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

合作研究：断层带粉岩成因的实验室研究

• 批准号: 0711047
• 负责人: Terry Tullis
• 金额: $ 26万
• 依托单位: Brown University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-09-01 至 2012-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0711047&HistoricalAwards=false
• 关键词: Collaborative; Research; Laboratory; Investigations; Origin

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.

关于地震的一个重要问题是，在地震中发生的快速滑动中，抵抗断层滑动的应力大小。如果这个应力很小，由于各种提出的高速弱化机制，那么这意味着地震期间的应力下降可能很大，因为在许多情况下，地震前的应力被设想为很大。大的应力降导致大的加速度，而加速度又导致大的破坏性地面运动。该项目正在评估一种高速弱化机制的可能运作，该机制可能在活动断裂带沿线留下证据。这种机制涉及高速滑动时断层带的动态打开，使滑动发生时接触压力降低或没有接触压力。有人提出，这种快速打开可能导致断裂带内岩石的粉碎，这可能是这种动态断裂弱化机制的诊断。事实上，最近在圣安地列斯断层和南加州其他断层的活动痕迹沿线发现了不寻常的粉状岩石。这些岩石似乎是在没有经历明显的应变的情况下被粉碎的，并且具有细小的晶粒尺寸，主要在30-200微米（微米）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，并被认为经历了与附近断层的滑动方向平行的剪切应变，但事实似乎并非如此。一个重要的问题是，它们是否能诊断出断裂期间断层开口的弱化机制，这种机制可能会导致比通常假设的更大的地震破坏。一些工作人员认为它们是在几公里深处发生的过程的诊断，而另一些人则认为它们只是在地球表面附近形成的。在这个项目中，研究人员正在进行一系列实验，以调查在实验室控制良好的条件下是否可以生产出粉状岩石，以及它们是否可以诊断任何特定的形成过程。各种准静态和动态加载实验正在进行中，以确定是否需要动态条件才能产生粉碎，如果需要，那么控制岩石破碎的起始过程和从初期破碎到完全粉碎的过渡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的莫哈韦部分的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。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状岩石是否实际上是动态破裂的诊断，以及它们是否可以诊断任何特定的地层深度。如果调查成功，他们可能会得出结论，沿活动断层表面轨迹发现的粉碎是否是同震滑动期间动态断层张开减弱的诊断。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。