Pictorial : Rock Avalanches Occurred in the Snow-melt Season at the Karasawa Cirque in Mt. Hotaka-dake, The Japanese Alps

Abstract

Based on the direct observation of two rock avalanches occurred in the snow-melt seasons of 1993 and 1995 at Karasawa Cirque in Mt. Hotaka-dake (3, 190 m a.s.l., 36°15' N, 137°39' E), central Japan, the characteristics of rock-avalanche processes were elucidated in detail.Observed rock avalanches occurred after the snow on rock walls melted. Exsistence of seasonal frozen layers in the bedrocks was considered to play an important role of the occurrence of rock avalanches. The frozen layers had close relation with seasonal freeze-thaw actions. Moreover, water-regime changes in the rock walls were also controlled by the frozen layers as impermeable layers in and/ or after heavy rains. Rock masses separated from rock walls fell and/or jumped down to snow-covered talus slopes and, after being mixed up much snow, flowed on the slopes with reducing its speed. Finally, the rock masses and snow formed dirty avalanche tongues in deposition areas. In the rock avalanche occurred on July 10th, 1993, its travel distance and duration of processes indicate the veocity of about 70 km/h in average and about 100 km/h in the maximum.Volume of debris of a rock avalanche with frequency of once in tens of years is more than 103 m3, and one with frequency of once in several years is about 102 m3. Relative vertical mass transfer in unit area, which is estimated for quantitative comparison of process intensity, has the rate of 106 t · m/ km2 ·year.Rock avalanches occur in a cyclic pattern which fundamentally depends on the structure of rock walls. Low magnitude rockfalls and small rock avalanches occur on the denselyjointed rock walls under the control of diurnal freeze-thaw actions and water-regime change affected by intense rainfall. Its occurrence is more frequent than once in several years. Erosion on densely jointed walls induces gradual instabilization of neighboring sparselyjointed walls, and finally provokes higher magnitude rock avalanches there. The latter takes place less frequent than once in several decades.