismic stress meter is well established. Numerous laboratory studies over several decades have shown that seismic velocities clearly exhibit stress dependence, usually attributed to changes in the physical characteristics of cracks (e.g. crack density, crack orientation). Our current experiment is built on a previous experiment we conducted at the SAFOD site in 2005 and 2006. Over a two-month period, we observed a negative correlation between changes in the time required for a shear wave to travel through the rock between the pilot and main hole (a few microseconds) and variations in barometric pressure (about 1 kilopascal). This result is a "calibration" of the stress sensitivity of seismic velocity at our experiment site. We also observed two large excursions in the travel-time data that are coincident with two earthquakes, a magnitude 3 and a magnitude 1 earthquake, that occurred sufficiently close to produce large coseismic stress changes at the SAFOD site. The two excursions started approximately 10 and 2 hours before the magnitude 3 and 1 earthquakes, respectively, suggesting that they may be related to pre-rupture changes in crack properties, as observed in the early laboratory studies. In the current experiment, we are using a similar equipment configuration to collect data that sample 10-15 magnitude 2 to 3 local earthquakes. We are using a cross-correlation based method and the coda wave interferometry technique to image systematic changes in medium near the SAFOD site, and use them to monitor temporal changes in fault zone processes near Parkfield, CA. This project has important implications for the study of earthquakes and for the EarthScope program. It will demonstrate whether there are measurable changes in seismic velocity structure near the source region of an impending earthquake immediately preceding the rupture. As such, this work could lead to significant improvement in our understanding on physical processes prior to earthquakes. It will represent significant progress toward measuring stress transients associated with earthquakes and other processes, demonstrating that tectonic stress could be continuously monitored with continuous active source borehole observations.

非技术性概述地震是由应力沿断层沿着突然释放引起的。板块构造描述了长期应变积累的过程，但应力释放的具体细节-最终导致断层破坏以及地震（小或大），震颤或缓慢滑动的结果-仍然没有得到很好的理解。在地震开始的深度处的随时间变化的应力/应变场是控制这些地震/地震事件的顺序和成核的最重要的性质之一。变形瞬变的重要性已经说明了成功的预测应力变化占空间分布的余震，以及提供一个解释的集群的大地震事件，如附近的1992年兰德斯和1999年赫克托矿地震。然而，应力的测量是出了名的困难，特别是在深度处。 大地测量提供了重要的限制，表面变形场，这可以通过一个假设的流变应力。 然而，大地测量学对应力和应变深度分布的限制是有限的。 这些表面约束需要与其他技术相结合，这些技术虽然与应力和应变不直接相关，但具有上级深度分辨率。 目前最有希望的技术似乎是地震技术。 例如，地震活动的模式长期以来一直被用来推断地震事件前后的应力状态。然而，这种方法工作的一个基本要求是，应力随时间的变化产生可以在地表观察到的地震波，这意味着不能以这种方式观察到应力和/或应变的非稳态变化。评估这一完全地震成分的方法之一是通过观测地壳弹性性质的时间变化。 事实上，这种与地震有关的变化早已被预测，有时也被观察到，这是由于应力引起的流体填充裂缝的特性和/或分布的变化。然而，由于信号电平很小，以及考虑可能产生明显变化的其他系统效应（如震源位置变化或浅层环境影响）的问题，很难最终观察到介质中的时间变化。技术描述我们正在进行一个连续的跨井主动源地震实验，利用SAFOD（圣安德烈亚斯断层观测深度）的试点和主要孔。这项实验的广泛、长期目标是开发一种工具，通过检测孕震深度地壳速度结构的时间变化，监测与地震和其他应力相关的地球过程（如非地震滑动和非火山震颤）相关的时变应力场。这种技术将是一种“压力计”。这种监测系统也许是了解地震和地震事件触发过程的唯一最重要手段。地震应力计背后的基本物理原理已经很好地建立起来。几十年来的大量实验室研究表明，地震速度明显地表现出应力依赖性，通常归因于裂缝的物理特性（例如裂缝密度、裂缝方向）的变化。 我们目前的实验是建立在2005年和2006年我们在SAFOD现场进行的先前实验的基础上的。在两个月的时间里，我们观察到剪切波穿过先导孔和主孔之间的岩石所需时间（几微秒）的变化与大气压力（约1千帕）的变化之间存在负相关关系。这一结果是对我们实验场地地震速度应力敏感性的一次“校准”。我们还观察到两个大的偏移的走时数据，是一致的两个地震，3级和1级地震，发生足够接近，产生大的同震应力变化在SAFOD网站。这两次偏移分别在3级和1级地震前约10小时和2小时开始，表明它们可能与早期实验室研究中观察到的破裂前裂纹特性变化有关。在目前的实验中，我们正在使用类似的设备配置来收集10-15次2至3级当地地震的样本数据。我们正在使用互相关为基础的方法和尾波干涉测量技术的图像系统的变化，在介质附近的SAFOD网站，并使用它们来监测的时间变化，在帕克菲尔德，CA附近的断层带过程。该项目对地震研究和地球镜计划具有重要意义。它将证明在即将发生的地震的震源区附近是否有可测量的地震速度结构的变化。因此，这项工作可能会大大提高我们对地震前物理过程的理解。它将代表测量与地震和其他过程相关的应力瞬变的重大进展，表明可以通过连续的活动源钻孔观测来连续监测构造应力。