噴火後20年を経過した雲仙普賢岳の火砕流堆積斜面における土石流発生を助長する地形・地質的条件

抄録

In Mt. Unzen-Fugen-dake, debris flows occurred frequently after the Heisei eruption from 1990 to 1995 due to fine volcanic ash with a low infiltration rate. While the occurrence rate of debris flows decreased with the lapse of time after that, in recent years, debris flows have been occurring about once or twice a year at some streams. We researched the topographical and geological characteristics of recent major source area of debris flows in a previous work. As a result, a hypothesis was established positing that debris flows occur mainly because of the groundwater depending on the discontinuity of the infiltration rate and the differential erosion on the boundary of new pyroclastic-flow deposits and pre-eruptive ground surface below. The current situation in which debris flows are caused by these factors is considered to be the second stage, which follows the first stage in which low infiltration rate of volcanic ash caused debris flows. In this study, we verified that the hypothesis is generally consistent with the actual occurrence condition of debris flows at every stream covered with pyroclastic-flow deposits on eastern slope of Mt. Unzen-Fugen-dake, by researching the relationship among the location of pre-eruptive ground surface, the depth of pyroclastic-flow deposits, bed slope, catchment area, erosion depth and the actual occurrence conditions of debris flows, using the longitudinal and transverse cross sections from terrain models before and after the Heisei eruption. The topographical and geological factors that advance debris flows occurrence in the current situation were summarized as follows : (1) exposure of pre-eruptive ground surface with a low infiltration rate at a site on the stream bed, (2) a continuous steep stream with abundant pyroclastic-flow deposits below the site (1), and (3) exposure of pre-eruptive ground surface at stream bank of section (2). Furthermore, after suggesting the three factors above are likely to be completed when the lava landform is buried by pyroclastic flows, we proposed a simple guide for forecasting the source areas of debris flows at the second stage, namely “pyroclastic-flow deposits with a depth of less than around 50 m and a slope greater than 15 degrees that have buried the lava landform”.