Debris Flow Induced by Deep-Seated Landslides at Minamata City, Kumamoto Prefecture, Japan in 2003

Takashi Jitousono, Etsuro Shimokawa and Yukiyoshi Teramoto

A large scale landslide occurred in the Atsumari River Catchment, Minamata City, Kumamoto Prefecture on July 20, 2003. This type of landslide is called a “deep-seated landslide.” The debris flow induced by this landslide killed 15 people. The deep-seated landslide was caused by the rising of groundwater level due to heavy rainfall, hydrogeomorphological formation of the underground area prone to storage of groundwater, and deeply weathered volcanic rocks. Some debris-flow disasters induced by the deep-seated landslides have occurred around this disaster site, such as the disasters at Nishiuchitate in Ebino City, Miyazaki Prefecture in 1972 and at Harihara in Izumi City, Kagoshima Prefecture in 1997. These areas are underlain by quaternary volcanic rocks, andesite, and tuff breccia. The characteristics of the deep-seated landslides and the debris flow induced by it were examined based on a field study in the Atsumari River Catchment. On the basis of the topographical, geological, and hydrological data of the volcanic areas around these sites, this study describes a method of predicting potential deep-seated landslide sites.