Wednesday, September 7, 2011

07/09 Typhoon No. 12 caused deep-seated landslides

This photo taken Monday shows a mud slide in Tanabe, Wakayama Prefecture.

Large mud slides in Wakayama and Nara prefectures resulting from Typhoon No. 12 are suspected of being deep-seated landslides, according to a specialist.
Unlike shallow landslides in which topsoil 0.5 meters to 2 meters deep comes loose, the bedrock beneath the topsoil also moves in deep-seated landslides, causing enormous damage.
Typhoon No. 12 brought record rainfall to the Kii Peninsula, where fragile bedrock allowed massive amounts of rainwater to permeate underground. This has sparked concerns that more deep-seated landslides could occur in the region.
Kyoji Sassa, a professor emeritus of Kyoto University, has warned the peninsula will frequently be hit with similar landslides in the future due to its geological features.
Sassa analyzed news photos following Typhoon No. 12 of a mud slide in the Fudono district of Tanabe, Wakayama Prefecture, and suspects it is a deep-seated landslide. He also believes a mountain collapse in Gojo, Nara Prefecture, could be a deep-seated landslide.
Asahiko Taira, a geologist and executive director of the Japan Agency for Marine-Earth Science and Technology, said the Kii Peninsula is formed with Shimanto-tai strata, which are highly prone to landslides.
The strata began forming 20 million to 100 million years ago by colliding tectonic plates. In a process of repeated collisions and transformations, land mass was pushed upward and cracks dozens of meters deep were formed, which is where rainwater can accumulate.
"The deep-seated landslides were caused as rainwater infiltrated deep cracks over long periods of time," Sassa said. "Similar large-scale collapses may occur when [the strata] absorb large quantities of water."
According to a map of areas that could be hit by deep-seated landslides drawn up by the Land, Infrastructure, Transport and Tourism Ministry in August last year, about 34 percent of the land in Nara Prefecture is at particularly high risk.
The map also indicates the remaining areas of the prefecture and neighboring Wakayama Prefecture are also at high risk of deep-seated landslides.
(Sep. 7, 2011)

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