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

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

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.