[H-DS10] Geohazards in humid, tectonically active countries and their precursors

[HDS10-04] Geological and geomorphological causes of the landslide in Ashikita district, Kumamoto Prefecture induced by the heavy rain in July 2020

*Hidehisa Nagata¹ (1.Fu-Sui-Do Company Limited)

Keywords:heavy rain, Kumamoto, accretionary complex, paleo-weathering

The heavy rain in July 2020 induced disasters in many places in Japan. Here I review the rapid landslides that occurred on the rocks of accretionary complex in Ashikita district, Kumamoto prefecture facing the Yatsushiro Sea, and examine the geological and geomorphological causes of the landslide. The investigation data are taken from the four cases of the Sasiki River valley (Ushibuchi, Miyaura, Yahata, and the north of the quarry) and Fukuhama near the sea. In every case, the depth is about 10 m and the volume ranks 10⁴ m³ order. Bedrocks are composed of sandstone and mudstone of the Chichibu Belt, a Jurassic accretionary complex. The common feature through these landslides is the strong weathering of rocks. At the south of the landslides area, the Plio-Pleistocene Hisatsu Volcanic Rocks are distributed. Because the volcanos acted on the land, the rocks of the Chichibu Belt underlying the volcanics have been situated near the past earth surface and weathered. Although the distribution of the Hisatsu Volcanic Rocks is narrowed by denudation, originally they reached the area of the landslides. At the Fukuhama Landslide, andesite boulders were found in the deposit, which indicates that volcanic rocks remain at the top of the failure. Also, the other landslides locate under the unconformable plain in 100 m, which means the rocks suffered paleo-weathering. Generally, long-term weathering can be a major cause of landslides.