In-situ Analysis of Hydraulic and Petrophysica Properties of a Strike-slip Fault

We characterize the chemical, physical, and mechanical properties of fault rock samples from the 80-100 m wide Mozumi strike-slip fault, Japan, as sampled in the Active Fault Survey Tunnel at a depth of 350 m, and combine these data with wellbore-based geophysical logs to determine the seismic properties of the fault zone. The fault has 125 500m of slip, and cuts siltstones and sandstones. Microstructures in the fault rocks record syn-tectonic fluid flow, brittle and plastic deformation concentrated in a sericite-rich matrix. Clay mineralogy varies throughout the fault zone, with kaolinite and illite dominant in host rock and fault breccia, and clay-rich fault breccia containing increased amounts of smectite, illite, and kaolinite dominant relative to the protolith. Whole-rock geochemistry shows that the damage zone is depleted in Fe, Na, K, Al, Mg, and Si oxides relative to the host rock, with a loss of up to ~90% of the major cations in clay-rich fault breccia. Electrical resistivity values are depressed by up to 50 ohm-m across the main fault zone relative to the protolith. The values of vp and vs decrease by 0.30 -0.40 km/s across the main fault relative to the surrounding damaged wall rock, and an average of ~0.70 km/s and ~1.0 km/s relative to host rock, respectively. We use the well-bore based measurements of vp and vs to calculate Young's modulus and Poisson's ratio of fault rocks in the damage zone which range from 16.2 to 44.9 GPa and 0.263 to 0.393, respectively. The protolith has a calculated Young's modulus of 55.4 GPa and a Poisson's ration of 0.242. Lowest calculated values of Young's modulus and highest calculated values of Poisson's ratio correspond to fault damage zone that has increased porosity, high fluid content, and low resistivity values. The physical and mechanical properties vary across the fault zone and demonstrate that the complex distribution of elastic properties corresponds to the permeability structure inferred to be a combined fault-parallel fluid conduit and fault-perpendicular fluid barrier. Fluid-rich pockets of fault rock also migrate through the fault zone with time, and may facilitate creep on the Mozumi fault.