Hydrologic properties and structure of the Mozumi Fault, central Japan

Analyses of rocks from the Active Fault Survey Tunnel (AFST) provides insight into the structure and hydrogeology of the northeast‐trending Mozumi‐Sukenobu fault, an active strike‐slip fault with 125 to 500 m of right‐lateral slip in central Japan. Interlayered regions of sub‐vertical permeability zones formed by cataclasis and slip on clay‐rich foliated zones. Core samples range from 10−19 m2 to almost 10−13 m2. CFC analyses of waters from the fault zone show that water entering the tunnel is 27–36 years old, yielding a bulk fault permeability of 10−14 to 10−15 m2. The data support a fault zone model of a fluid‐saturated, low‐velocity zone with a sharp velocity contrasting with the protolith. Pore‐fluid pressures could build and dissipate in isolated high permeability pods 10's–100's m in dimension.