Tokyo, March 9 (IANS): Over 12,000 aftershocks were felt in the coastal areas of Japan since a powerful earthquake struck the region in March 2011, the Meteorological Agency said.

Quakes that are considered aftershocks of the magnitude 9.0 temblor are becoming less frequent but their frequency is still at double the level before the great quake five years ago and is expected to remain so for some time, Xinhua cited the agency as saying on Tuesday.

As of Sunday, 12,077 quakes that could be felt by humans had occurred in coastal areas stretching from Chiba to Aomori prefecture and farther offshore.

During the past year, 615 quakes occurred in the areas, compared with 306 on average between 2001 and 2010. During the first year since the powerful quake, 8,112 temblors occurred, followed by 1,583 quakes in the second year, 1,023 quakes in the third year and 744 quakes in the fourth year.

Many of the quakes that still hit have originated in coastal areas. Occasionally, however, a quake with a magnitude of seven or greater hit farther offshore.

The agency has not detected a significant change in the number of quakes originating on land before and after the March 11, 2011, quake.

After Japan was hit by the powerful quake in 2011, the agency designated as an aftershock zone an offshore area in eastern Japan stretching about 600 km from north to south and around 350 km from east to west.