Exhumed fault zones offer insights into near-field earthquake processes in unparalleled spatial resolution. Unfortunately, differentiating seismic slip from slow or aseismic slip based on evidence preserved in the rock record is not trivial. Until a few years ago, pseudotachylyte (solidified frictional melt) was broadly considered the only reliable indicator of past earthquake rupture, because frictional heat production can only achieve melting temperatures during rapid slip. Significant progress in fault rock studies over the past two decades has revealed a range of reaction products which can be used to detect frictional heating at peak temperatures less than the melt temperature of the rock. We have also learned that, in addition to on-fault products of rapid slip, transient perturbations in the stress state adjacent to the fault are capable of producing fault damage zone structures diagnostic of earthquake rupture. In this talk I will discuss the process of coseismic fracture and fragmentation of rocks to form pulverized fault damage zone rocks. Pulverized rocks have several peculiar features that make them unique fingerprints of past earthquake rupture. We have devised several new experimental rock mechanics techniques to simulate complex, impulsive loading histories expected near earthquake rupture tips, and we are using them to place constraints on the stress and strain rate fields associated with coseismic rock fragmentation and the potential energy sink that this process represents for earthquake ruptures. Our ongoing work is focused on using fragmented and pulverized rocks to determine past earthquake rupture directivity, to better understand how fault damage zones evolve, and to develop metrics for determine the maximum credible earthquake magnitude (Mmax) possible on active faults by studying damage zone structure.
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