熱海市逢初川盛土崩壊の地質的原因について

抄録

On 3 July 2021, a heavy rainstorm induced landslide of an embankment in Atami, Shizuoka Prefecture; it transformed into debris flow along the Aizome River hit a town of Izusan causing heavy damage including 26 fatalities and 1 missing. The cause of the landslide of the embankment was first attributed to probable inappropriate workmanship and poor drainage control of the embankment. We performed careful field survey after the landslide, and found that the area used to be covered by fluvial deposits (chaotic debris flow deposits). These fluvial deposits overlie hydrothermally altered andesite rich in smectite, which likely caused their landslide with the sliding surface in between since geological past. The fluvial deposits covered not only the Narusawa River but also the upstream of the Aizome River, where the shape of fluvial cone has been erased because of recent artificial modification. The embankment was made above the fluvial deposits. The landslide scar after the event suggested that water had gushed out from the basal parts of the old landslides. The water is assumed to be not only from the catchment of the Aizome River but also from that of the Narusawa River through the fluvial deposits, particularly along the basal part of the old landslides on the impervious clayey materials.