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Two electrical conductors beneath Kusatsu-Shirane volcano, Japan, imaged by audiomagnetotellurics, and their implications for the hydrothermal system

Earth, Planets and Space volume 58, pages 1053–1059 (2006)Cite this article

Abstract

Kusatsu-Shirane volcano, Japan, is known for its active phreatic eruptions. We have investigated its hydrothermal system by conducting audio-magnetotelluric soundings at 22 stations along a profile that extends across the volcano. The final two-dimensional model is characterized by two conductors. One is a 300- to 1000-m-thick conductor of 1–10 Ωm, which is located on the eastern slope and covered with 200-m-thick resistive layers of Kusatsu-Shirane lava and pyroclastics. This conductor indicates the presence of a Montmorillonite-rich layer of Pliocene volcanic rocks that may function both as an impermeable floor for the shallow fluid path from the peak to the hot springs to the east and as an impermeable cap for the deeper fluid path from the summit region to the foot of the volcano. The second conductor is found at a depth of 1–2 km from the surface, at the peak of the volcano, and its resistivity is as low as 1 Ωm or less. This low resistivity can be explained by fluids containing high concentrations of chloride and sulfate which were supplied from the magmatic gases. Micro-earthquakes cluster above this conductor, and the cut-off of the earthquakes corresponds to the top of the conductor. This conductor infers the presence of the fluid reservoir, and the upward release of these fluids from the reservoir through the conduit presumably triggers the micro-earthquakes at the peak area of the volcano. Crustal deformation modeling using GPS and leveling data of the past 10 years revealed that the center of the deflation coincides with the top of the second conductor, indicating that the fluid reservoir itself can be hosting the deformation.

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Author notes

  1. Naoto Ujihara

    Present address: Hydrographic and Oceanographic Department, Japan Coast Guard, Tokyo, Japan

  2. S.Bulent Tank

    Present address: Kandilli Observatory and Earthquake Research Institute, Boğaziçi University, İstanbul, Turkey

Authors and Affiliations

  1. Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, 152-8551, Japan

    Nurhasan, Naoto Ujihara, S.Bulent Tank & Yoshimori Honkura

  2. Volcanic Fluid Research Center, Tokyo Institute of Technology, Tokyo, 152-8551, Japan

    Yasuo Ogawa, Shin’ya Onizawa & Takehiko Mori

  3. Geological Survey of Japan, AIST, Tsukuba Ibaraki, 305-8567, Japan

    Shin’ya Onizawa, Takehiko Mori & Masahiko Makino

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  1. Nurhasan
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  2. Yasuo Ogawa
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  5. Yoshimori Honkura
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Correspondence to Yasuo Ogawa.

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Nurhasan, Ogawa, Y., Ujihara, N. et al. Two electrical conductors beneath Kusatsu-Shirane volcano, Japan, imaged by audiomagnetotellurics, and their implications for the hydrothermal system. Earth Planet Sp 58, 1053–1059 (2006). https://doi.org/10.1186/BF03352610

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