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

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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.