231 strong aftershocks recorded after March 11

As of Wednesday, 231 aftershocks measuring 4 or higher on the Japanese seismic scale of 7 had occurred since the Great East Japan Earthquake, the Japan Meteorological Agency said.

According to the agency's announcement on Thursday, the number is about 24 times the recorded average for the area, which is estimated at about 9.8 earthquakes per year for the decade through 2010.

The aftershock zone covers an area measuring about 600 kilometres north to south from the coast of Iwate Prefecture to Ibaraki Prefecture and about 350 kilometres from east to west. The agency has defined earthquakes with a focus in this region as aftershocks of the March 11 earthquake. It also warned that aftershocks measuring lower 5 or stronger may occur in the area in the future.

In the wake of the disaster, there were 109 aftershocks with an intensity of 4 or higher last March, followed by 50 such aftershocks in April, and six last month.

Although the frequency of aftershocks is on the decline, seismic activity is still occurring in the area. An aftershock measuring lower 5 with a focus off Ibaraki Prefecture occurred on March 1.

In the period from March 11 to Wednesday, aftershocks with an intensity of 1 or higher occurred 7,225 times in the area. These aftershocks accounted for about 70 per cent of the 10,120 earthquakes measuring 1 or stronger during the same period nationwide.

Outside of the aftershock zone, 18 earthquakes measuring lower 5 or stronger were recorded nationwide, with focuses in Shizuoka, Nagano, Akita prefectures and other locations.

The agency believes seismic activities have become more frequent across the country.

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