DEBRIS AVALANCHE AND LAHAR DEPOSITS IN THE YATSUGATAKE VOLCANIC CHAIN AND MYOKO VOLCANO GROUP, GENTRAL JAPAN

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The occurrence and characteristics of 14 debris avalanche and 4 lahar deposits from the Yatsugatake volcanic chain and the Myoko volcano group in central Japan are described. The number of debris avalanche and lahar deposits by volume are 5 (>1 km^3), 9 (1-0.1 km^3), and 4 (<0.1 km^3). Most of the deposits, of more than 0.3 km^3,have horseshoe-shaped calderas at the source. Although the largest is the Nirasaki debris avalanche deposit (9 km^3; Pleistocene) from the Yatsugatake volcanic chain, there is a large-scale unconformity at the corresponding source. Steam explosions were the possible trigger for the debris avalanches in the area studied, heavy rain or rapid melting of snow for the lahars. Large debris avalanches (>0.01 km^3) occurred 5 times within the past 10,000 years in both volcano groups, the frequency of such avalanches being one per 2,000 years. The potentiality of the next large debris avalanche in the Yatsugatake volcanic chain appears to be low. For Myoko volcano group, however, the potentiality is high because the latest event in the group occurred 2,700 years ago.
日本自然災害学会の論文