Big quake off Japan but no tsunami risk

Authorities say there is no threat of a tsunami from a magnitude-7.0 earthquake that has struck off the coast of Japan.

A magnitude 7.0 earthquake has struck off the southern coast of Japan. (AAP)

A magnitude-7.0 earthquake has struck off the southwestern coast of Japan, the US Geological Survey says.

The USGS put the epicentre of the shallow quake about 160km from the town of Makurazaki.

The Pacific Tsunami Warning Center said there was no tsunami threat from the quake, which struck just before 6am local time on Saturday.

Japan's islands are at the juncture of several tectonic plates and experience several relatively violent quakes every year.

But building codes are rigorous and regular disaster drills are held, helping to ensure that despite their frequency and magnitude, quakes usually pass without loss of life or significant damage to property in Japan.

A massive undersea quake that hit in March 2011 sent a tsunami barrelling into Japan's northeast coast.

As well as killing thousands of people and destroying communities, the waves also swamped the cooling systems at the Fukushima nuclear plant, sending three reactors into meltdown.

The nuclear disaster, the world's worst since Chernobyl, displaced tens of thousands of people and rendered tracts of land uninhabitable, possibly for decades.