平成24年7月九州北部豪雨による阿蘇山カルデラ壁および中央火口丘での斜面崩壊

Abstract

The north-Kyushu heavy rainfall on July 2012 triggered severe shallow landsliding of hillslopes of caldera wall and volcanic central cones located in the Aso caldera area. The sliding materials are mainly tephra deposits with stratified structure covering this area, bearing a mechanically weak layer at the depth of sliding. Shear strength of soils extracted from the two types of typical slip scars shows clear difference in their cohesive component. The silty loam obtained at the Naka-sakanashi area (a part of the west-facing caldera wall) has a higher cohesion than the sandy loam from the Sensui-kyo area (in the central cone). This soil property seems to be reflected to the shape of landslides, which exhibits a deeper (~3 m) and gentler (~30°) sliding scar in the Naka-sakanashi area, than a shallower (< 1 m) and steeper (~40°) scar in the Sensuikyo area. We carried out an infinite planer slope stability analysis to reconstruct conditions for landslide initiation. The results demonstrate that an increase in pore water pressure in the mechanically-weak potential slip plane triggered the landslides.