Properties of sector-collapse and debris avalanches on Quaternary volcanoes in Japan

Abstract

Many types of mass movements frequently occur on volcanic edifices. The most dangerous is a gigantic sector-collapse with its following debris avalanche, as in the Mount St. Helens and Mt. Bandai cases. A total of 128 debris avalanches are described for 67 Quaternary volcanoes in Japan. Two-fifths of Japanese volcanoes show evidence of debris avalanches. Debris avalanches have occurred at the rate of one every 60 years during the last 500 years in Japan. Most are larger than 0.2km3. Deposits usually spread widely around the foot of volcanoes. One-third of debris avalanche deposits are accompanied by hummocky hills, which provide a good index of debris avalanches. Only 48 debris avalanches have obvious amphitheaters. Geological or geomorphological evidence indicates source positions of 65 debris avalanches. Gigantic sector-collapses tend to occur on the upper part and crest of volcanoes. Two-thirds of debris avalanches did not have a clear source area. Topographic features of amphitheaters are easily eroded or buried. The causes of destruction of volcanic bodies are not well known. Most height-to-length ratios (H/L) of debris avalanches range from 0.2 to 0.08 and indicate high mobility. Gigantic sector-collapses and debris avalanches are usual processes of destruction of volcanic edifices. Debris avalanches tend to occur repeatedly on active stratovolcanoes, which have the cone shape characteristic of Mt. Fuji. Debris avalanches on Quaternary volcanoes are usually larger than those in other geologic regions.