hydraulic tests in the active fault survey tunnel, Kamioka mine, excavated through the active Mozumi-Sukenobe fault zone and their hydrogeological significance

茂住-祐延断層を貫く活断層調査トンネルにおける原位置水理試験とそれらの水理学的重要性

野原 壯   ; 田中 秀実*; 渡辺 邦夫*; 古川 昇*; 高見 明*

Nohara, Tsuyoshi; Tanaka, Hidemi*; Watanabe, Kunio*; Furukawa, Noboru*; Takami, Akira*

陸域地下構造フロンティア研究の一環として、茂住-祐延断層を貫く活断層調査トンネルにおいて、活断層の水理地質構造の調査を行った。茂住-祐延断層は、地下では分岐断層として観察される。地表から地下約300mのトンネルまで、主要な水みちとなる導管様の構造が、断層近傍に偏在する。この水みちは、破砕の進んだ断層角礫帯に一致せず、また、断層粘土とも一致しない。共役リーデル剪断面を構成する高角断層が観察されるダメージゾーンと断層角礫の混在部分が、この主要な水みちとなる構造を提供している。この構造は、跡津川断層系における"conjugate Riedel shears"として形成されたことが示唆される。

The spatial hydrogeological and structural character of the active Mozumi-Sukenobu Fault (MSF) was investigated along a survey tunnel excavated through the MSF in the Kamioka Mine, Central Japan. Major groundwater conduits on both sides of the MSF are recognized. One is considered to be a sub-vertical conduit between the tunnel and the surface, the other is estimated to be a major reservoir of old meteoric water alongside the MSF. Studies indicate that part of the MSF is a sub-vertical continuous barrier that obstructs younger meteoric water observed in the southeastern part of the Active Fault Survey Tunnel (AFST) and recharge to the rock mass intersected by the northwestern part of the AFST. It is considered that the MSF is a continuous barrier resulting in the storage of a large quantity of older groundwater to the northwest. The observations indicate that the major reservoir is not the fault breccia associated with the NE-SW oriented faults of the MSF. Rather, the reservoir is considered to be the zone in which blocks of fractured rocks occur beside high angle faults corresponding to X shears that coincide in orientation with the present-day regional stress field and antithetic Riedel shears of the MSF. It is considered that secondary porosity is developed in the major reservoir by the destruction of filling minerals and fracture development beside these shears. The results of borehole investigations in the AFST indicate that increase in hydraulic conductivity is not directly related to increased density of fractures around the MSF. Our results suggested that minor conduits are sporadically distributed in the sedimentary rocks around the MSF in the AFST.