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

关于地震的一个重要问题是，在地震中发生的快速滑动过程中，阻止断层滑动的应力的大小。如果这种应力很小，由于各种建议的高速弱化机制，那么这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很大。大的应力降导致大的加速度，这反过来又导致大的破坏性地面运动。该项目正在评估一种可能在活动断层带沿着留下证据的高速弱化机制的可能运作。该机制涉及在高速滑动期间断层带的动态打开，使得滑动在接触压力减小或没有接触压力的情况下发生。有人提出，这种快速打开可能会导致断裂带中岩石的粉碎，而这可能是这种动态断层弱化机制的诊断。事实上，最近沿着圣安德烈亚斯和南加州其他断层的活动痕迹描述了不寻常的粉碎岩石。这些岩石似乎是在没有经历显著应变的情况下就地破碎的，并且具有细粒度，主要在30-200 μ m（微米）范围内。这些岩石的来源不明。在过去，它们可能被称为断层泥，并假定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，它们是否诊断了破裂期间断层开放的拟议弱化机制，该机制将允许地震造成比通常假设的更大的破坏。一些研究人员认为，它们可以用来诊断发生在几公里深处的过程，而另一些研究人员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，以研究在实验室控制良好的条件下是否可以产生粉碎的岩石，以及它们是否可以诊断任何特定的形成过程。各种各样的准静态和动态加载实验正在进行中，以确定是否动态条件实际上是需要产生粉化，如果是这样，什么是临界动态加载条件，控制的过程中开始破碎的岩石和过渡从初期破碎到完全粉化。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地区沿着几个地方收集的粉碎花岗岩样本，以便比较自然和实验变形的岩石。研究的结果应该使他们能够辨别出这些沿着活动断层沿着发现的粉碎岩石实际上是否是动态破裂的诊断，以及它们是否是任何特定深度的地层的诊断。如果调查是成功的，他们可能能够得出结论，是否粉化发现沿着表面痕迹的活动断层是诊断削弱的动态断层开放期间同震滑动。因此，这些结果可以让我们更好地了解地震造成的破坏性地面运动可能会有多大。

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.