MARGINS FELLOWSHIP: Fluidization of Very Thick Granular Layers in Ancient Thrust Faults

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Seismogenic faults - that is, faults that generate earthquakes of significant magnitude, are of particular interest because the earthquakes they produce can cause appreciable damage and loss of life. It is thus very important to understand the mechanisms by which they fail in the brittle regime - i.e., by fracture as opposed to flow. Granular layers are ubiquitous features of faults that form in the brittle regime, and their evolution and behavior have been shown to be critically related to strength and strain rate relationships. In the laboratory, the behavior of both synthetic and natural samples has been shown to be affected by a variety of factors, such as grain size and size distribution, layer thickness, and fluid saturation. Theoretical mechanisms for dynamic weakening of these materials have been proposed and explored, but not examined in the field. Field observations of faults suggest that granular layers fluidize or deform as a suspension that behaves like a viscous fluid. It is therefore essential to examine the role of these factors in natural fault settings and constrain these parameters with observations of in situ granular layers in faults. This study by two female researchers will compare two large, mature brittle faults (one in Alaska, and one in Africa) that contain very thick fluidized granular layers that contain evidence for significant flow during fault activity. The structure of fluidized granular layers has the potential to elucidate the rate at which the fault slipped, as well as the changes in overall strength of faults during the seismic cycle. Among its broader impacts, this study will advance the career development of a young female researcher, Dr. Christie Rowe, including her reintegration into the US academic community. In addition, there will be significant international collaboration with South African and Namibian scientists. The project has high societal relevance in that it addresses the causes of earthquake risk.