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Decomposed granite soil may have caused Hiroshima Pref. 'surface landslides': experts


After torrential rains triggered a series of deadly landslides in Hiroshima Prefecture, experts pointed to the possibility that they were so-called "surface landslides," in which soil about 0.5 to 2 meters thick breaks away and falls.

    Hiroshima Prefecture has repeatedly suffered landslides caused by downpours due to weak ground in many areas. It is said that roughly 50 percent of the prefecture's land area sits on granite layers. In many of these areas, decomposed granite soil deposits on hard bedrock. Due to limited flatland in the prefecture, slope ground with such soil at the foot of mountains was cleared for developing residential areas.

    However, decomposed granite soil is fragile and prone to landslides, raising the risks of heavy rain triggering such disasters in areas with such soil deposits. There are an estimated 49,500 locations in Hiroshima Prefecture that are designated as landslide disaster warning zones -- the highest figure in the country.

    In July 1967, heavy rain triggered landslides in the city of Kure and other parts of Hiroshima Prefecture, leaving 159 people dead or missing. When torrential rain hit the prefecture in June 1999, landslides occurred at newly developed residential areas and elsewhere in the city of Hiroshima, leaving 32 people dead or missing. In August 2014, mudslides hit residential areas in Asaminami and Asakita wards in northern parts of Hiroshima city, claiming the lives of 77 residents, including related deaths.

    Rescue workers examine a collapsed structure that was caught up in a landslide in Kure, Hiroshima Prefecture, on July 8, 2018. (Mainichi)

    Professor Masahiro Chigira at Kyoto University's Disaster Prevention Research Institute, who is specializing in geology, commented based on news footage of the latest landslides and geological maps, "It is believed that surface landslides were triggered by weathered granite, like in the 2014 disaster." With regard to a landslide disaster on Nuwa Island in Matsuyama, Ehime Prefecture, which buried a mother and two children, he pointed out that the island also sits on granite layers. "There must have been more areas where surface landslides occurred than reported," the professor said.

    Meanwhile, with regard to a landslide in Ayabe, Kyoto Prefecture, which left three people missing, Chigira noted, "The area appears to sit on old sedimentary rock layers, not those of granite." In 2014, around the same time as the deadly landslides in Hiroshima, slope collapses took place in Fukuchiyama, Kyoto Prefecture, which is also home to sedimentary rock layers. It is believed that the landslides due to the record rainfall over the weekend occurred regardless of the strength of the ground.

    (Japanese original by Koki Matsumoto and Ryo Watanabe, Osaka Science & Environment News Department)

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