雲仙普賢岳1990-95年噴火堆積物の粒度組成と浸透能

Miyabuchi, Yasuo, Shimizu, Akira, Takeshita, Keiji, 竹下, 敬司

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Unzen volcano in wastern Kyushu, southwestern Japan, began a series of eruptions in November 1990. Repeated pyroclastic flows and debris flows distributed a vast quantity of sediment that devastated much of the area surrounding Unzen volcano. Distribution, grain size characteristics and infiltration rates of deposits associated with the eruption were examined in order to assess potential geomorphic and sedimentologic impacts. Pyroclastic flow deposits generated by dome collapse, which are confined to valleys, are poorly sorted. Pyroclastic surge deposits, which are distributed more widely than pyroclastic flow deposits, consist mainly of well-sorted sand. Ash-fall deposits are very fine grained and well sorted. Infiltration rates in pyroclastic flow deposits were lower than rates in ash-fall deposits. The rates in pyroclastic surge deposits were highest according to the well-sorted sandy texture. Hillslopes covered with fine ash-fall deposits were the source areas of many debris flows owing to low infiltration rates at the early stage of these eruptions. However, on forested slopes infiltration rates of ash-fall deposits have increased as a result of bioturbation and intermixing of little fall.

雲仙普賢岳1990-95年噴火堆積物の粒度組成と浸透能

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