The quake which struck late Saturday night was preliminarily measured with a magnitude of 7.1 and was later revised up to 7.3, the Japan Meteorological Agency (JMA) said.
The injuries were reported in Miyagi and Fukushima prefectures, Xinhua news agency quoted the authorities as saying.
The temblor occurred at 11.08 p.m., with its epicentre at a latitude of 37.7 degrees north and a longitude of 141.8 degrees east, and at depth of 60 km, according to the JMA.
So far no tsunami warning has been issued.
The quake logged Upper 6 in some parts of Fukushima prefecture on the Japanese seismic
Chief Cabinet Secretary Katsunobu Kato said about 950,000 households were left without electricity following the strong earthquake.
The blackout affected 860,000 homes under the area covered by
Following the quake, the Japanese government set up a task force at the Prime Minister's Office.
Prime Minister Yoshihide Suga instructed the government to quickly survey the damage from the quake, conduct rescue efforts in areas necessary, and to relay information to the public as soon as possible.
The quake was also felt in the capital Tokyo where an intensity scale of 4 was logged, with many reporting that their houses and furniture underwent strong shaking and some saying they felt dizzy because of the quake.
Tokyo Electric Power Company said it has found no abnormalities with the reactors at the Fukushima Daiichi and Daini nuclear power plants after the quake.
Meanwhile, no abnormality has been found at Japan Atomic Power Co.'s inactive Tokai No. 2 nuclear power plant in the village of Tokai in Ibaraki prefecture, according to the operator.
The quake is believed to be an aftershock of the earthquake that triggered a massive tsunami in 2011 in the same area, said Kenji Satake, a professor at the University of Tokyo's Earthquake Research Institute.
"Because (the 2011 quake) was an enormous one with a magnitude of 9.0, it's not surprising to have an aftershock of this scale 10 years later," Satake said.
According to the professor, although the quake was relatively large in scale with the focus off Fukushima prefecture, it was unlikely to cause tsunami because it had a deep epicentre of about 55 km below the sea surface.
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