Map pinpoints risks of deep-seated landslides

The Yomiuri Shimbun

The land ministry has released the first ever map indicating the probability of deep-seated landslides throughout the nation.

A deep-seated landslide--in which the main collapse occurs deep below ground--occurs when a large volume of rainwater penetrates bedrock that has weakened over the years, causing a collapse as deep as five meters.

Only 122 deep-seated landslides have been observed--a relatively low number--since 1868.

According to the analysis by the Land, Infrastructure, Transport and Tourism Ministry, deep-seated landslides occur more frequently in regions where the ground was raised during the formation of the archipelago, or which comprise fragile sedimentary or volcanic rock.

The ministry used the results of its analysis to categorize areas into one of four likelihoods: five times or greater than the national average is "particularly high," between one and five is "high," and so on, based on number of occurrences.

Nagano Prefecture tops the ranking, with 48 percent of the prefecture's total area categorized as "particularly high," followed by Miyazaki Prefecture (38 percent), Nara Prefecture (34 percent), Yamanashi Prefecture (28 percent) and Gunma Prefecture (26 percent).

The ministry plans to conduct further research on very high regions by fiscal 2011.

According to records, one deep-seated landslide in the past involved 100 million square meters of dirt--enough to cover the entire area encompassed by the JR Yamanote Line in dirt 1.5 meters deep.

Kazuyuki Sato, chief of the ministry's Erosion and Sediment Control Office, said: "[Deep-seated landslides] can happen even on a clear day after a heavy rain. We need to work with local governments to set up evacuation plans."