SHIZUOKA -- A massive earth mound on a hill is believed to have collapsed, exacerbating the damage from a mudslide triggered Saturday by torrential rainfall in Atami, Shizuoka Prefecture.
The Shizuoka prefectural government on Sunday released photographs, taken by drone, of the point where the mudslide began. The spot is about 2 kilometers upstream from the mouth of the Aizome River, which runs through the city.
Based on data from 2010 and later, the prefecture estimated that an embankment created by piling up earth and measuring about 54,000 cubic meters existed near the point where the mudflow began. Of this figure, more than 50,000 cubic meters -- the amount required to elevate one hectare of land by five meters -- are believed to have collapsed.
"It is not known whether the collapse of this embankment triggered the landslide, but it can be said to have added to the damage," the prefectural government said.
According to the Shizuoka prefectural and Atami city governments, a company based in Kanagawa Prefecture submitted an application to the city in 2007 regarding development of the area, in compliance with a Shizuoka prefectural ordinance regulating soil gathering.
Construction was reported to have been completed around 2011. The embankment made from this construction is believed to have collapsed on Saturday.
Experts are questioning the stability of the embankment.
"In general, embankments are more prone to collapse than natural slopes. The embankment may have become extremely unstable due to the heavy rain, because it was built in a valley where water accumulates easily," said Norifumi Hotta, a University of Tokyo associate professor specializing in erosion control engineering.
Based on the possibility that multiple mudslides occurred, Mie University Prof. Daizo Tsutsumi, who also specializes in erosion control engineering, said, "It cannot be ruled out that the slope below the embankment collapsed first or was eroded by heavy rain, thereby creating a mudslide that caused the embankment to collapse because it had lost its support."
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