Japan’s government has warned of a heightened risk of a “megaquake” after an earthquake with a magnitude of 7.5 struck off the nation’s north-east coast.

Residents were urged to stay away from coastal areas where tsunami waves of ‌up to three metres were expected on Monday afternoon.

Two hours after the tremor, which struck at 4.52pm (0752 GMT), tsunami waves as high as 80 centimetres had been detected.

A tsunami warning was later downgraded to a tsunami advisory.

There were no immediate reports of ‌casualties or major damage, ‌Japan’s top ⁠government spokesman Minoru Kihara said as night fell in ​the capital Tokyo.

Several ⁠port towns, including Otsuchi and Kamaishi — both hard-hit by a massive earthquake and tsunami ​in 2011 — earlier issued evacuation orders for thousands of residents, according to public broadcaster NHK.

Bullet train services were halted and some motorways closed due to the tremors.

The government urged people to take steps to “protect one’s own life” amid a heightened risk of a follow-up “mega-quake”.

Normally, the probability of an earthquake of magnitude 8.0 or stronger striking along the Japan Trench and Kuril Trench in the Pacific off northern Japan in a week is ⁠about 0.1 per cent.

But for the week that followed Monday’s quake, it would ‌be higher, at about 1.0 per cent, a government official said.

“Please take anti-disaster steps, while embracing the idea that one must protect ​one’s own ‌life,” the official said.

The ​quake measured an “upper five” on Japan’s seismic intensity scale — strong enough to make ​it difficult for ‌people to move around and cause unreinforced concrete-block walls to collapse.

The tremor had an epicentre in the Pacific Ocean and ‌was 20 kilometres deep, the Japan Meteorological Agency said.

A three-metre ‌tsunami could cause damage to low-lying areas, flooding buildings, and anybody exposed would be caught in its currents, the agency said.

Located in the “Ring of Fire” of volcanoes and oceanic ​trenches ​partly encircling the Pacific Basin, Japan is one ​of the world’s most earthquake-prone countries, with a tremor at least every five minutes.

It ​accounts for about 20 per cent of the world’s ‌earthquakes of magnitude 6.0 or more. Among them are the 2011 disaster that caused nuclear meltdowns at a Fukushima power ​plant.

There are no nuclear power plants in operation in the affected areas and Hokkaido Electric Power Co and Tohoku ​Electric Power Co said no abnormalities had been reported at idled facilities there.

-with AAP