869 Tohoku tsunami parallels stun

Research team's efforts set precedent to add history to other quake-study disciplines


Staff Writer

When the Great East Japan Earthquake hit Tohoku on March 11 last year, quake researcher Masanobu Shishikura grabbed a tablet computer and called up the website of the U.S. Geological Survey in Virginia to search for information.

Seeing projections of a preliminary magnitude of 8.8 and its location off Miyagi Prefecture, he immediately realized that what his team had long dreaded had finally become reality: a recurrence of the 869 Tohoku megaquake.

He also knew what was coming next — massive tsunami that would engulf coastal areas in Iwate, Miyagi and Fukushima prefectures in 30 to 60 minutes, just as the previous disaster did more than 1,100 years ago.

“I knew that the people would not be saved unless they immediately started running,” Shishikura said in a recent interview with The Japan Times. “I just kept praying, wondering if people there were evacuating properly.”

Shishikura, who heads a quake research team at the National Institute of Advanced Industrial Science and Technology (AIST) in Tsukuba, Ibaraki Prefecture, now regrets he was unable to save any lives despite his team’s findings.

But the March 11 tsunami and his team’s research have been influential in initiating a paradigm shift in Japanese seismological studies over the past year.

The disaster lent credibility to his team’s research methodology, prompting others to launch interdisciplinary studies integrating seismology, geology and philology to examine the history of earthquakes over hundreds of years.

Shishikura’s team took soil layer samples along the Tohoku coast and checked them against disaster accounts recorded in ancient chronicles. This helped them conclude that major temblors and tsunami repeatedly slam Tohoku over a cycle of between 500 and 1,000 years.

Shishikura had planned to visit the Fukushima Prefectural Government last March 23 to explain this danger.

Shishikura’s team even drew up maps of the areas flooded by the 869 tsunami and planned to distribute copies to residents along the coast. The maps turned out to be nearly identical to the areas that were inundated on March 11.

“We could have saved some lives if the tsunami had come just a month later,” he lamented.

Shishikura said his team’s approach — studying both soil layers and historical accounts to extrapolate quake cycles over hundreds of years — was regarded as a “minor” technique by seismologists.

Before March 11, mainstream seismologists had mostly focused on watching and analyzing real-time data from observation equipment tracking tectonic plate and ground movements.

This led them to focus only on temblors from the past 100 years or so, because that’s as far back as the available data went, Shishikura said.

But in the wake of March 11, a panel under the government’s Central Disaster Management Council in June adopted a new interim report signaling a clear departure from traditional seismology.

The report recommended that the government prepare for a “worst case” earthquake and tsunami combo that could strike in a cycle extending over hundreds or even thousands of years.

“To confirm occurrence of gigantic tsunami over thousands of years, it’s important to strengthen integrated research from not only seismology, but also archaeology, historical science and surveys of tsunami sediment, soil and animal fossils,” the report said.

Prompted by this shift, many seismologists are now using interdisciplinary methods to determine the worst-case quake-tsunami scenario.

Among them, Shishikura and other experts are particularly concerned about gigantic tsunami that could be striking Hokkaido’s east coast about every 400 years.

Far before its Tohoku surveys, his research team at AIST had taken soil layer samples from Hokkaido’s coastline and found traces of multiple tsunami that were probably caused by powerful earthquakes that originated in the Kuril Trench off Kushiro.

The most recent one is believed to have occurred in the 17th century and to have spawned tsunami at least 10 meters high, Shishikura said.

Since about 400 years have passed, the same thing could happen again at any time, he warned.

The eastern coast of Hokkaido has been sinking at a rate of roughly 1 cm a year, an unusually fast pace. This may be a sign of an impending quake.

“The tsumami could be the same size as (those on) March 11,” he said.

Shishikura also believes the March 11 quake changed the distortion balance of the plates near Tohoku to a great extent, possibly increasing the risk of another major temblor occurring in the region.

For example, after the magnitude 9.1 megaquake off Sumatra in 2004 spawned catastrophic tsunami, quakes with magnitudes of 7 or 8 started occurring.

“You can’t deny that the Japanese archipelago could be in a similar situation as well,” he said.

Shishikura, however, also said March 11 might have instilled “some sense of panic” in quake researchers, causing them to focus too much on the worst-case combo scenario.

After all, while researchers can only point out the risk of a potential quake, they can’t predict exactly when it will come, he said.

“So the important thing is to learn about the risks and get prepared for disaster, not worry too much about when it will come,” he added.

In the past year, the public has become hypersensitive to new quake predictions.

The daily Yomiuri Shimbun reported Jan. 23 that the risk of southern Kanto, the region encompassing Tokyo, being hit by a magnitude 7 earthquake in the next four years is as high as 70 percent. This kicked off a string of sensational media reports.

The Yomiuri report was based on an estimate by the University of Tokyo’s Earthquake Research Institute, which immediately played down the significance of its own estimate. The institute said its margin of error was too wide and its calculations were based on data from the multitude of aftershocks that followed the March 11 quake through September.

“The figures themselves do not have much meaning,” the institute said on its website.

But it also urged Tokyoites to brace for a major quake, emphasizing that many experts have said one is long overdue in the capital.

Indeed, the science ministry’s Headquarters for Earthquake Research Promotion is fond of repeating that the probability of a magnitude 7 earthquake hitting southern Kanto is “70 percent within 30 years,” without stating the specific year that period starts from.

Separately, the government’s Central Disaster Management Council has predicted that if a magnitude 7.3 quake hits Tokyo, it will kill up to 11,000 people, destroy 850,000 structures and cause ¥112 trillion in economic damage.

“It’s not just Tokyo. If you live anywhere in Japan, there is the possibility of being hit by an earthquake with a magnitude of around 7,” the research center said.

“As long as you live in Japan, you should prepare for a 7-magnitude quake. That’s the minimum necessary thing you should do.”