The number of noticeable aftershocks related to the Great East Japan Earthquake that rocked northeast Japan in March 2011 had reached 9,577 as of Monday, the Meteorological Agency said.
The tremors are continuing to rattle the country two years on, though with lessening frequency, the agency said.
Yoshimori Honkura, chairman of the government’s Earthquake Research Committee, said it will take several more years before the rate returns to the predisaster levels.
By scale, quakes registering upper and lower 6 on the Japanese seismic intensity scale of 7 were observed twice each, while those in the upper and lower 5 range were logged on 12 and 44 times, respectively. Those rating 4 were recorded on 226 occasions.
By month, 2,941 aftershocks struck in March 2011, the same month as the main shock. That number hovered between slightly over 100 and several hundred from May 2011 to December 2012.
In January this year, it stood at 93, falling short of 100 for the first time since the disaster. It then leveled off in February at 92.
The aftershock zone linked to the March 2011 quake is believed to stretch about 600 km north to south from Aomori to the waters off the Boso Peninsula in Chiba Prefecture, and around 350 km east to west.
In that area, quakes with preliminary reading of 5 or more occurred 84 times a year through last Saturday, or more than four times the annual average of 19 in the same area over the decade before the disaster.
One of the biggest aftershocks so far took place as recently as December and had a preliminary magnitude of 7.3, making it a lower 5 on the Japanese scale. This sent nearly 1 meter-high tsunami toward the coastline of Tohoku.
Quakes believed to have been induced by the main shock also continued to take place outside the aftershock area, like one with a preliminary magnitude of 6.3, or upper 5, in northern Tochigi Prefecture in February.
Quakes outside the aftershock zone are believed to be induced by crustal movements, among other causes.
According to observations by the Geospatial Information Authority of Japan, a measuring spot in Ichinoseki, Iwate Prefecture, moved 1.5 cm eastward within a month to late February, with crustal movements also taking place in wide areas of the northeast.