Increasing weight is being given to the view that the damage from a July 3 mudslide disaster in the city of Atami in central Japan's Shizuoka Prefecture was exacerbated by the collapse of large mounds of soil piled up near the hilltop.
The Shizuoka Prefectural Government estimates that about half of the approximately 100,000 cubic meters of material that came down in the disaster is soil that was used to raise the ground level.
The place where the landslide occurred was originally a valley, which was later filled with huge amounts of soil. Experts have asserted that it is possible groundwater that built up over the long period of rainfall was dammed by the mounds of soil, triggering the mudslide.
It is not known at this stage whether the soil mounds gave way before other ground. But artificial mounds are thought to be weaker than naturally formed layers of soil. In the present case, a thick artificial layer of soil was situated above houses built onto a sloping area.
Prefectural government investigations into whether this caused the mudslide are continuing. The site must be thoroughly surveyed, and questions have to be answered regarding whether there were problems with construction methods and if the mounds were properly managed.
Some reports say a real estate firm transported excess soil up to the site around 2009. Residents who spotted dump trucks driving in and out of the area voiced their concerns, including that it could be dangerous in the event of rainfall.
The soil was carried to the site in accordance with a Shizuoka Prefecture ordinance regarding rules on the handling of earth, and documents were filed with the Atami Municipal Government. Questions are being asked as to whether the authorities' oversight was sufficient.
Since 2006, the central government has strengthened its safety measures against the creation of soil mounds during large-scale residential developments. Damage from landslides, soil liquefaction and other issues in the 1995 Great Hanshin Earthquake that struck west Japan, and the 2004 Chuetsu Earthquake affecting central Japan's Niigata Prefecture, led to these changes.
The site of the most recent mudslide, however, was not subject to the limits because it is not a residential property. The central government has indicated it is considering nationwide inspections to ascertain whether other regions face similar risks. It is imperative that dangerous areas are exposed.
Across Japan there are around 660,000 places classed as sediment disaster-prone areas, like the Atami site, and roughly 180,000 mountain streams where mudslides could occur.
About 70% of Japan's land is covered in forest. Because there are few natural plains, many people live in areas where mountains have been cleared and developed on.
The effects of global warming have increased the risk of landslides caused by torrential rains. The national and local governments have a responsibility to thoroughly implement measures to protect residents' lives.