Journal of Natural Disaster Science

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

Shimpei KAWACHI
Professor, Faculty of Education, Shinshu University
and
Kenji HAYATSU
Director, Myoko Volcano Research Laboratory, Arai City

(Received 13 June, 1994 and in revised from 16 November, 1994)

Abstract

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 cubic kilometers), 9 (1 -0.1 cubic kilometers), and 4 ( <0.1 cubic kilometers). Most of the deposits, of more than 0.3 cubic kilometers, have horseshoe-shaped calderas at the source. Although the largest is the Nirasaki debris avalanche deposit (9 cubic kilometers; 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 cubic kilometers) 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.

Key words

debris avalanche, lahar, Yatsugatake, Myoko