日本における流れ山のサイズ分布

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Many hummocky terrains are found preserved along the courses of debris avalanches. This research investigates the functional relation between the size (plane area) and the run-out distance of hummocks on debris avalanche depositional surfaces in Japan. The geomorphic meanings of coefficients of the function expressing this size-distance relation are discussed, along with the geomorphological role of catastrophic volcanic sector collapses. The sizes of hummocks decrease with distance and can be expressed by an exponential function of distance for individual debris avalanches, although their distribution patterns vary from one debris avalanche to the next. Concerning the function expressing the size-distance relation, slope coefficients, implying the rate of decrease of the sizes of hummocks with distance, show a correlation with the equivalent coefficients of friction. This indicates that the downsizing rate of hummocks is controlled by the mobility of debris avalanches. Intercept coefficients, which mathematically indicate the initial average size of hummocks at a distance of 0 km, show a high correlation with the volumes of collapsed masses. The initial average sizes of hummocks at the source area are potentially determined by the volume of collapsed mass of the volcanic body. During the last 500 years in Japan, catastrophic collapses at volcanoes have accounted for an annual denudation rate of ca. 0.5 mm/y per unit area of Quaternary volcanoes, which is comparable to the contemporary denudation rate of non-volcanic mountains in Japan. Furthermore, accompanying debris avalanches cause severe disasters in and around the volcanoes. In such situations, the size-distance relation of hummocks, even for part of a debris avalanche, would contribute to revealing the magnitude and damage of debris avalanches.

著者

吉田英嗣
関東学院大学経済学部

日本における流れ山のサイズ分布

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