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Japan weather agency to stop announcing quakes as aftershocks in 2011 disaster area

The Japan Meteorological Agency is seen in Tokyo's Minato Ward. (Mainichi/Shinji Kurokawa)

TOKYO -- The Japan Meteorological Agency (JMA) revealed on April 1 that from 10 a.m. that day it plans not to announce earthquakes as aftershocks even if they occur in the aftershock area of the 2011 Great East Japan Earthquake.

    This is because 10 years have passed since the tremor, and that the number of temblors in the aftershock area is approaching pre-earthquake levels. In the Tohoku region in northeastern Japan, there is a high probability of a large quake striking again, and the JMA plans not to use the term "aftershock" as it could lower people's disaster prevention awareness for a possible next disaster.

    After the Great East Japan Earthquake, the JMA designated an area of about 600 kilometers north to south and around 350 kilometers east to west between off the coast of Aomori and Chiba prefectures as an "aftershock area." Tremors that occurred in this zone have been announced as aftershocks, regardless of the mechanism.

    After the 2011 quake, the aftershock area became seismically active. There was a yearly average of 138 aftershocks with a magnitude of 4 or more in the 10 years before the disaster, but the figure jumped to 5,387 in the year after the Great East Japan Earthquake.

    Yet the number of aftershocks decreased as time passed, and only 212 were recorded between March 11, 2020 and the same day in 2021. This was more than before the Great East Japan Earthquake but was only about 1/25 of the figure recorded in the year after the disaster.

    In the aftershock area, a magnitude 7.3 tremor that registered as high as an upper 6 on the Japanese seismic intensity scale of 7 occurred off the coast of Fukushima Prefecture on Feb. 13 this year, and a magnitude 6.9 temblor that registered as high as an upper 5 took place off the coast of Miyagi Prefecture on March 20 -- both in the aftershock area.

    The JMA had announced both as aftershocks, but recently it has become difficult to determine whether such quakes were caused by the effects of the 2011 earthquake. The JMA decided to review the announcement of aftershocks, also based on this situation.

    According to the long-term evaluation of earthquakes that is expected to occur in the Tohoku region and elsewhere within the next 30 years, released by the government in 2019, the probability of a strong tremor followed by a big tsunami is as high as 30% in the entire area between off the coasts of Aomori and Chiba prefectures.

    A JMA official called for caution, saying, "It does not mean the seismic activity has ended in the aftershock area, so please continue to prepare for major earthquakes and tsunamis."

    (Japanese original by Mayumi Nobuta, Science & Environment News Department)

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