A powerful earthquake in northeast Japan rocked a nuclear plant, causing a small amount of radioactive water to spill, but the operator said there was no immediate danger.

Thursday's 7.1-magnitude aftershock resulted in water flowing from containers onto the floor in all three reactor buildings at the Onagawa plant, the Nuclear and Industrial Safety Agency said on Friday.

This comes as three people were confirmed dead and about a hundred were injured following the quake.

Meanwhile authorities reported there was no new damage at the stricken Fukushima Daiichi nuclear power plant, about 100km away from Onagawa.

Thursday's aftershock was the strongest since the devastating quake and tsunami that flattened the country's northeastern coast last month.

"Due to the earthquake, there were a few abnormalities..." said Hidehiko Nishiyama, the Nuclear and Industrial Safety Agency's spokesman.

"As we continue to inspect the plant, I think we will see more of these abnormalities," he said of the radioactive water spill at the Onagawa plant.

Most seriously affected was reactor number two, where 3.8 litres from the spent fuel pool ended up on the floor of the operation room.

"We're currently investigating where the water came from. The radiation levels in the wet areas are far below the level that would require us to report to authorities," said a spokesman for the plant's operator Tohoku-Electric Power Company.

No abnormal readings

The Tokyo Electric Power Company (TEPCO) this week began pumping lower-level radioactive water into the sea from the Fukushima Daiichi plant, to free up urgently needed storage space for water so toxic that it is hampering crucial repair work.

The move drew fire from some, most recently China which on Friday expressed concern and urged its neighbour to take concrete steps to protect the marine environment.

At the Fukushima plant, following the quake, a nuclear safety agency official told reporters: "There are no abnormal readings at the Fukushima Daiichi's monitoring posts."

"Local power plants were designed to sustain 7.9 earthquakes. So this kind of earthquake - a 7.1 or 7.4 - they sustained [it] without damage," Jasmina Vujic, professor of nuclear engineering at the University of California, Berkeley, told Al Jazeera.

"What I suspect is simply that there was splashing of water in spent fuel pools and water in the spent fuel pool is really mildly radioactive so if there were splashes it is inside of the plant. So I don’t see any ways the level of radiation might have increased."

A day earlier, TEPCO had begun pumping nitrogen, an inert gas, into Fukushima's reactor No. 1, where engineers were concerned a build-up of hydrogen might react with oxygen to cause an explosion.

Work at the plant was remotely controlled and had continued uninterrupted, the company said, with a spokesman adding that there was "no information immediately indicating any abnormality at Fukushima Daiichi plant".

But Japan's nuclear safety agency said workers there had retreated to a quake-resistant shelter in the complex. No one there was injured.

Expressing concerns

Shares in TEPCO vaulted 23.52 per cent to close at 420 yen on Friday.

This comes after shares in the utility had hit record closing lows this week on concerns the company will face soaring compensation bills from the crisis at its plant.

Following the aftershock, the Japan meteorological agency briefly issued a tsunami warning following the tremor, but later cancelled it.

Al Jazeera's Harry Fawcett, reporting from Misuzawa, said a 50cm wave was observed, according to local media, but this was not confirmed by officials.

Al Jazeera's Marga Ortigas, also in Mizusawa, said there had been reports of gas leaks and power cuts following the quake.

Officials said Thursday's aftershock hit 50km under the water and off the coast of Miyagi prefecture.

The quake struck at 11:32pm local time and initially measured 7.4 on the Richter Scale, however, the US Geological Survey in Golden, Colorado, later downgraded it to 7.1. Buildings as far away as Tokyo shook for about a minute.

Source: Al Jazeera and agencies