Japan's 'megaquake' warning explained

TOKYO: Japan's earthquake scientists say the country should prepare for a possible «megaquake» one day 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 2011 earthquake, tsunami and nuclear disaster killed around 18,500 people.

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.

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 800km undersea trough runs from Shizuoka, west of Tokyo, to the southern tip of Kyushu island.

It has been the site of destructive quakes of magnitude eight or nine every century or two.

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 has a roughly 70 per cent probability of striking within the next 30 years.

In the worst-case scenario 300,000 lives