Collapsed black embankment sediments "definitely originated from elsewhere": Shizuoka University and others perform compositional analysis to verify landslide in Atami City

A research group from Shizuoka University, the Japan Atomic Energy Agency, the Museum of Natural and Environmental History, Shizuoka, Ehime University, and the University of Tokyo conducted a geochemical and particle composition analysis of the embankment and debris flow deposits at the site of the landslide that occurred in Atami, Shizuoka Prefecture, in July 2021. The group made important findings that can be used to evaluate the disaster hazard of embankments by examining whether chert fragments are present in the sediment.

At around 10:30 a.m. on July 3, 2009, a landslide occurred along the Aizome River in the Izusan district of Atami City that extended to Izusan Port and flowed into Sagami Bay. Subsequent investigations revealed that a large amount of the embankment at the head of the Aizome River had collapsed. Comparing topographic survey data from 2009 and 2019, the Geospatial Information Authority of Japan (GSI) estimated the volume of embankment formed during the same period to be about 56,000 cubic meters, of which Shizuoka Prefecture reported about 55,000 cubic meters had collapsed. Based on the calcium content of the debris flow sediments and embankment, they estimated that the mix ratio of sediment washed down as debris flow was 75-85% black sediment and 15-25% brown sediment.

Based on the Shizuoka Prefecture report, the research group collected the brown and black sediment and debris flow deposits of the embankment and surrounding soil from the site and analyzed it for grain size composition, major and trace elements of mud particles, mineral composition, identification of mineral and rock species and particles, and radiolarian fossils in chert fragments. They found that the black sediment in the embankment came from elsewhere, and that chert fragments were found in the black sediment and debris flow deposits. The radiolarian fossils and dolomite grains contained in the chert were identified as dating from around the boundary of the Paleozoic Mesozoic periods to the Early Cretaceous period. Chert fragments are an effective material for tracing black sediments and were key to identifying black sediment collection sites.

The black sediment and debris flow deposits in the embankment contained chert fragments, but no chert rock bodies are distributed around the surveyed area, indicating that the black sediment came from elsewhere. Chert fragments contain radiolarian fossils and dolomite (CaMg(CO₃)₂) particles, a carbonate mineral.

Based on the radiolarian fossil types and dolomite grains, the age at which the chert was deposited ranges from near the boundary of the Paleozoic Mesozoic periods to the Early Cretaceous period. Assuming that the chert fragments in the mudslide deposits and embankment are all of the same origin, the Chichibu Belt is the only geologic body for which chert of the relevant depositional age has been reported in the Kanto region. Thus, some of the black sediments collected could have come from the lower reaches of the Chichibu Belt in the Kanto mountains. Therefore, chert fragments are effective in tracing the black sediments and an important key to identifying black sediment collection sites.

Professor Akihisa Kitamura of the Department of Geosciences, Faculty of Science, Shizuoka University, said, "This mudslide was caused by the collapse of the embankment, and marine shellfish were found in the embankment and mudslide deposits. Chert fragments containing radiolarian fossils were found in the deposits as well, but they were mixed with a variety of other materials. Moving forward, we would like to identify where the embankment collected and how the debris flow occurred based on chert fragments and marine shellfish."