JMA will stop calling temblors an aftershock of 3/11 quake

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Ten years after the Great East Japan Earthquake, the Japan Meteorological Agency will no longer describe a quake occurring off Tohoku as “likely an aftershock,” starting from April 1.

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That’s because the frequency of such quakes occurring has declined over the years and reached the level of the annual average recorded before the massive 2011 quake.

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“It is now difficult to clearly determine if a quake is an aftershock of the 2011 one or not,” the JMA said.

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After the Great East Japan Earthquake, the JMA designated a rectangular-shaped region stretching from Tohoku to the offing of Chiba Prefecture as an “aftershock area.”

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All temblors in the lower 5 or greater on the Japanese seismic intensity scale of 7 occurring within the region were called a possible aftershock of the 2011 quake.

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In general, it is difficult to scientifically determine if an earthquake is an aftershock. But the JMA decided to continue using the description as a means of warning the public.

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However, as it marked the 10th anniversary of the disaster in March, the agency decided to stop using the description. 

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From now on, the Headquarters for Earthquake Research Promotion, set up by the central government, will alert the public based on a long-term evaluation that covers seismic probability and estimated scale in each coastal area.

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