Collaborative Research: Laboratory Study of the Mechanics and Physical Properties of the San Andreas Fault and 3D SAFOD Volume

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0545548TobinThe principal goal of the SAFOD borehole, and one of the main motivations for the NSF EarthScope initiative, is to gather critical data needed to understand fault mechanics and earthquakes. Through sampling, down-hole measurements, and long-term monitoring, the SAFOD experiment will provide data to test key hypotheses regarding long-term fault strength, earthquake nucleation, and fault slip behavior. However, the borehole itself will penetrate only a small part of the crustal volume surrounding the San Andreas Fault zone, and will sample only a subset of lithologies present in the subsurface. Although SAFOD will provide observations of the shallow seismogenic zone of a major plate bounding fault in unprecedented detail, additional characterization of rock physical properties for the 3-D volume containing SAFOD and the San Andreas Fault are critical for addressing several of the most important outstanding questions in fault mechanics and earthquake physics. These include: 1) What causes spatial variability in fault slip behavior and seismicity? 2) Are elevated fluid pressures within the SAFOD 3-D volume plausible? 3) How are geophysical observations such as low velocity or resistivity linked to in situ conditions of stress and fluid pressure? 4) What do thermal data in the shallow subsurface tell us about the fault energy budget? We are conducting a comprehensive study of the processes and properties that affect mechanical behavior and transport properties of fault zones. The research involves laboratory measurements of SAFOD core and outcrop samples. These measurements are designed to characterize the deformation processes and physical properties of rocks from the 3-D SAFOD volume. This work complements ongoing work on SAFOD samples. The data we collect will inform regional geologic, hydrologic, and thermal models. Our research is designed to address the following key objectives: - Determine the frictional strength and constitutive properties for SAFOD core material and host rock adjacent to the San Andreas Fault. - Test the hypothesis that the upper stability transition from aseismic to seismic faulting is associated with a change in mineralogy of fault gouge and/or host rock. - Develop experimental constraints necessary to test 1) the hypothesis that the San Andreas Fault is weak in an absolute and relative sense, and 2) models of long-term pore pressure generation and dynamic fault weakening. - Provide constraints on processes relevant to the energy budget of faulting: including frictional heat generation, advective heat transport, and thermal refraction. - Investigate the relationship between frictional strength (including healing and steady-state velocity dependence), seismic wave speed, and permeability. - Investigate the stress and pore pressure dependence of P and S wave speeds and their anisotropies in fault zone and wall rock, to evaluate and improve seismic-attribute proxies for pore pressure, effective stress, frictional strength, fluid content, and other properties inferred from borehole log and surface seismic data. This research will provide an understanding of processes that govern the strength and stability of major faults. In addition, we will measure those properties of fault rock that determine remotely sensed geophysical signatures, which is important for better assessment of earthquake hazard and for linking observations of fault behaviors with fundamental physical processes.