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Landslides in quake-hit Hokkaido triggered by waterlogged pumice crust

Houses hit by a landslide caused by an earthquake are seen from a Mainichi Shimbun airplane in the town of Atsuma, Hokkaido, on Sept. 6, 2018. (Mainichi)

Deadly landslides in the town of Atsuma in southern Hokkaido Prefecture following an intense earthquake on Sept. 6 occurred because the crust of hills that gave way consisted largely of volcanic pumice stones, experts say.

The light, porous stones are easily moved by earthquakes. Continuing rains around the town this summer prior to the temblor may have contributed to the disaster, which killed at least 16 people and left dozens missing.

According to Yoshihiro Ishizuka, who heads the volcanic activity research group at the National Institute of Advanced Industrial Science and Technology, huge volcanic eruptions some 40,000 years ago that created Lake Shikotsu 40 kilometers west of Atsuma caused a massive amount of pumice stones to fall over the town, which accumulated to a depth of 4 meters. Moreover, eruptions of Mount Eniwa to the north of Lake Shikotsu, and Mount Tarumae to the south also each brought 50-centimeter layers of the lightweight stones.

These soil strata are considered to contain a plenty of water from the long, heavy rains of this summer, and crumbled as the earthquake hit the area. "There are no big rocks visible there. The surface pumice stones alone crumbled while the rocky foundation below remained intact," explained Ishizuka.

According to Ishizuka, pictures of the landslide sites show the colors of pumice stones -- beige ones from the Shikotsu eruption, orange ones from Mt. Eniwa and whitish stones from Mt. Tarumae -- as well as the black surface soil.

These strata of pumice stones are called tephra layers. Similar landslides of such layers occurred when massive earthquakes hit the southern prefecture of Kumamoto in April 2016, said professor Masahiro Chigira, a specialist in applied geology at Kyoto University's Disaster Prevention Research Institute. This area also has pumice layers from Mount Aso. According to Chigira, pumice stones become more porous and more easily give way to tremors as they weather.

During the Kumamoto earthquakes, more than 400 landslides hit areas around Mount Aso. Professor Chigira said that judging from the photos from the landslide sites in Atsuma, the slips could be attributed to "the collapse of tephra layers." The professor added that the damage was more widespread than that caused by the 2016 quakes. He warned that tephra layers exist near volcanoes across Japan, and tend to give way when hit by an earthquake measuring 5 or higher on the 7-point Japanese seismic intensity scale.

The Tokachi-oki Earthquake of 1968 in the northern prefecture of Aomori triggered landslides from strata of volcanic ash originating in the Towada caldera, killing 48 people.

(Japanese original by Tomohiro Ikeda and Toru Watanabe, Science & Environment News Department)

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