Changes around the Kuril Trench are similar to those observed before the 2011 Great East Japan Earthquake, and a government panel is warning that another tsunami-spawning megaquake could devastate eastern Hokkaido.

Scientists failed to warn against the magnitude-9.0 earthquake and tsunami that struck on March 11, 2011, and killed around 18,000 people on the coast of the northeastern Tohoku region.

With indications that the next “big one” is imminent, they are determined not to make the same mistake.

That sentiment was expressed when Ritsuko Matsuura, who heads the analysis division of the public-interest entity Association for the Development of Earthquake Prediction, addressed a gathering of seismologists in August.

“You should go as far as possible to do thorough research” on the next powerful earthquake striking in the Kuril Trench, she said at a meeting of the government’s Coordinating Committee for Earthquake Prediction. “You cannot say that you have actually identified signs only after the event occurs.”

After the Great East Japan Earthquake struck, a flurry of reports emerged noting changes in seismic activity off the Tohoku region that experts said could have been a “predictor” of the disaster.

The scientists knew that the Tohoku region had been flooded by great tsunami in the distant past. But they could not issue the warning in a timely manner to mitigate the impact of the towering waves that hit the coast eight years ago.

Now, with similar changes in earthquake data from the Kuril Trench running northeast of Hokkaido to the south of the Kamchatka Peninsula in Russia, Matsuura and other scientists are taking no chances.

Many scientists agree that an unusually powerful earthquake could occur in the Kuril Trench in the near future.

The long-term assessment of future earthquakes released in December 2017 by the Headquarters for Earthquake Research Promotion, an expert panel within the science ministry, said there was a “high” likelihood of a magnitude-8.8 or larger quake striking the trench in the near future.

The last known quake of that scale occurred in the first half of the 17th century.

The average interval between such megaquakes is estimated at 340-380 years, which was calculated based on a study of tsunami deposits in eastern Hokkaido and elsewhere.

That means the Kuril Trench is overdue for a huge quake.

A study of earthquakes estimated at magnitude-5.7 or larger since 1965 showed that the occurrence of such temblors had dwindled since around October 2008. But it has been increasing since the latter half of 2015.

A similar quiescence trend was also reported before the Great East Japan Earthquake and other major quakes.

However, a scientific correlation between a decline in seismic activity and a subsequent huge earthquake has not been established. There are also cases in which a major temblor does not follow a “quiet” period.

Experts have also cited crustal movements on the coast of Hokkaido as a sign that the next big earthquake is on the way.

In eastern Hokkaido, the land sank after earthquakes of magnitude-8.0 or larger, including the Tokachioki earthquake of 2003.

But historical traces of the coastline, including sand and other deposits, have been found at elevated areas of the region.

To explain the existence of the coastline on higher ground, scientists hypothesized that unusually large earthquakes caused the land to rise considerably.

Some seismologists say the fast pace of land sinking observed in recent years could be a precursor of an enormous earthquake in the Kuril Trench.

In fact, land in the Tohoku region had been sinking until the Great East Japan Earthquake caused it to rise.

“(Eastern Hokkaido) is in a situation resembling the Tohoku region before the Great East Japan Earthquake,” said Masanobu Shishikura, an expert on tsunami deposits who heads a group of researchers at the government-affiliated National Institute of Advanced Industrial Service and Technology. “We should be fully prepared.”

The expected focal point of the megaquake stretches more than 300 kilometers from the coast of the Tokachi region in eastern Hokkaido to waters northeast of the South Kuril Islands.

A tsunami triggered by the powerful temblor that occurred in the first half of the 17th century in the trench had inundated Hokkaido up to 4 km inland, according to a study.

A similar tsunami could also swamp the Tohoku region, according to some seismologists.

The central government’s Central Disaster Management Council is now reviewing its projected damage assessments for powerful earthquakes along the Kuril Trench and the Japan Trench to the south.

The Hokkaido government is expected to update its anti-disaster measures based on the council’s latest finding.

Quake experts admit they have no clue when an unusually big quake will strike, but they are taking a “better safe than sorry” approach with the expected temblor in the Kuril Trench.

“We should convey to the public our sense of alarm,” Kazuro Hirahara, professor emeritus at Kyoto University, said at a news conference.

Hirahara chairs the Coordinating Committee for Earthquake Prediction, which met twice last year to assess how imminent the temblor is off Hokkaido.

Matsuura called for people living on the coast to immediately flee to higher ground whenever they feel a big jolt.

Tokyo Electric Power Co., which is still battling the tsunami-triggered disaster at its Fukushima No. 1 nuclear plant, plans to install a levee by fiscal 2020 to protect the nuclear complex from another towering tsunami.

TEPCO announced in December that the new levee is designed for a scenario in which a magnitude-9.4 earthquake hits in a focal area stretching along a 1,400-kilometer portion of the Kuril Trench.

The structure will stand at a height of 11 meters above the sea surface and 2.5 meters from the ground of the plant’s core facilities.

The plant sat about 10 meters above sea level before the Great East Japan Earthquake struck.

After the Fukushima disaster unfolded, the company installed a makeshift levee in the southern section of the ground of the plant to prepare for anticipated tsunami caused by aftershocks of the Great East Japan Earthquake.

But no barrier was erected around the nuclear plant itself.

The utility fears that if the plant is hit by another tsunami like the one in 2011, the event would cause leaks of highly radioactive water stored in rows of tanks on the grounds of the plant, resulting in a major delay in decommissioning work.

“We decided to build the barrier based on the long-term assessment” by the science ministry’s Headquarters for Earthquake Research Promotion, TEPCO said.

But the utility did not take action for the same panel’s assessment released in 2002, and the Fukushima No. 1 plant was swamped by the towering tsunami that led to the triple meltdown.