Composition, Microstructures, and Petrophysics of the Mozumi Fault, Japan: In Situ Analyses of Fault Zone Properties and Structure in Sedimentary Rocks from Shallow Crustal Levels

[1] We characterize the chemical, microstructural, and geophysical properties of fault-related rock samples from the 80–100 m wide Mozumi fault zone, north central Honshu, Japan. The fault is exposed in a research tunnel 300–400 m below the ground, and we combine geological data with borehole geophysical logs to determine the elastic and seismic properties of the fault zone. Detailed mapping within the tunnel reveals that the fault zone consists of two zones of breccia to foliated cataclasites 20 and 50 m thick. Two narrow (tens of centimeters wide) principal slip zones on which most of the slip occurred bound the central fault zone. The dominant deformation mechanisms within the fault zone were brittle fracture, brecciation, slip localization, plastic deformation, and vein formation in a sericite-calcite rich matrix. Clay alteration patterns are complex within the fault zone, with clay-rich fault breccia enriched in smectite, illite, and kaolinite relative to the kaolinite and illite dominant in the host rock. Whole rock geochemical analyses show that the fault-related rocks are depleted in Fe, Na, K, Al, Mg, and Si relative to the host rock. The fault zone exhibits depressed electrical resistivity values by 10–100 ohm m relative to the wall rock, values of Vp and Vs values that are ∼0.30 and ∼0.40 km/s (10–20%) less than protolith values. The spontaneous potential logs indicate that the fault zone has increased freshwater content relative to formation waters. Wellbore-based measurements of Vp and Vs in fault-related rocks to enable us to calculate values of Young's modulus from 16.2 to 44.9 GPa and Poisson's ratio for the fault zone of 0.263 to 0.393. The protolith has Young's modulus of 55.4 GPa and a Poisson's ratio of 0.242. Lowest calculated values of Young's modulus and highest calculated values of Poisson's ratio correspond to fault breccia with increased porosity, high fluid content, and low resistivity values. Taken together, these data show that the shallow portion of the Mozumi fault consists of a complex zone of anastomosing narrow slip zones that bound broad zones of damage. Fluid-rock alteration and deformation created altered fault-related rocks, which have resulted in overall reduced interval velocities of the fault zone. These data indicate that seismic waves traveling along the interface or internally reflected in the fault zone would encounter rocks of differing and reduced elastic properties relative to the host rocks but that in detail, material properties within the fault may vary.