Rainfall and Its Infiltration Conditions for Landslide Occurred at Edge of Pyroclastic Flow Plateau in the Kyushu Island, Southwestern Japan

The torrential rains of July 2012 caused many landslides at the edge of the Aso pyroclastic flow plateau, consisting of the Aso-3 and the Aso-4 pyroclastic flow deposits in Kyushu Island, Japan. These landslides can be classified into two types: the caprock type, which occurred in Aso-4 areas, and the rainwater direct infiltration type, which occurred in Aso-3 areas. In both types, the landslide sliding surface is located in the lower pyroclastic fall layer of the pyroclastic flow deposit. The maximum hourly precipitation and effective precipitation with a half-life of 1.5 h control landslide occurrence and differ between landslide-prone and non-prone areas and between caprock and direct infiltration types. These parameters are larger in landslide-prone areas than in non-prone areas and are more significant in the caprock type than in the direct infiltration types. In a caprock type landslide, the effective stress in the pyroclastic fall layer estimated by theoretical calculations decreases with increased pore water pressure due to rainwater infiltration. Therefore, the landslide at the edge of the Aso pyroclastic flow plateau was caused by a decrease in effective stress in the pyroclastic fall layer.