Magnetization Intensity Distribution in the Western Unzen Region, Kyushu, Japan, Inferred from a Helicopter-borne Aeromagnetic Survey

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  • OKUBO Ayako
    Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University
  • TANAKA Yoshikazu
    Aso Volcanological Laboratory, Graduate School of Science, Kyoto University
  • KAGIYAMA Tsuneomi
    Aso Volcanological Laboratory, Graduate School of Science, Kyoto University
  • UTSUGI Mitsuru
    Aso Volcanological Laboratory, Graduate School of Science, Kyoto University
  • KANDA Wataru
    Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University

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  • ヘリボーン空中磁気探査から推定される雲仙西部地域の磁化強度分布
  • ヘリボーン クウチュウ ジキ タンサ カラ スイテイ サレル ウンゼン セイブ チイキ ノ ジカ キョウド ブンプ

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Abstract

A helicopter-borne, aeromagnetic survey was performed in October 2004 over the western Unzen region, Kyushu, Japan. The survey was flown at an altitude of about 1100m above sea level along north-south survey lines and east-west tie lines, spaced 500m and 1000m apart, respectively. The observed aeromagnetic anomalies were reduced onto a constant surface of 1500m above sea level. Then, a magnetization intensity distribution was estimated, assuming that it varies only laterally. The characteristics of the magnetization intensity distribution are summarized as follows;(1) The hydrothermally altered areas of Unzen hot spring, Ii-Dake, Shimo-Dake, and Kami-Dake show magnetization lows, which indicates that high temperature fluids associated with hydrothermal activities reduce the rock magnetization. These fluids are likely to migrate through E-W trending faults developed inside the Unzen graben.(2) Magnetization lows lie outside of the Unzen graben, corresponding to debris flow deposits and pyroclastic-flow deposits of the Older Unzen (500-150ka). In contrast, magnetization highs are distributed in the northern part region of Azuma-Dake and Hachimaki-Yama and around Takaiwa-Yama, where the lava of the Older Unzen (500-300ka) is exposed. (3) Magnetization lows, corresponding to fan deposits of the Older Unzen (500-300ka), are distributed along the Chijiwa fault.

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