2020 Volume 29 Issue 1 Pages 33-40
Kumamoto Prefecture in Japan was struck by two huge earthquakes of Japanese seismic intensity scale 7 in April 2016. This paper describes the following sediment-related disasters caused by the 2016 Kumamoto Earthquake: a debris flow, a landslide and a collapse. We discuss debris flow runoff characteristics based on relationship between the torrent gradient and the sediment concentration and landslide and collapse occurrences based on slope stability analyses. Especially, we set both rainfall and earthquake calculation conditions for a semi-infinite slope and conducted slope stability analyses that focused on the outbreak factor of slope disasters. According to the results, when the debris flow structure consisted of a high sand-water mixture, the material of a small particle size raised the pore water density. An earthflow and a mudflow might form in the mild gradient torrent, arrive downstream and overflow even if the bed slope gradient is very small. For a landslide and a collapse, earthquakes caused a repeated load and generate excess pore water pressure, and since they reduced the shear strength near the sliding surface, the mountain slope lost stability and collapsed rapidly. If we could appropriately estimate the loss of the shear strength caused by the earthquake, we might be able to explain mass movement occurrences on a mountain slope that didn't collapse due to rainfall. Depending on the earthquake's scale, mass movements might not occur. Our result suggests that the enormity of the 2016 Kumamoto Earthquake was a primary factor in its destruction.
