September 2005 rain-induced catastrophic rockslides on slopes affected by deep-seated gravitational deformations, Kyushu, southern Japan

Abstract

River rejuvenation forms convex slope breaks that can destabilize nearby slopes, leading to gravitational deformation and, in some cases, catastrophic sliding of the slopes during heavy rainstorms. Such landslides were induced by Typhoon 0514, which passed to the west of Kyushu Island, Japan, on 6 September 2005. Five large, catastrophic rockslides occurred, aligning along convex slope breaks 150 to 300 m above the present river bed of the Mimi River; the slope breaks are inferred to have been formed by rejuvenation of the river. Geological and geomorphological investigation revealed that these slides occurred in sandstone and mudstone of the Cretaceous Shimanto Group and were preceded by slope deformation, which generated gravitational folds and faults that appeared as scarplets at the tops of the deformed areas. The slopes seem to have been in critical condition just before catastrophic failure, which was triggered by the rainstorm.