An earthquake with a preliminary magnitude of 6.2 jolted eastern Japan on Wednesday and was felt in Tokyo, but no tsunami warning was issued, the Japan Meteorological Agency said.
The quake, at around 8:44 pm (1144 GMT), shook buildings in the Japanese capital, but there were no reports of injuries or damage.
Runways at Tokyo's Narita international airport were closed for checks right after the quake but soon reopened, while service on some train lines was briefly halted.
The focus of the tremor was 40 km (25 miles) below the sea off Ibaraki prefecture, north-east of Tokyo, the agency said.
"It was a slow but powerful shake. It was shaking for a while, but not so badly that objects fell off the shelves," Toru Saito, an Ibaraki prefectural government official, told state broadcaster NHK.
About 600 households experienced a brief power outage. "The building was shaken from side to side, but nothing fell," Hiroshi Okasato of the fire department in Rokkou town, Ibaraki, told NHK.
An experimental nuclear reactor in Ibaraki shut down automatically and safely, but there were no problems at any nuclear power plants in the area, a prefectural official said. There were also no reports of any damage at oil refineries and facilities, the official said.
The magnitude of the earthquake was measured according to a technique similar to the Richter scale, but adjusted for Japan's geological characteristics.