北海道南西部クッタラカルデラの内部構造 Internal Structure of Kuttara Caldera, Hokkaido, Japan

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Abstract

北海道南西部に位置するクッタラカルデラは,クッタラ火山の山頂部に形成された径約3kmの小型カルデラである.このカルデラの内部構造を解明するため,CSAMT法による比抵抗構造探査を行った.探査はクッタラカルデラを東西方向に横断する測線上(測線長12km,受信点23箇所)で行い,カルデラ内のクッタラ湖では小型ボートを用いた湖上測定を行った.データ解析は,有限要素法を用いた2次元逆解析を用いた.その結果,深度1000mまでの比抵抗構造が得られた.クッタラカルデラの下には,鉛直方向に伸長する低比抵抗領域(<30Ω・m,深度1000m以上,東西径1300-1500m)が存在し,この領域はカルデラ形成により落ち込んだ溶岩ブロックや火山砕屑物等が熱水変質を受けたものであると推定される.この低比抵抗領域は,クッタラ湖の中心からやや東に位置する湖盆地形の直下にあり,カルデラ陥没は主にこの地点で行われたと考えられる.カルデラの西部は地滑り等により拡大された可能性が高い.クッタラカルデラの内部構造は東西非対称であり,非対称なカルデラ陥没はクッタラ火山の非対称な地質に起因すると考えられる.

A controlled-source audio-frequency magnetotelluric (CSAMT) survey was conducted across the caldera of Kuttara volcano, Hokkaido, Japan, to investigate its subsurface structure. The caldera is 3 km in diameter and contains a circular lake (Lake Kuttara) 2.5 km across. The CSAMT survey was conducted along a 12-km-long east-west-trending transect that crossed the volcano and passed over the caldera. A total of 23 receiver stations were distributed along the survey line, including 7 stations in the caldera. Unique on-boat measurements were obtained at the surface of Lake Kuttara. A two-dimensional inversion of the CSAMT data, which revealed the resistivity structure to depths of up to 1000m beneath the caldera, suggested the existence of a low-resistivity region(<30Ω・m) beneath the eastern caldera floor, extending subvertically for >1000m and with a width of 1300-1500 m. The structure is interpreted to be a region filled with lava blocks and pyroclasts, which subsided during collapse of the caldera and which has been affected by hydrothermal alteration. The location of the low-resistivity region corresponds to an oval basin in the eastern part of the lake, implying that caldera subsidence occurred mainly beneath this basin. The western part of the caldera floor tilts gently to the east and has a rugged surface, suggesting that the western caldera rim was significantly enlarged as a result of landsliding during caldera formation. We thus infer that the Kuttara caldera was produced by asymmetric caldera collapse, which is attributed to the geological heterogeneity of the Kuttara volcano, with an eastern part comprised mainly of andesitic lavas and a western part comprised mainly of dacitic pyroclastic deposits.

Journal

  • BULLETIN OF THE VOLCANOLOGICAL SOCIETY OF JAPAN

    BULLETIN OF THE VOLCANOLOGICAL SOCIETY OF JAPAN 60(1), 35-46, 2015

    The Volcanological Society of Japan

Codes

  • NII Article ID (NAID)
    110009919296
  • NII NACSIS-CAT ID (NCID)
    AN10512786
  • Text Lang
    ENG
  • ISSN
    0453-4360
  • NDL Article ID
    026262606
  • NDL Call No.
    Z15-101
  • Data Source
    NDL  NII-ELS  J-STAGE 
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