北海道北部,幌延地域における大曲断層の三次元分布と水理特性

Three-dimensional distribution and hydrogeological properties of the Omagari fault in the Horonobe area, northern Hokkaido, Japan

石井 英一   ; 安江 健一; 田中 竹延*; 津久井 朗太; 松尾 公一*; 杉山 和稔*; 松尾 重明*

Ishii, Eiichi; Yasue, Kenichi; Tanaka, Takenobu*; Tsukui, Rota; Matsuo, Koichi*; Sugiyama, Kazutoshi*; Matsuo, Shigeaki*

北海道北部、第三紀珪質岩分布域において地表踏査,反射法地震探査、及びAMT探査を実施し、当域に分布する大曲断層の位置,連続性,構造,水理特性,変位センス、及び断層形態などについて検討した。その結果、大曲断層は、(1)ダメージゾーンを主体とした幅120m程度の断層帯であり、その透水性は高く、透水性異方性は弱い。(2)後期鮮新世前期更新世以降に形成された東側隆起の左斜めすべり断層であるが、それ以前の活動可能性についても今後検討する必要がある。(3)地表ではover-stepし、地下では、flower structureの形態をなして収斂する特徴を持つものと推定された。

Fault zones have significant influences on ground water flow. It is important to understand the characteristics of fault zones for hydrogeological modeling. We investigated the hydrogeological property and the geological structure of the fault in Neogene siliceous sedimentary rocks of northern Hokkaido, using geological mappings, reflection seismic surveys, and audio-frequency magneto telluric surveys. The result of description at a fault outcrop suggests that the Omagari Fault is a fault zone of about 120 m width that consists of mainly damage zone and that is a permeable structure without the strong anisotropy in permeability. In addition, the result of magneto telluric surveys shows high-resistivity zones having a flower structure, and one of the high-resistivity zones corresponds to the position of the Omagari fault inferred from the results of the reflection seismic surveys and of the borehole investigations. These high resistivity zones overlap with the low saline concentration zones, and the overlapping suggests the infiltration of the fresh water from the surface into the deeper part through the Omagari Fault as permeable structures. Based on these results and on study of the hydrogeological property, the geological structure, and petrological characteristics in and around the fault, it seems that the fault is a sinistral oblique-slip fault with eastside uplift sense that is formed after late Pliocene to early Pleistocene, but the possibility of the previous activity needs to research, and that the fault is over-stepping at the surface and is converging in deep underground like a flower structure shape.