Resistivity characterisation of Hakone volcano, Central Japan, by three-dimensional magnetotelluric inversion

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  • Yoshimura, Ryokei
    Disaster Prevention Research Institute, Kyoto University
  • Ogawa, Yasuo
    Volcanic Fluid Research Center, Tokyo Institute of Technology
  • Yukutake, Yohei
    Hot Springs Research Institute of Kanagawa Prefectural Government
  • Kanda, Wataru
    Volcanic Fluid Research Center, Tokyo Institute of Technology
  • Komori, Shogo
    Aso Volcanological Laboratory, Institute for Geothermal Sciences, Graduate School of Science, Kyoto University・Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology
  • Hase, Hideaki
    Volcanic Fluid Research Center, Tokyo Institute of Technology・Geothermal Energy Research & Development Co., Ltd., Tokyo
  • Goto, Tada-nori
    Graduate School of Engineering, Kyoto University
  • Honda, Ryou
    Hot Springs Research Institute of Kanagawa Prefectural Government
  • Harada, Masatake
    Hot Springs Research Institute of Kanagawa Prefectural Government
  • Yamazaki, Tomoya
    Disaster Prevention Research Institute, Kyoto University
  • Kamo, Masato
    Disaster Prevention Research Institute, Kyoto University
  • Kawasaki, Shingo
    Disaster Prevention Research Institute, Kyoto University
  • Higa, Tetsuya
    Graduate School of Science, Kyoto University
  • Suzuki, Takeshi
    Graduate School of Science, Kyoto University
  • Yasuda, Yojiro
    Graduate School of Engineering, Tottori University
  • Tani, Masanori
    Graduate School of Engineering, Kyoto University
  • Usui, Yoshiya
    Volcanic Fluid Research Center, Tokyo Institute of Technology

Description

On 29 June 2015, a small phreatic eruption occurred at Hakone volcano, Central Japan, forming several vents in the Owakudani geothermal area on the northern slope of the central cones. Intense earthquake swarm activity and geodetic signals corresponding to the 2015 eruption were also observed within the Hakone caldera. To complement these observations and to characterise the shallow resistivity structure of Hakone caldera, we carried out a three-dimensional inversion of magnetotelluric measurement data acquired at 64 sites across the region. We utilised an unstructured tetrahedral mesh for the inversion code of the edge-based finite element method to account for the steep topography of the region during the inversion process. The main features of the best-fit three-dimensional model are a bell-shaped conductor, the bottom of which shows good agreement with the upper limit of seismicity, beneath the central cones and the Owakudani geothermal area, and several buried bowl-shaped conductive zones beneath the Gora and Kojiri areas. We infer that the main bell-shaped conductor represents a hydrothermally altered zone that acts as a cap or seal to resist the upwelling of volcanic fluids. Enhanced volcanic activity may cause volcanic fluids to pass through the resistive body surrounded by the altered zone and thus promote brittle failure within the resistive body. The overlapping locations of the bowl-shaped conductors, the buried caldera structures and the presence of sodium-chloride-rich hot springs indicate that the conductors represent porous media saturated by high-salinity hot spring waters. The linear clusters of earthquake swarms beneath the Kojiri area may indicate several weak zones that formed due to these structural contrasts.

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