The Kuju volcano group in the southwestern part of the Oita prefecture, Japan, consists of more than 20 volcanoes distributed over an area of approximately 400 sq. km. On October 11, 1995, a phreatic eruption occurred on the eastern flank of Mt. Hossho, in the central part of the Kuju volcano. A second eruption occurred in mid-December 1995, producing an ash volume of approximately 5000 m3. Therefore, clarifying the thermal and hydrologic processes occurring beneath the entire Kuju volcano area is important because the geothermal areas beneath active volcanoes may be important to the development of geothermal energy. The largest geothermal power plant in Japan (Otake–Hatchobaru power plant; 120 MW) is located 5 km west of Mt. Hossho and Mt. Iwoyama. To identify the structures associated with the hydrothermal systems beneath the Kuju volcano area, we performed a wide-band magnetotelluric survey. The survey data were collected at 63 observation points around the Kuju volcano from September–October 2015. The resulting primary three-dimensional resistivity structure model clarified that a resistivity structure (~50 Ω·m) predominates the Kuju volcano area. A highly conductive structure (~10 Ω·m) exists around the Iwoyama–Kuju area, extending in depth towards the northeastern part of Iwoyama. Based on the results of previous studies, the former can be interpreted as a lava and pyroclastic deposit whereas the latter corresponds to a magmatic fluid pathway and hydrothermal altered layers.
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