Prediction of potential landslide sites on Shirasu steep slopes applying the landslide periodicity

本研究的目的是根据日本九州南部地区的滑坡伤痕年代学、坡面土层深度和滑坡的周期性，编制一张实用的地图，用于预测九州南部石拉苏(来自阿拉火山的火山碎屑流沉积)陡坡上的高潜在浅层滑坡地点。主要研究成果如下：1.利用全球定位系统(GPS)和光电测量仪器分别进行了大比例尺的骨架地形图和细节地形图的绘制。每一张浅层滑坡伤痕、表土深度和植被的地图都叠加在地形图上。年龄在60年以下、表土较薄、年龄在60年以上、表土较厚的老滑坡上的森林植被，在树种组成和植物演替上存在差异。这表明，通过对坡面上生长的植被进行区分，可以间接预测高潜力浅层滑坡地段。基于大比例尺航片的图像分析，可以区分年轻滑坡伤疤上的森林植被和老滑坡伤疤上的森林植被。根据坡体形态特征，将研究区石拉苏坡体划分为6个坡段。在这些部分中，由坡度超过40度的最陡峭的斜坡组成的部分被选为高潜在浅层滑坡现场。这部分坡度陡峭，广泛分布在峡谷和河道边的山坡上。根据坡面形态划分预测的高潜在浅层滑坡点位上，新老浅层滑坡的大部分已形成伤痕。

The purpose of this study is to prepare a practical map for predicting high potential shallow landslide sites on Shirasu (pyroclastic flow deposits from Aira volcano) steep slopes based on the landslide scar chronology and topsoil depth on the slopes and the landslide periodicity in the southern part of Kyushu, Japan. The results are as follows :1. A large scale topographic map of the study area was made based on skeleton and detail topographical surveying using the global positioning system (GPS) and electro-optical survey instrument, respectively. Each map of the shallow landslide scars, topsoil depth and vegetation was superimposed on the topographic map.2. The forest vegetation on younger landslide scar of less than 60 years in age with thinner topsoil and older landslide scar of more than 60 years in age with thicker topsoil differ from each other in tree species composition and plant succession. This suggests that the high potential shallow landslide sites can be indirectly predicted through distinction of the vegetation grown on the slopes.3. It is possible to distinguish between the forest vegetation on the younger landslide scar and the one on the older scar based on the image analysis using large-scale aerial photographs.4. The Shirasu slopes in the study area were divided into six parts based on the slope morphology. Of these parts, a part consisting of steepest slopes of over 40 degrees in gradient with a form of concave or straight was selected as the high potential shallow landslide site. This part with steep slopes are widely distributed on the slopes along ravines and river channels. A most parts of the younger and older shallow landslide have been scarred on the high potential shallow landslide sites predicted based on the division of slope morphology.