Hokkaido underprepared for offshore winter quakes and 'ice tsunami' destruction

Eleven years have passed since the 2011 Great East Japan Earthquake, and the Kushiro and Nemuro regions of eastern Hokkaido still urgently need to draw up disaster prevention measures for the midwinter period — including ways to avoid damage from so-called ice-tsunamis, or giant waves carrying floes of ice.

According to earthquake damage estimates released by the government in December, up to around 200,000 people could die if a megaquake struck in the Japan Trench or the Chishima Trench off the coasts of Hokkaido and the northeastern region of Tohoku during winter, when it becomes more difficult for people to evacuate. But the report said the death toll could be greatly lowered if people are able to evacuate quickly. Experts point to the need to consider disaster prevention measures in detail, particularly for winter.

Mitsugu Kawamorita, 80, of Hamanaka in Kushiro, recalls the ice tsunami caused by an earthquake off the coast of Tokachi, Hokkaido, on March 4, 1952. “Tsunami containing drift ice came and went many times, breaking down houses and engulfing a boat — splitting it into two,” he said.

The magnitude 8.2 quake caused ice tsunamis three to four meters high that hit areas including Hamanaka. Due to the disaster, 33 people died or went missing in Hokkaido and 815 houses were completely destroyed. In Hamanaka alone, ice tsunamis completely destroyed 62 houses and killed three people.

Mitsugu Kawamorita speaks about how tsunamis carrying ice hit the town of Hamanaka in Hokkaido in March 1952 after a major earthquake.

Two meters high

Kawamorita, then a fourth-grader at Kiritappu Elementary School in Hamanaka, was in a classroom when “the school building shook like it would collapse,” he said.

He escaped from a window with his indoor shoes on, climbed up a mountain at the back of the school building and watched the ice tsunami approaching.

After the waves receded, massive ice floes, each more than 2 meters high, were left on streets and strewn among the rubble. Aftershocks continued throughout the day. During the night, Kawamorita and some 10 family members and relatives found shelter in a ditch alongside a street in the neighborhood that lead to high ground.

“I was bundled up and wearing a blanket to get through the night,” Kawamorita said, looking back on the experience. “But it was still freezing cold, and I really felt the need to be prepared.”

The government has estimated that together with the roughly 200,000 deaths that could be caused in Hokkaido and the Tohoku region combined by a huge quake off northern Japan, a quake along the Chishima Trench could completely destroy some 77,000 houses, while a quake along the Japan Trench could cause some 211,000 houses to collapse.

If the effects of drift ice are considered in combination with a tsunami, the number of destroyed houses could increase further by 3,000 to 5,000.

The volume of sea ice floes routinely pushed ashore at Hamanaka and other coastal areas facing the Pacific in eastern Hokkaido is on a declining trend due to global warming, but a certain amount still reaches the shores every year.

According to Shinji Kioka, researcher at the Civil Engineering Research Institute for Cold Region in Sapporo, an ice tsunami can cause a phenomenon called an “ice jam” in which floating sea ice accumulates in the spaces between buildings and in streets.

The tsunami is then blocked by the accumulated ice and overflows into open spaces, resulting in faster and higher levels of flooding than would otherwise be expected.

“You should never feel you are safe enough even after you have escaped,” he stressed. “You should always be thinking about evacuating to an even higher place.”

Evacuation drills

In an effort to make clear the challenges of disaster prevention during winter, the town of Hamanaka conducted an evacuation drill last fall and relocated people to three locations in the town, including the three-story Kiritappu High School.

The Hamanaka Municipal Government asked 53 residents who took part in the drill to wear GPS devices, to measure the speed and duration of their movements.

Under the assumption that a tsunami would reach the town in 26 minutes and that the people would start evacuating five minutes after the alert was issued, everyone in the drill was able to arrive at shelters before the estimated time the waves would have reached them.

But when the drill was conducted with people evacuating 10 minutes after the alert, one person wasn’t able to reach the shelter before the assumed tsunami arrival time. When evacuations began 20 minutes after the alert, only about 10 people managed to reach the shelters in time.

Emiko Konno, a 62-year-old housewife who arrived at an evacuation shelter 21 minutes after leaving her home, said, “I gave up carrying a rucksack that contained emergency food and water because it was heavy. It’s hard for elderly people to walk in wintertime.”

The municipal government is considering building an evacuation tower or preparing lifeboats, but it would likely take at least 10 years to equip them.

Residents “would basically have to evacuate by car,” said an official at the municipal government’s disaster prevention office. But even evacuating by car would be difficult in winter.

In Hamanaka’s coastal Shinkawa district, the roughly 10-kilometer-long prefectural road that is the main route for evacuation by car is mostly mountainous and the road surface tends to freeze during winter, increasing the risk of accidents due to slippage. In March last year, slipping caused a pileup on the road involving more than 20 vehicles.

“Driving to evacuate from tsunami threatens the lives of other people,” said Eiji Konno, the 66-year-old former chairman of the district’s neighborhood association, which represents 124 households. He plans to evacuate to Kiritappu High School, which is around 20 minutes walk from his house.

Munehito Ishiwari, head of a network of disaster prevention experts in Kushiro, said, “There is an urgent need to build an evacuation tower that can withstand the impact of ice floes and where people can keep out of the cold.

“Residents should also be prepared, with raincoats to prevent hypothermia and headlamps to evacuate at night.”


Hamanaka Town