Over 12,000 aftershocks have occurred in the coastal areas of eastern Japan since a powerful earthquake struck the region in March 2011, the Meteorological Agency said Tuesday.

Quakes that are considered aftershocks of the magnitude-9.0 temblor are happening less often, but their frequency is still at double the level of quakes before the big one five years ago and is expected to remain so for some time, according to the agency.

As of Sunday, 12,077 quakes that could be felt by people had occurred in coastal areas stretching from Chiba Prefecture 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, temblors with a magnitude of 7 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 2011 quake.

After the nation was hit by the powerful quake, 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.