An earthquake measuring 7.4 on the Richter scale hit the sea near Miyagi Prefecture in northeastern Japan at 23:32 p.m. (1432 GMT) local Time Thursday, according to the Japan Meteorological Agency.
Tsunami warnings have been lifted so far, the agency said.
The focus of the quake was located some 40 km under the sea off the northeastern prefecture ,said the agency.
Xinhua's office building in Tokyo swayed for a while when the quake occurred.
The quake has not brought any major problems to the nuclear power plants in Fukushima Prefecture so far, according to Tokyo Electric Power Co.
The weather agency considered the quake as the aftershock of the devastating March 11 quake.
According to Kyodo news reports, blackouts were seen all over Aomori, Iwate and Akita prefectures as well as several parts of Miyagi and Yamagata
prefectures, according to the nuclear regulator.
Nuclear power stations in Ibaraki Prefecture were operating normally after the quake, while at the Onagawa nuclear plant in Miyagi Prefecture, which
has been suspended, two external power supply units among three have failed, according to the nuclear regulator.
Tohoku Electric Power Co. said, meanwhile, external power supply was disrupted at the No. 1 reactor of the Higashidori nuclear station. The
emergency generator is being used to cool the spent fuel pool.
In Rokkasho, Aomori Prefecture, external power supply was disrupted at the spent nuclear fuel reprocessing plant of Japan Nuclear Fuel Ltd. The
emergency generator is being used there, Kyodo said.
In Miyagi Prefecture, expressways were closed due to the quake.
The crisis at the Fukushima Daiichi power station started after the March 11 killer earthquake and tsunami. Strong aftershocks continued after the quake.
A 9.0-magnitude earthquake shook the Pacific coastal areas of northeastern and eastern Japan on March 11, triggering enormous tsunami.
According to the National Police Agency, the twin disasters had left 12,468 people dead and 15,091 others unaccounted for in Japan by 10: 00 a.m.
Wednesday.
The Japanese authorities have been stepping up efforts to fix the troubled Fukushima nuclear power plant crippled by the disasters.