Unusually strong rains caused heavy damage in western Japan and the Chubu region this summer. For example, an evacuation order was issued to 110,000 residents of some 48,000 households in Hiroshima. In mid-July, record precipitation was recorded at six points in Japan, including rainfall of 107 mm per hour in Tokyo's Itabashi Ward and 108.5 mm per hour in Minami, Tokushima Prefecture.
As hard-to-predict localized torrential rains are becoming rampant, the land and infrastructure ministry is calling attention to "deep landslides," which involve both surface soil and bedrock. Such landslides are triggered when a large volume of water from torrential rain seeps into the cracks of bedrock two meters to several dozens of meters below the Earth's surface. This may occur a few days after a torrential rain.
The ministry has created a nationwide map showing the estimated deep-landslide frequency, which is divided into four stages. The ministry will spend three more years examining places where the estimated frequency is high. It has found that deep landslides each involving 100,000 cubic meters or more of mud and rock occurred nine times in the 1980s, 19 times in the 1990s and 24 times in the 2000s.