The Meteorological Agency has stopped using the term “aftershocks” for strong earthquakes occurring in areas around the focus of the Great East Japan Earthquake in March 2011, which devastated Japan’s northeast.
When tremors measuring lower 5, the fifth-highest level on the country’s shindo seismic intensity scale, or higher occurred in areas designated as the 2011 earthquake’s aftershock zone, the agency had noted the possibility that they were aftershocks of the 2011 quake, which had measured up to 7, the highest level on the shindo scale.
The decision to avoid describing such quakes as aftershocks came as it became more difficult to make clear judgments on the matter, with 10 years having passed since the massive jolt, and also because earthquakes with a magnitude of 7 to 8 have repeatedly been occurring along the Japan Trench off the coast of eastern Japan.
But the agency will continue to disclose data on earthquakes occurring in the aftershock zone because the impact of the 9.0-magnitude quake has lasted over 10 years.
“I want people to prepare for possible powerful earthquakes and tsunamis regardless of whether they are aftershocks or not,” an agency official said.
The aftershock zone is a rectangular area measuring some 600 kilometers in a north-south direction and 350 kilometers east-west, covering coastal areas from Aomori Prefecture to Chiba Prefecture, as well as offshore waters east of the Japan Trench.
On Feb. 13 this year, a 7.3-magnitude earthquake struck off Fukushima Prefecture, measuring up to upper 6, the second-highest level on the shindo scale. On March 20, a quake with a magnitude of 6.9, measuring up to upper 5, occurred off Miyagi Prefecture, adjacent to Fukushima.
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