Weak 'masado' soil in disaster

Weak 'masado' soil in disaster
A local resident stands on rubble at a site where a landslide swept through a residential area at Asaminami ward in Hiroshima, western Japan, August 21, 2014.

HIROSHIMA, Japan - Difficulties in accurately predicting where heavy localized downpours will strike and a type of soil that becomes fragile when wet appear to be the main factors behind Wednesday's deadly mudslides in Hiroshima, according to experts.

The disaster, which left about 40 people dead and several others missing, apparently involved a sudden surface failure on several hills after hours of torrential rain. The soil in mountainous areas of Hiroshima Prefecture has a fragile composition, and there are about 32,000 locations at risk of landslides-the most of any prefecture in the nation.

All the same, Kansai University Prof. Yoshiaki Kawata, an expert on weather-related disasters, believes such a tragedy could happen almost anywhere in Japan.

"In recent years, the weather has become more unseasonable across the whole nation. It wouldn't be strange for such intense rain to fall anywhere in Japan," Kawata said.

According to the Land, Infrastructure, Transport and Tourism Ministry, from 700 to 1,400 landslides are reported in Japan annually. A total of 497 such incidents had been reported so far this year as of Aug. 7.

Potential danger in many areas

Japan's topography is a major reason why the nation is prone to landslides. Being a mountainous country, the surface of many regions is covered by volcanic ash or "masado," a type of fine soil made from weathered granite that easily breaks apart when it absorbs water.

Hirotaka Ochiai, director of the Research Planning and Coordination Department at the Forestry and Forest Products Research Institute and an expert on forestry engineering, said last month's mudslide that killed or injured four people in Nagiso, Nagano Prefecture, occurred in an area with extensive amounts of weathered granite soil.

The collapse of a layer of volcanic ash during heavy rain was the cause of a massive debris flow that left 39 people dead or missing on Izu-Oshima island just south of Tokyo in October 2013.

Prefectural governments can designate about 520,000 locations across Japan that are deemed to be at risk of landslides or mudslides as "caution zones" or "special caution zones," and restrict the development of residential areas in these zones.

However, this process has only been completed for about 350,000 locations, or 70 per cent of the total. Most of Wednesday's landslides in Hiroshima happened in areas that fell outside these designated locations.

"Surveying the land takes time," an official of the Land, Infrastructure, Transport and Tourism Ministry said.

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