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

地震是由沿断层的应力突然释放引起的。板块构造学描述了长期应变积累的过程，但应力释放的细节——最终导致断层破裂的原因，以及是地震（小地震或大地震）、震颤还是缓慢滑动的结果——仍然没有得到很好的理解。地震发生深度的时变应力/应变场是控制这些地震/地震事件顺序和成核的最重要的性质之一。变形瞬态的重要性已经通过成功地预测应力变化来说明余震的空间分布，并为大型地震事件的聚集提供了一种解释，例如附近的1992年兰德斯和1999年赫克托雷地震。然而，测量压力是出了名的困难，尤其是在深度处。大地测量学对地表变形场提供了重要的约束，地表变形场可以通过假设的流变学与应力有关。然而，大地测量对应力应变深度分布的约束是有限的。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没有直接关系，但具有更好的深度分辨率。目前最有前途的技术似乎是地震技术。例如，地震活动性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，应力随时间的变化会产生可以在地表观察到的地震波，这意味着应力和/或应变的地震变化不能用这种方式观察到。评估这种完全抗震成分的一种方法是通过观察地壳弹性特性的时间变化。事实上，这种与地震有关的变化早已被预测到，有时也被观察到，这是由于应力引起的充满流体的裂缝的特征和/或分布的变化。然而，由于信号水平小，以及考虑可能产生明显变化的其他系统影响（如源位置的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。技术描述我们正在利用SAFOD （San Andreas Fault Observatory at Depth）先导孔和主孔进行连续的井间活源地震实验。该实验的长远目标是开发一种工具，通过探测发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（如地震滑动和非火山震动）相关的时变应力场。这种技术将是一种“压力计”。这样一个监测系统可能是理解地震和地震事件触发过程的唯一最重要的手段。地震应力计背后的基本物理原理已经得到了很好的证实。几十年来的大量实验室研究表明，地震速度明显表现出应力依赖性，通常归因于裂缝物理特征的变化（例如裂缝密度、裂缝方向）。我们目前的实验是建立在我们2005年和2006年在SAFOD站点进行的先前实验的基础上的。在两个月的时间里，我们观察到横波穿过导孔和主孔之间岩石所需的时间变化（几微秒）与气压变化（约1千帕）之间呈负相关。这一结果是对我们实验场地地震速度应力敏感性的“校准”。我们还观察到两次旅行时间数据的大偏移，与两次地震同时发生，一次是3级地震，一次是1级地震，这两次地震发生得足够近，在SAFOD站点产生了大的同震应力变化。这两次偏移分别发生在3级和1级地震前10和2小时左右，这表明它们可能与早期实验室研究中观察到的裂缝破裂前的变化有关。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级2到3级局部地震的数据。我们正在使用基于互相关的方法和尾波干涉测量技术来成像SAFOD站点附近介质的系统变化，并使用它们来监测加利福尼亚州帕克菲尔德附近断裂带过程的时间变化。该项目对地震研究和EarthScope项目具有重要意义。它将证明在破裂之前即将发生地震的震源区域附近是否存在可测量的地震速度结构变化。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。它将代表着在测量与地震和其他过程相关的应力瞬态方面取得的重大进展，表明构造应力可以通过连续的活源钻孔观测来连续监测。

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.