Photo/IllutrationA landslide carried this corestone to a residential area in Hiroshima’s Aki Ward. (Naoko Kawamura)

  • Photo/Illustraion
  • Photo/Illustraion
  • Photo/Illustraion

Rain-triggered landslides in Hiroshima Prefecture unearthed boulders known as “corestones,” whose rounded surfaces accelerated their descent and exacerbated damage in residential areas.

Granite corestones up to 3 meters in diameter were found in Hiroshima’s Aki Ward, which was hit by a landslide that flowed beyond a mud-control dam.

Hiroshi Takebayashi, associate professor of Kyoto University’s Disaster Prevention Research Institute who specializes in river engineering, found many granite corestones in damaged areas of Hiroshima Prefecture on July 9.

“Large craggy rocks do not turn over so easily,” Takebayashi said. “When mixed with fine-grain soil, the corestones slid down the side, which led to larger damage.”

The corestones in Hiroshima Prefecture were formed after huge slabs of granite cracked. Over time, erosion from water and weathering created the smooth granite boulders known as corestones, according to Makoto Saito, innovation coordinator who specializes in geology at the National Institute of Advanced Industrial Science and Technology.

Granite bedrock, which can easily become corestones, had spread to many of the slopes where landslides occurred, including Hiroshima’s Aki Ward, Saito said.

In 2009, a mudslide hit a residential area in Hofu, Yamaguchi Prefecture. Corestones worsened the damage to the area, Saito said.