[S-VC29] 火山の熱水系

[SVC29-07] The plumbing system of a Miyakejima volcano: insights into magmatic-hydrothermal interactions

*Marceau Gresse¹、Makoto Uyeshima¹、Takao Koyama¹、Hideaki Hase²、Koki Aizawa³、Yusuke Yamaya⁴、Yuichi Morita¹、Derek Weller¹、Tawat Rung-Arunwan⁵、Takayuki Kaneko¹、Yoichi Sasai⁶、Jacques Zlotnicki⁷、Tsuneo Ishido⁸、Hideki Ueda⁹、Maki Hata¹ (1.Earthquake Research Institute, The University of Tokyo, Tokyo, Japan、2.Geothermal Energy Research & Development Co., Ltd., Tokyo, Japan、3.Institute of Seismology and Volcanology, Faculty of Science, Kyushu University, Fukuoka, Japan、4.National Institute of Advanced Industrial Science and Technology (AIST), Renewable Energy Research Center, Fukushima, Japan、5.Curl-E Geophysics Co., Ltd., Thailand、6.Earthquake Prediction Research Center, Tokai University, Shizuoka, Japan、7.CNRS, EMSEV, OPGC-UMR6524, Aubière, France、8.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan、9.National Research Institute for Earth Science and Disaster Resilience (NIED), Tsukuba, Japan)

キーワード：hydrothermal system, magnetotellurics, electrical resistivity, seismicity, self-potential, remote-sensing

Phreatic and phreatomagmatic eruptions are difficult to predict with accuracy on volcanoes due to complex interactions at depth between heat, water, and magmatic fluids. To better understand such multifaceted interactions, we present here a multidisciplinary geophysical approach performed on Miyakejima, a 10-km wide stratovolcano in the Izu Bonin arc. Its plumbing system was highlighted by combining four geophysical methods: magnetotellurics, seismicity (hypocenters), self-potential, and thermal image (remote sensing). We thus propose the first large-scale interpretation of the volcanic structure in terms of rock properties, temperature, fluid content, and fluid flow. Our findings indicate that hot volatiles released from a deep magmatic reservoir (> 350 °C, 2.5–4.5 km depth) rise through a narrow permeable path, interact with the conductive hydrothermal system beneath the 2000 A.D. caldera (<250 °C, 0–2 km depth). This mixture of fluid is finally released in the fumarolic area in the southern part of the caldera at 181°C. This combined approach allow us to: 1) delineate the water table of the volcano (300–700 m depth), 2) determine the general fluid flow circulation beneath the island, 3) characterize seismic signatures of long-period and volcano-tectonic events, and 4) elucidate the origin of the high water content of fumaroles developed since the last eruption in A.D. 2000.