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High probability of magnitude-7.0 earthquake off Tohoku coast

  • February 27, 2019
  • , The Asahi Shimbun , 3:05 p.m.
  • English Press

Given that the Great East Japan Earthquake struck just eight years ago, the probability of another temblor of that scale hitting the Tohoku region anytime soon is close to nil.


But that still leaves room for earthquakes of magnitude-7.0 or greater jolting the northeastern part of the main Honshu island, according to the latest assessment by the Headquarters for Earthquake Research Promotion.


The Feb. 26 announcement covered probabilities for undersea quakes striking within 30 years along the Japan Trench that stretches east of Honshu from the northernmost prefecture of Aomori all the way south to off the Boso coast of Chiba Prefecture.


The last time similar probabilities for a quake in those areas was released was in November 2011.


The new assessment was based on subsequent seismic activity, crust movements and data from sediment produced by past tsunami.


The panel said the probability of another magnitude-9.0 temblor like the Great East Japan Earthquake striking the region was “close to zero.”


That earthquake, which hit on March 11, 2011, affected a wide area extending from the southern part of coastal Iwate Prefecture to off the coast of Ibaraki Prefecture, producing towering tsunami that devastated the Tohoku region and claimed close to 16,000 lives. Those two events also triggered the disaster at the Fukushima No. 1 nuclear power plant.


The latest assessment said there was a 90 percent probability of a magnitude-7.0 scale earthquake striking off the coast of Miyagi Prefecture. Temblors of that scale could still cause considerable damage, and experts urged no let-up in caution.


With regard to the probability of earthquakes of magnitude-7.0 or greater, the area off the coast of eastern Aomori Prefecture to the northern part of Iwate Prefecture was considered at high risk. That region was said to have a probability of 90 percent or greater. It said there was an 80 percent probability that a magnitude-7.0 or greater quake would hit off the coast of Ibaraki Prefecture.


The area off the Miyagi coast closer to the shore was given a probability of 50 percent. In the past, scientists said they were unsure of the probability for that area. They noted that recent observations of crust movements led them to believe the region had entered the next cycle for a possible quake.


Past quakes in the region that had magnitudes of at least 7.0 triggered tsunami that were often dozens of centimeters in height. While such tsunami are much smaller than those triggered by quakes of magnitude-9.0, which can rise to more than 10 meters, or of magnitude-8.0, which can lead to tsunami of several meters, there is still the possibility that people along the coast could be swept away.


In 1978, an undersea earthquake of magnitude 7.4 struck off Miyagi Prefecture, causing the collapse of concrete walls and other damage that killed 28 people. It led to a review of safety standards for such concrete block walls.


Naoshi Hirata, chairman of the headquarters’ Earthquake Research Committee, said, “There is a high probability of a magnitude-8.0 or -7.0 quake hitting the Pacific coast of the Tohoku region. People need to prepare for strong shaking or tsunami.”

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