Collaborative Research: Developing a Link between Dynamic Friction and Fracture Mechanics Models of Earthquake Rupture using a New Dynamic Double-direct Shear Apparatus

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There is broad agreement amongst researchers in the geophysics community that similar rocks may undergo very different weakening processes in different normal stress and/or slip velocity regimes. Consequently, inference of weakening behavior of fault rocks in situ from laboratory experiments at interfacial conditions of relevance to earthquake physics cannot be done simply by scaling exercises, and relatively small changes in normal stress and/or the slip speed can result in changes in the slip weakening distance of an order of magnitude. Motivated by these observations, the investigators propose to advance the current state of our understanding regarding the frictional constitutive behavior of earthquake faults using two principal approaches: (1) implementing a new dynamic shear friction testing apparatus by synergistically combining the split-Hopkinson pressure bar and the double-direct shear friction apparatus to the study of dynamic friction in both intact and granular geo-materials; and (2) developing a methodology for testing the efficacy of parameters extracted from dynamic friction experiments in dynamic rupture models. The intellectual merit of this proposal is strengthened by the fact that it addresses some of the outstanding problems in earthquake-physics, including the influence of slip and slip-velocity on fault strength during a typical fault rupture event. No laboratory experiments to-date combine the large displacement, high slip rates, and normal stresses that are understood to characterize dynamic earthquake slip at natural fault interfaces. These failings mean that processes that may occur during dynamic slip in earthquakes have not been explored experimentally. The new experimental configuration proposed in here, which is a modification of the well-established experimental procedures employed routinely in engineering for investigating high-strain-rate behavior of engineering materials (split Hopkinson pressure bar) and quasi-static friction studies in geo-materials (double-direct shear apparatus), has the potential to provide friction data in the slip-speed and normal stress range of direct relevance to earthquake physics. Furthermore, the two-pronged methodology of our proposed work aims to fundamentally change the way we approach studying the frictional resistance of faults. The first task guarantees significant results that will advance the state of understanding of dynamic friction during earthquake rupture under relevant conditions, whereas the second approach will further constrain the inferred frictional constitutive models by comparing predictions of dynamic rupture models that incorporate lab-derived frictional slip constitutive behavior with laboratory rupture experiments. The proposed research will contribute toward our understanding of earthquakes in several ways. To construct theoretical models of the earthquake process, we must understand how frictional resistance on faults changes during an earthquake. In particular, the weakening mechanism that we propose to study have profound implications for the magnitude of stress-drops during earthquakes and consequently for the magnitude of strong ground shaking. The manner in which fault strength varies with displacement and rupture velocity, as well as the rate at which healing occurs as the slip velocity decreases behind the rupture tip, can control the mode of rupture propagation, i.e. as a crack or a pulse. Thus, understanding dynamic friction is important not only for practical matters related to predicting strong ground motions and resulting damage, but also for answering major scientific questions receiving considerable attention, e.g. the strength of the San Andreas fault/the heat-flow paradox, the question that ultimately is responsible for the San Andreas Fault Observatory at Depth (SAFOD) project. The proposed program also provides exciting opportunities for interdisciplinary research and educational interactions by involving faculty and graduate students from two neighboring institutions.. Both universities are strongly encouraging the involvement of undergraduate students in cutting edge faculty research, and this would occur for the proposed work as well. Special attention will also be given to recruitment of underrepresented minority students for the project. Dissemination of research results is planned by conference presentations and publications in relevant peer-reviewed journals. The investigators will also employ internet and mass-media-based information dissemination to increase awareness of the potential impact of the proposed research in earthquake hazard mitigation.