Japanese scientists say the country should prepare for a possible 'megaquake' that could kill hundreds of thousands of people—although they stress the warning does not mean a colossal tremor is imminent.

The Japan Meteorological Association (JMA) warning is the first issued under new rules drawn up after a major earthquake.

The JMA's 'megaquake advisory' warns that 'if a major earthquake were to occur in the future, strong shaking and large tsunamis would be generated'.

'The likelihood of a new major earthquake is higher than normal, but this is not an indication that a major earthquake will definitely occur during a specific period of time,' it added.

Long after rescue teams gave up searching for survivors from the March 2011 tsunami, farmer Norio Kimura continued to painstakingly dig for his seven-year-old daughter, Yuna.

The advisory concerns the Nankai Trough subduction zone between two tectonic plates in the Pacific Ocean, where massive earthquakes have hit in the past.

The 800-kilometre undersea trough runs from Shizuoka—west of Tokyo—to the southern tip of Kyushu island.

These so-called 'megathrust quakes', which often occur in pairs, have been known to unleash dangerous tsunamis along Japan's southern coast.

In 1707, all segments of the Nankai Trough ruptured at once, unleashing an earthquake that remains the nation's second-most powerful on record.

That quake—which also triggered the last eruption of Mount Fuji—was followed by two powerful Nankai megathrusts in 1854, and then a pair in 1944 and 1946.

Japan's government has previously said the next magnitude 8-9 megaquake along the Nankai Trough had a roughly 70 percent probability of striking.

In the worst-case scenario, experts estimate, with some engineers saying the damage could reach $US13 trillion, with infrastructure wiped out.

'The history of great earthquakes at Nankai is convincingly scary,' geologists Kyle Bradley and Judith A.

Hubbard wrote in their Earthquake Insights newsletter.

And 'while earthquake prediction is impossible, the occurrence of one earthquake usually does raise the likelihood of another', they explained.

'A future great Nankai earthquake is surely the most long-anticipated earthquake in history—it is the original definition of the 'Big One'.

'Japan is reminding people living in quake zones to take general precautions, from securing furniture to knowing the location of their nearest evacuation shelter.

Many households in the country also keep a disaster kit handy with bottled water, long-life food, a torch, radio and other practical items.

Professor Meghan Miller, a seismologist at Australia National University, says Japan has 'one of the best' earthquake monitoring systems in the world.

'They have fairly densely spaced instruments including seismometers that record the ground shaking and can tell us more about why faults rupture, where they rupture,' she told ABC News Channel.

There is only a 'small probability' that Thursday's magnitude 7.

1 earthquake is a foreshock of a megaquake, according to Mr.

Bradley and Ms.

Hubbard.

'One of the challenges is that even when the risk of a second earthquake is elevated, it is still always low,' they said.

'For instance, in California the rule of thumb is that any given earthquake has around 5 percent chance of being a foreshock.

'We acknowledge Aboriginal and Torres Strait Islander peoples as the First Australians and Traditional Custodians of the lands where we live, learn, and work.

This service may include material from Agence France-Presse (AFP), APTN, Reuters, AAP, CNN and the BBC World Service which is copyright and cannot be reproduced.