Simulating Alternative Conceptual Models of Fault Controls on Regional Ground Water Flow near the Horonobe Underground Research Laboratory, Hokkaido, Japan

The Horonobe Underground Research Laboratory (URL) is being constructed by the Japan Atomic Energy Agency (JAEA) in northwestern Hokkaido on the northeastern margin of a large-scale back-arc sedimentary basin formed during the opening of the Sea of Japan. The URL is being constructed in fine-grained, marine Tertiary sediments near the Omagari fault zone, a major thrust fault running approximately north-south to the east of the URL site. Numerical models by JAEA have indicated that the Omagari fault zone has an important effect on ground water flow in the vicinity of the URL and multiple conceptual models of the geometry and hydraulic properties of the fault zone have been developed. A particular hydrologic feature believed to be influenced by the fault zone is a strong upward hydraulic gradient in some of the deeper test borings near the URL site. We conducted an independent analysis of regional ground water flow system to further investigate the influence of the fault zone on ground water flow. We combined geologic framework modeling and ground water flow modeling to elucidate the geometry of the fault zone and its hydraulic influence. Four models were constructed using alternative conceptualizations of the fault zone geometry and its hydraulic influence: two fault surfaces with either low or high permeability, and three fault surfaces with either low or high permeability. The conceptual models were compared on the basis of the difference between the simulated hydraulic heads and measured heads. The three fault surface models performed better, but there was almost no difference in the calibration error between the low and high permeability fault zone conceptualizations. None of the conceptualizations could fully reproduce the measured upward hydraulic gradients.