A tsunami warning for Japan's east coast has been downgraded following a magnitude 6.9 earthquake that struck off the coast of Fukushima.
A powerful earthquake rocked northern Japan on Tuesday, briefly disrupting cooling functions at a nuclear plant and generating a tsunami that hit the same region devastated by a massive quake, tsunami and nuclear disaster in 2011.
The earthquake, which was felt in Tokyo, had a magnitude of 7.4, the Japan Meteorological Service said, and was centred off the coast of Fukushima prefecture at a depth of about 10 kilometres.
There were no reports of deaths or serious injuries several hours after the quake hit at 5.59 am (2059 GMT Monday).
A tsunami of up to 1.4 metres had been observed around Sendai, about 70 km (45 miles) north of Fukushima, with smaller waves hitting ports elsewhere along the coast, public broadcaster NHK said.
Television footage showed ships moving out to sea from harbours as tsunami warning signals wailed, after warnings of waves of up to 3 metres were issued.
"We saw high waves but nothing that went over the tidal barriers," a man in the city of Iwaki told NTV television network.
The Pacific Tsunami Warning Center in Hawaii said the tsunami threat had now largely passed.
"Sea level fluctuations may continue along some coasts of Japan over the next few hours," it said.
The US Geological Survey measured Tuesday's quake at magnitude 6.9, down from an initial 7.3.
The Japan Meteorological Agency issued a tsunami warning for waves of up to 3 metres in Fukushima, and a tsunami advisory for much of the rest of northeast Japan's Pacific coast.
The Japan Times reported a 1.4-metre tsunami had been observed at Sendai Port, the largest since March 11, 2011.
Tsunamis of up to 90 centimetres were recorded about an hour after the 6am earthquake in the area which is home to the nuclear power plant that was destroyed by a huge tsunami following an offshore earthquake in 2011.
All Japan's nuclear power plants on the coast threatened by the tsunami are shut down in the wake of the March 2011 disaster, which knocked out Tokyo Electric Power Co's Fukushima Daiichi nuclear plant, spilling radiation into the air and sea.
A spokeswoman for Tokyo Electric Power, known as Tepco, said the cooling system for a storage pool for spent nuclear fuel at the reactor at its Fukushima Daini Plant had been halted. A spokesman said the cooling system had restarted soon after.
No other damage from the quake has been confirmed at any of its power plants, although there have been blackouts in some areas, the spokeswoman said.
Japanese residents are being reminded of the smaller waves that preceded the tsunami that destroyed the Fukushima nuclear plant in 2011.
There were no immediate reports of damage or injury, which struck at 5.59 am (2059 GMT).
Only two reactors are operating in Japan, both in the southwest. Even when in shutdown, nuclear plants need cooling systems operating to keep spent fuel cool.
Tohoku Electric Power Co said there was no damage to its Onagawa nuclear plant, while the Kyodo news agency reported there were no irregularities at the Tokai Daini nuclear plant in Ibaraki prefecture.
SBS News journalist James Elton-Pym is in Tokyo and is monitoring the aftermath of the earthquake.
He said the Japanese Meteorological Agency had said there was a risk the height of tsunami waves would continue to rise, but the agency had given a timely evacuation order throughout Fukushima.
The US Geological Survey measured the magnitude at 6.9 as tremors were felt in buildings 240 kilometres southwest of the epicentre in Tokyo.
One woman suffered cuts to her head from falling dishes, Kyodo news agency reported, citing fire department officials.
Japanese Minister for Disaster Management Jun Matsumoto told reporters about three hours after the quake that there had been no reports of significant injuries so far.
NHK showed footage of residents of Ishinomaki, a city badly hit in 2011, standing on a hill dressed in hats and heavy coats, staring down at the ocean. Several thousand people along the coast evacuated or were told to evacuate.
Earthquakes are common in Japan, one of the world's most seismically active areas. Japan accounts for about 20 per cent of the world's earthquakes of magnitude 6 or greater.
The March 11, 2011, quake was magnitude 9, the strongest quake in Japan on record. The massive tsunami it triggered caused the world's worst nuclear crisis since Chernobyl a quarter of a century earlier.
Associate Professor Mark Quigley, who specialises in earthquake science at Melbourne University, told SBS News Tuesday's quake could be related to the devastating 2011 earthquake and tsunami.
"I think they’re certainly related in the sense that that magnitude 9 earthquake would have shifted [the earth] around quite a lot in that particular area, so they are potentially related," he said.
However, he said it could not be confirmed whether it was related to the November 19 earthquake that severely damaged infrastructure on New Zealand's south island.
"They’re not large enough to have a real, strong causal linkage but we know that when earthquakes of this size do occur they send waves out around the planet and it could be that there just enough to trigger a fault that’s close to breaking and it might be just enough to push it over the edge," he said.
Nissan Motor Co said it would suspend work at its engine factory in Fukushima at least until the latest tsunami warning was lifted. A spokesman said there were no injuries or damage at the plant, which was badly damaged in the 2011 disaster.
Separately, Toyota Motor Corp said all its factories in northeastern Japan were operating as usual.
An Iwaki city fire department official said there was smoke or fire at Kureha's research centre in a petrochemical complex in Iwaki city at 6.17 am (2117 GMT Monday) but it was extinguished soon after. Other details were not clear, he said, but no other major damage had been reported in the city so far.
Japan's famous Shinkansen bullet trains were halted along one stretch of track and some other train lines were also stopped.
One hotel in Ofunato, also badly hit by the 2011 quake, initially told guests to stay in the facility but later bussed them to higher ground.
Japanese financial markets were little affected, with Nikkei futures recovering after a brief fall and the yen up a touch against the US dollar, although still near a five-month low hit earlier in the session.
- with Reuters