Morphological Characteristics of Hummocks Originating from the 24ka Sector Collapse Event of Asama Volcano, Central Japan

  • YOSHIDA Hidetsugu
    JSPS Research Fellow, Department of Natural Environmental Studies, The University of Tokyo
  • SUGAI Toshihiko
    Department of Natural Environmental Studies, The University of Tokyo

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Other Title
  • 24,000年前の浅間火山大規模山体崩壊に由来する流れ山地形の特徴
  • 24 000ネンマエ ノ アサマ カザン ダイキボサンタイ ホウカイ ニ ユライ スル ナガレ ヤマチケイ ノ トクチョウ

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Asama volcano, whose present highest altitude is 2568 m above sea level, and which is a typical Quaternary stratovolcano in Japan, collapsed at approximately 24 ka. The sediment originating from this collapse event has been recognized in far distant areas by mineralogical, lithological, and tephrochronological research. A collapsed sector often moves as a debris avalanche, which consists of debris-avalanche blocks and debris-avalanche matrix. The debris-avalanche blocks form a hummocky topography, which indicates the flow under relatively dry conditions. To deduce the nature of the debris transport event to be clarified, this paper shows the morphological characteristics of hummocks. Hummocks formed by the Asama volcano collapse at 24 ka are distributed over Ohkuwa, Karuizawa, Saku, and Nakanojo districts within 45 km from the source. Their sizes are larger as approached to the source, indicating the debris-avalanche blocks were disaggregated irreversibly according to travel distance. This means the debris flowed as a debris avalanche in Nakanojo district, even though the debris had channelized into the Agatsuma River valley. Also, there is expected to be an exponential relation between distance from source and length of major axis of a maximum hummock in each region. Although no hummocks are recognized in the northwestern corner of the Kanto Plain, some blocks at least 5-15 m across are observed in the deposits. If we extrapolate downstream the above exponential relation, similar values for the sizes of these blocks can be acquired in the northwestern corner of the Kanto Plain. This size-distribution shows the possibility that the collapsed materials reached the northwestern corner of the Kanto Plain as a single debris transport event.


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