On July 3 2021, a debris flow caused by a landslide from an embankment occurred along the Aizome River in the Izusan area of Atami City, Shizuoka Prefecture, central Japan, and destroyed numerous houses and flowed into Sagami Bay. In this study, two samples of embankment, three debris flow deposits and two soil samples from around the Aizome River valley were analyzed for grain size, and geochemistry and grain compositions for each mud and sand particles. The most important result was the discovery of radiolarian chert fragments in black embankment sediments and debris flow deposits. These chert fragments are useful as tracer material for black embankment sediments. Furthermore, the radiolarian chert was found to have been deposited near the Permian/Triassic boundary and later into the Jurassic and Early Cretaceous. This is an important finding for identifying the source of black embankment sediments.
