Landslide Disaster Triggered by July 2020 Heavy Rainfall in the Southern Part of Kumamoto Prefecture, Southwestern Japan

Abstract

 Intense rainfall related to a stationary front on July 3-4, 2020 (maximum total 513 mm; maximum 4-hour rainfall 260 mm) triggered many landslides and floods in the southern part of Kumamoto Prefecture, Kyushu, southwestern Japan. The distribution of landslides coincided with the highest concentration of rainfall in the region. The July 4, 2020 landslides were concentrated mainly in Ashikita and Tsunagi towns by the Yatsushiro Sea. Most of the landslides occurred on forested slopes steeper than 20°. They occurred in thick weathered soil layers overlying sedimentary and volcanic rocks; the thickness of the slides was generally less than 20 m. Some of those occurring on steep upper slopes along valleys mobilized completely into debris flows, traveling along stream channels and flooding at the lower reaches. The landslides and debris flows transported sediments and woody debris, which damaged houses and caused fatalities at some sites. The July 4, 2020 landslides and associated debris flows occurred mainly in areas underlain by Jurassic accretionary complexes of the Chichibu Belt. Some landslides were affected by fault fracture zones with the formation of thick weathered soil layers. These geologic characteristics of landslides provide important information for preventing or mitigating similar future disasters in accretionary complex zones.