Japan's Meteorological Agency has decided not to use the term "aftershocks" for post-quake tremors near the epicenter to keep people from letting their guard down.
The agency designated a zone 600 kilometers long and 350 kilometers wide off northeastern Japan as an "aftershock area," where seismic activity had increased since a powerful earthquake ten years ago.
But aftershocks have decreased over the years. Monthly averages in the year through March were close to those during the decade before the quake.
Meanwhile, the government's Earthquake Research Committee says there's more than a 90 percent chance of an earthquake with a magnitude of 7 to 7.5 occurring within 30 years along the Japan Trench, which lies off northeastern to eastern Japan.
The Meteorological Agency says it will avoid describing tremors originating off northeastern Japan as "aftershocks," which could give the impression that a huge quake is not imminent.