Journal of the Japan Society of Engineering Geology

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It is well known that high-salinity groundwater（deep-seated fluid）spouts out in the limited inland areas of Japan. This high-salinity groundwater is thought to originate from the dehydration of subducted slab. Such spouts are considered to be one of the important criteria indicators for site selection and safety assessment of candidate locations for underground facilities（e.g., high-level radioactive waste disposal facilities）. Therefore, it is necessary to investigate the ascending and advective transport processes and distribution of deep-seated fluid.

Deep-seated fluid corresponding to Arima-type geothermal water spouts in southwestern Shimane Prefecture and northeastern Yamaguchi Prefecture. We conducted geological/hydrological surveys and CSAMT geophysical exploration in the Tokusa Basin of Yamaguchi Prefecture. On the basis of the three dimensional distribution of resistivity of rocks and sediments obtained by the CSAMT exploration, we conclude that deep-seated fluid with low resistivity ascends through the Tokusa-Jifuku fault zone in the basement rock, and penetrates to the unconsolidated surface sediments, and is diluted by the shallow groundwater, and flows into the river. The results suggest the possibility of the existence of deep-seated fluid along the concealed fault zones even if there are no surface traces of underground deep-seated fluid by a thick cover of younger sediments.

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