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Hokkaido quake points to loose volcanic soil on bedrock as landslide trigger: seismologist

Mud and sand from landslides triggered by the Sept. 6 earthquake have been removed from major roads in the southern Hokkaido prefectural town of Atsuma on Sept. 12, 2018. (Mainichi)

TOKYO/ATSUMA, Hokkaido -- Weak volcanic soil was to blame for deadly landslides triggered by the strongest earthquake on record to hit the northernmost prefecture of Hokkaido on Sept. 6, according to a seismologic expert.

Of the 41 victims killed by the quake, 36 died in landslides in the southern Hokkaido town of Atsuma, and 19 of them passed away in its Yoshino district. The soil layers that slipped along hard rock slopes after the quake was made of pumice stones spewed by nearby volcanos millennia ago.

According to professor Takashi Furumura of the University of Tokyo, jolts in extremely short cycles of about 0.5 seconds, which tend to cause landslides and damage small structures, likely hit Atsuma and unleashed the landslides.

The seismologist bases his estimate on the recording of similar tremblers by a National Research Institute for Earth Science and Disaster Resilience (NIED) seismometer in the Oiwake district of the town of Abira sitting right next to Atsuma to the east. The Oiwake district has thin pumice stone layers covering hard rock soil, the same ground structure that existed in the neighboring Yoshino district, according to Furumura.

Furumura's analyses also show that strong oscillations with cycles of 1 to 2 seconds, which have a tendency to cause wooden homes to collapse, were recorded on a seismometer set up in the Shikanuma district in southern Atsuma. The intensity of the jolts was stronger than those in the 1995 Great Hanshin Earthquake, and rivaled those that struck the Kumamoto prefectural town of Mashiki in the 2016 Kumamoto earthquakes.

Such strong shaking, however, caused relatively few houses to collapse in the Hokkaido quake. "This could be because there were relatively few homes in the area and those homes had very sturdy structures to endure heavy snowfalls," Furumura suggested. He went on to warn, "Even if the homes are sturdy, they will collapse if their foundations are weak. Earthquakes and the ground must be considered as a package. As buildings become increasingly more quakeproof, the problem with earthquakes will become more and more about the strength of the foundations for those structures."

Meanwhile, Atsuma Mayor Shoichiro Miyasaka said he did not foresee a major earthquake triggering landslides.

According to the Atsuma Municipal Government, there are no records of a disaster in which an entire swath of mountainside collapsed since the Meiji Period (1868-1912). Even the 10th typhoon of the season in 1992, which caused the most costly damage in the town's history, and the 2003 Tokachi-Oki earthquake, which measured an intensity of 5 on the 7-point Japanese seismic intensity scale, did not trigger any landslides that destroyed homes. In the latest disaster, however, numerous residents who lived outside of areas marked "dangerous" in the town's disaster prevention hazard map, fell victim to landslides.

The damage was concentrated in the marshy Atsuma River Basin, and communities in central to northern Atsuma were located along the mountain where the landslides occurred. Flood-control and irrigation projects were promoted there after World War II, and roads and agricultural waterways were built along the Atsuma River. But to avoid damage from flooding in the river basin, which occurred once every four to five years, many of the homes of local residents remained alongside the mountain.

According to 41-year-old Kiyoto Nakamura, who lost both his parents and his grandmother to the landslides in Atsuma, the homes closer to the foot of the mountain were less affected by the winds that blew down the mountain in the winter. He suggested that keeping the homes so close to the mountains was a way to avoid the effects of those winds.

The Atsuma Municipal Government is set to review its disaster-prevention policies, including its hazard map.

(Japanese original by Tomohiro Ikeda, Science & Environment News Department; and Jun Konno and Kotaro Adachi, Hokkaido News Center)

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