Stress Analysis and Fracture-Based Fluid Transmissivity at the Underground Research Laboratory Site at Horonobe, Hokkaido, Japan

Faults and fractures are important conduits for subsurface flow. The ambient stress state is now well-recognized to exert a primary control on fluid transmissivity of faults and fractures. Using fault geometries based on available map and seismic reflection data, and fracture simulations based on well data from the Underground Research Laboratory (URL) site at Horonobe, Hokkaido, Japan, we investigated the sensitivity of fracture-based transmissivity to estimates of in situ stress. In the Horonobe area, NNW-SSE trending and west-verging faults and folds formed with maximum principal compressive stress (σ₁) horizontal and approximately ENE-WSW from Late Pliocene to present. The Omagari fault system, which is one of the major thrust faults in the area, passes close to the URL site, so the shape and properties of the Omagari fault and related fracture systems potentially affect the subsurface ground-water flow in the URL area. Our analysis shows that although the fault and fracture systems that are likely to be present are sensitive to variations in stress state, the currently available stress estimates indicate that they are unlikely to have a significant influence on ground-water flow in the area.