Photo/Illutration Landslide-hit areas in Atami, Shizuoka Prefecture, on April 6 (Provided by the Geospatial Information Authority of Japan)

A new aerial analysis suggests that roughly 85 percent of an overbuilt dirt landfill had given way in a deadly landslide in Atami brought on by heavy rains.

The Geospatial Information Authority of Japan (GSI) used a drone to inspect and analyze the site of the landslide disaster at the seaside city in Shizuoka Prefecture.

Some 56,000 cubic meters of soil had been added over the course of a decade, from 2009 to 2019, at the most upstream area of an infilled valley, it found. The bulk of it--about 47,000 cubic meters--is estimated to have collapsed and gushed out in the mudslide, according to the analysis.

The Shizuoka prefectural government had also estimated that about 54,000 cubic meters of soil was added over a roughly 10-year period.

The GSI compared past aerial survey data and discovered that some places in the valley in 2009 had much higher altitudes in 2019.

On June 6, after the landslide occurred, the GSI Land Bird, a team of drone experts, collected the aerial data by using lasers to measure the site and compared it with data from 2019 to determine how much soil had flowed out.

However, it is possible that even more soil was added after that measurement in 2019. In that case, more soil must have gushed out of the landfill site, a GSI official said.