7.4 Japan earthquake: What we know so far

Residents were today urged to "evacuate immediately" after a 7.4-magnitude earthquake struck off Fukushima prefecture in Japan, sparking a tsunami warning.

Warnings were issued for waves of up to 3m. But the Pacific Tsunami Warning Centre says the tsunami threat has now passed.

The centre however said there was still the chance of fluctuations in sea levels along some coasts and people should remain alert.

Officials in Japan, who are keeping alerts in place, say the danger is not over and sea levels are still rising.

The cooling system of the number 3 reactor at the Fukushima power plant was temporarily shut down but has been restored and the reactor temperature is under control, according to Japanese broadcaster NHK.

The magnitude of the quake, which struck at 6am Japan time, was initially put at 7.3, before being revised down to 6.9 and then up again to 7.4 - at a depth of 25km.

NHK earlier warned that the tsunami had been observed 20km offshore near the southern city of Iwaki.

Tsunami waves ranging in height from 30cm to 1.4m have been observed in areas along the Fukushima coast.

Residents in coastal areas were urged to flee to higher ground or tall buildings.

Long lines of cars, with their headlights on, formed as coastal residents followed the government orders.

The earthquake shook buildings in Tokyo, 240 kilometres southwest of the epicentre.

Fukushima prefecture is home to the nuclear power plant that was destroyed by a huge tsunami following an offshore earthquake in 2011 that killed about 18,000 people.

Fukushima prefecture is north of Tokyo and home to the nuclear power plant that was destroyed by a huge tsunami following an offshore earthquake in 2011.

Tokyo Electric Power has checked its nuclear plants in Fukushima for damage, public broadcaster NHK said.

All nuclear plants on the coast threatened by the tsunami were shut down in the wake of the Fukushima disaster in 2011.

Only two reactors are now operating in Japan, both in the southwest of the country.

Television footage showed ships moving out to sea from Fukushima harbours, where Tepco's Daiichi nuclear plant was devastated in a March 2011 quake and tsunami.

Nuclear plants have been taken offline as a precaution.

There have been no changes to the nuclear radiation levels in Fukushima following the earthquake, according to NHK World News.

There is no tsunami threat to New Zealand, Civil Defence reported.

Victoria University earthquake scientist Dr John Townend says Kiwis shouldn't draw any connections between the Japan quake and last Monday's 7.8 Kaikoura quake.

"The basic explanation of the earthquake in Japan is they had a magnitude 9.0 quake in 2011 and a magnitude 9 generates large aftershocks and an extended aftershock sequence," he said.

"So we are just seeing that ongoing activity in that part of the world."

Fomer All Black Cory Jane was one of the first to tweet about the quake: "Early wake up call to a 7.3 earthquake here in Japan. Room got to wiggling. Hope everyone is safe."

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 in Fukushima was magnitude 9, the strongest quake in Japan on record.

The massive tsunami it triggered caused world's worst nuclear crisis since Chernobyl a quarter of a century earlier.

- News.com.au, staff reporters