The warning was later lifted and no tsunami was reported after the quake, which struck shortly before midnight.
No damage from the quake, measured at magnitude 7.4 by the Japan Meteorological Agency, was detected at the Fukushima Daiichi plant, operator Tokyo Electric Power Co (TEPCO) said.
Workers struggling to bring the plant under control were evacuated but returned once the tsunami warning was lifted, a TEPCO official said.
Large parts of northern Japan, whose infrastructure was severely damaged by the March 11 quake and tsunami, were without electricity following the latest of many aftershocks, the biggest since last month’s killer quake.
In the capital, Tokyo, buildings also shook.
“It started out as nothing much, then the building started swaying quite strongly,” a Reuters witness said.
By 1.30am (16.30 GMT) seven people were reported injured, two seriously, a spokesman for the National Police Agency said.
Last month’s 9.0 magnitude quake triggered tsunami waves which swept in along the coast, wiping out towns.
About 28,000 people were killed or are missing.
The disaster disrupted industry and affected supply chains around the world but it was not immediately clear if yesterday’s aftershock would compound those problems.
At the Fukushima nuclear plant, TEPCO said it was continuing to inject nitrogen into reactor No 1 after no irregularities were reported.
Engineers, who sealed a leak this week that had allowed highly radioactive water into the sea, are pumping nitrogen into one reactor to prevent the risk of a hydrogen gas explosion, and want to start the process in another two reactors.
There were no abnormalities in radiation levels around Tohoku Electric’s Onagawa nuclear power plant, where fuel rods are being cooled with just one outside power source, Japan’s nuclear safety agency said.
As well as Fukushima Daiichi and Onagawa, nuclear power plants Higashidori in Aomori prefecture, Tokai No 2 in Ibaraki prefecture, and Fukushima Daini have been out of operation since the March quake.
No abnormalities were reported at those plants after yesterday’s quake, which the meteorological agency said was an aftershock from last month’s quake.
“Due to the (March 11 quake), the risk of landslides or buildings collapsing is higher than usual and there are possibilities of further damage with aftershocks,” deputy chief cabinet secretary Tetsuro Fukuyama told reporters.
Japan’s neighbours have sounded increasingly alarmed over the risk of radiation from the damaged plant 240km north of Tokyo, while tourists are staying away in what should be the peak season, and the country seeks ways to cut power use.
The world’s worst nuclear disaster in 25 years is also raising concern over safety in the United States, which has more atomic reactors than any other country, especially at one plant which is similar to the Fukushima plant damaged last month.
TEPCO said yesterday it did not expect it would have to dump any more contaminated water into the ocean after tomorrow.
Earlier, TEPCO said the chance of a repetition of the gas explosions that damaged two reactors in the first days of the disaster was “extremely small”.
As engineers battle multiple crises — some the result of efforts to try to cool reactors — officials said it could take months to bring the reactors under control and years to clear up the toxic mess left behind.
“Data shows the reactors are in a stable condition, but we are not out of the woods yet,” chief cabinet secretary Yukio Edano said.
The government has already set up a 20km exclusion zone around the plant, banned fishing along much of the north-east coast and set up evacuation centres for the tens of thousands forced to leave their homes following the crisis.
Trace levels of radioactive material had been detected in the air in 22 Chinese provinces but the amounts did not pose a threat to health or the environment, China’s state news agency Xinhua said.
Earlier, China’s Health Ministry said traces of radioactivity in spinach had been found.
In South Korea, some schools were closed because parents were worried that rain could be toxic.
“We’ve sent out an official communication today that schools should try to refrain from outdoor activities,” an education official in South Korea said.
South Korea’s nuclear safety agency reported a low level of radioactive iodine and cesium particles in rain but said it was not enough to be a health concern.
The few schools that closed were expected to reopen today if the rain stopped.
India said a blanket ban on food items imported from Japan was not warranted, although authorities would monitor the situation every week, a source in the Trade Ministry said.
India said earlier in the month that it had imposed a three-month ban on imports of food from Japan on fears that radiation from an earthquake-hit nuclear plant was spreading to other parts of the country.