Collaborative Research: Laboratory Investigations of the Origin of Fault-Zone Pulverized Rock

与地震有关的一个重要问题是，在地震发生的快速滑动过程中，防止断层滑动的应力的大小。如果由于各种提出的高速衰减机制，该应力很小，则意味着地震期间的应力下降可能很大，因为地震前的应力在许多情况下都被认为是很大的。较大的应力降会引起较大的加速度，进而导致较大的破坏性地面运动。该项目正在评估一种高速减弱机制的可能性，该机制可能已经在活动断裂带上留下了证据。这种机制包括在高速滑动期间动态打开断裂带，以便在接触压力减小或没有接触压力的情况下发生滑动。有人提出，这种快速打开可能会导致断裂带中岩石的粉碎化，这可能是这种动态断裂减弱机制的诊断。事实上，最近在南加州圣安德烈亚斯和其他断裂的活动痕迹上描述了不寻常的粉碎岩。这些岩石似乎在没有经历重大应变的情况下就已经破碎到位，并且具有细小的颗粒尺寸，主要在30-200微米(微米)的范围内。这些岩石的来源尚不清楚。在过去，它们可能被称为断层泥，并被认为经历了平行于附近断层滑动方向的剪切应变，但情况似乎并非如此。一个重要的问题是，它们是否是对所提出的断裂过程中断层开放的减弱机制的诊断，该机制将允许地震造成比通常假设更大的破坏。一些工作人员提出，它们是对发生在几公里深的过程的诊断，而另一些人则认为它们只在地球表面附近形成。在这个项目中，研究人员正在进行一系列实验，以调查在实验室中是否可以在控制良好的条件下产生粉状岩石，以及它们是否对任何特定的形成过程具有诊断作用。各种准静态和动态加载实验正在进行中，以确定是否确实需要动态条件来产生粉碎，如果是，控制岩石开始破碎过程和从初始破碎到完全粉碎转变的临界动态加载条件是什么。作为这项研究的一部分，科学家们正在进一步表征在圣安德烈亚斯莫哈韦段沿线几个地方收集的花岗岩粉状样本，以便比较自然变形的岩石和实验变形的岩石。这项研究的结果将使他们能够辨别这些沿着活动断层发现的粉碎岩是否实际上是动态破裂的诊断，以及它们是否对任何特定的地层深度具有诊断作用。如果调查成功，他们可能能够得出结论，沿着活动断裂表面痕迹发现的粉碎化是否诊断为同震滑动期间动态断裂张开导致的减弱。因此，这一结果可能会让我们更好地了解地震造成的破坏性地面运动可能会变得多大。

An important question concerning earthquakes concerns the magnitude of stress that resists slip on faults during the rapid slip that occurs in an earthquake. If this stress is small, due to a variety of proposed high-speed weakening mechanisms, then it means that the stress drops during earthquakes could be large, since the pre-earthquake stress is envisioned to be large in many settings. Large stress drops cause large accelerations, which in turn cause large damaging ground motions. This project is evaluating the possible operation of one high-speed weakening mechanism that may have left evidence along active fault zones. This mechanism involves dynamic opening of a fault zone during high-speed slip so that the slip occurs with reduced or no contact pressure. It has been proposed that this rapid opening could cause pulverization of the rocks in the fault zone, and that these might be diagnostic of this dynamic fault weakening mechanism.In fact, unusual pulverized rocks have been described recently along the active traces of the San Andreas and other faults in Southern California. These rocks appear to have been shattered in place without having experienced significant strain, and have fine grain sizes, primarily in the 30-200 um (micrometer) range. The origin of these rocks is unknown. In the past, they might have been called fault gouge and presumed to have experienced shear strain parallel to the slip direction of nearby faults, but this does not appear to be the case. An important question is whether they are diagnostic of the proposed weakening mechanism of fault opening during rupture that would allow more damage from earthquakes than is typically assumed. Some workers have suggested that they are diagnostic of processes occurring at several kilometers depth, while others think they are only formed near the Earth's surface.In this project the researchers are conducting a series of experiments to investigate whether pulverized rocks can be produced under well-controlled conditions in the laboratory and whether they are diagnostic of any particular process of formation. A variety of quasi-static and dynamic loading experiments are underway to determine whether dynamic conditions are in fact required to produce pulverization, and, if so, what are the critical dynamic loading conditions that control the process of initiation of fragmentation in rocks and the transition from incipient fragmentation to complete pulverization. As part of this study, scientists are further characterizing samples of the pulverized granites collected in several places along the Mojave section of the San Andreas in order to compare naturally and experimentally deformed rocks. The results of the study should allow them to discern whether these pulverized rocks found along active faults are in fact diagnostic of dynamic rupture and whether they are diagnostic of any particular depth of formation. If the investigation is successful, they may be able to conclude whether or not the pulverization found along the surface traces of active faults is diagnostic of weakening by dynamic fault opening during coseismic slip. Thus, the results may allow us to understand better how large the damaging ground motions from earthquakes may become.