An earthquake with a preliminary magnitude of 6.4 shook the northeast Tuesday night, centering on Iwate and Miyagi prefectures, the Meteorological Agency said.
No tsunami warning was issued.
The temblor, which struck at 8 p.m., measured lower 5 on the Japanese intensity scale of 7 in a wide area of Iwate and northern Miyagi. The quake's focus was in the Pacific off Iwate at a depth of 10 km.
The agency said the nature of the temblor indicates it was an aftershock of the magnitude 9.0 earthquake that hit the region March 11, 2011.
Tohoku Electric Power Co. said the quake did not cause any abnormalities at its nuclear power plants in Onagawa, Miyagi Prefecture, and Higashidori, Aomori Prefecture, while Japan Nuclear Fuel Ltd. said it detected no trouble at its plant for reprocessing spent nuclear fuel in Rokkasho, Aomori Prefecture.
The temblor did not cause any new problems at the Fukushima No. 1 power plant, Tokyo Electric Power Co. said.