The view that the damage from a mudslide disaster in Atami city, central Shizuoka Prefecture, Japan, was compounded by the collapse of large mounds of earth near the hilltop on July 3.
The Shizuoka Prefecture government estimates that about half of the roughly 100,000 cubic meters of material that crashed in the disaster is soil used to raise the ground.
The place where the landslide took place was originally a valley that was later filled with huge amounts of earth. Experts have claimed that groundwater accumulated during the long rainy season may have been dammed up by the mounds of earth and triggered the mudslide.
At this point it is not known whether the mounds of earth gave way to other soil. However, artificial mounds are considered weaker than naturally formed soil layers. In the present case, there was a thick layer of artificial earth over houses that were built on a hillside plot.
The prefectural government’s investigation into whether this caused the mudslide is ongoing. The terrain must be thoroughly surveyed and questions must be answered as to whether there were any problems with the construction and whether the hills were properly cultivated.
According to some reports, around 2009 a real estate company moved excess land onto the site. Local residents who saw dump trucks drive in and out of the area voiced their concerns, including that it could be dangerous when it rains.
The soil was transported to the construction site in accordance with a Shizuoka Prefecture Ordinance on Rules for Handling Soil, and documents were submitted to the Atami City Government. It is asked whether the supervision of the authorities was sufficient.
Since 2006, the central government has increased its security measures against the formation of mounds in large housing estates. Damage from landslides, soil liquefaction, and other problems in the Great Hanshin Earthquake in 1995 that struck western Japan and the 2004 Chuetsu earthquake in Niigata prefecture in central Japan led to these changes.
However, the location of the recent mudslide was not subject to the limit values as it is not a residential property. The central government has announced that it will look to nationwide inspections to see if other regions face similar risks. It is imperative that dangerous areas are exposed.
Across Japan, there are about 660,000 places classified as sediment disaster prone areas, such as the Atami site, and about 180,000 mountain streams where mudslides could occur.
About 70% of the Japanese land area is covered with forest. Since there are few natural plains, many people live in areas where mountains have been cleared and built on.
The effects of global warming have increased the risk of landslides from torrential rains. National and local governments have a responsibility to thoroughly implement measures to protect residents’ lives.