Prediction of Deep-Seated Landslide Sites on the Basis of Hydrogeomorphological Study

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A deep-seated landslide occurred at Izumi City in northwestern Kagoshima Prefecture on July 10,1997. The purpose of this study is to prepare a practical map for predicting high potential deep-seated landslide sites on the flank of the old volcano of Mt. Yahazu-dake. The mechanism of this landslide, and the geological and hydrogeomorphological characteristics of the landslide slope sites are examined based on field study. The results are as follows :1. In the hydrological observatories installed in the basins of Mt. Yahazu-dake, discharge, temperature, and electrical conductivity of the stream water were measured continuously. Although these basins are located in the same flank, the runoff characteristics changes with the rates of gentle slope, the degree of weathered rocks, and the degree of dissection, etc.2. We measured the base flow, water temperature, and electrical conductivity at 17 streams around Mt. Yahazu-dake. Those values depend on the spatial distribution of gentle slope and deeply weathered rocks, and the degree of dissection.3. The discharge of groundwater at the foot of the hillslope where the landslide occurred were measured. The peak of groundwater flow is several hours behind the peak of rainfall. This result is harmony-like as the time of landslide in July, 1997.4. Base flow and electrical conductivity for every altitude at the streams around Mt. Yahazu-dake were measured. Consequently, these values changed at the altitude of the deep-seated landslide scar.The discharge and electrical conductivity of stream are the important factors in predicting the potential deep-seated landslide sites.