Fears Japan megaquake could kill 300,000 people

Japan's economy could lose as much as $US1.81 ($A2.88) trillion if a long-anticipated megaquake strikes off its Pacific coast, potentially triggering devastating tsunamis and killing an estimated 300,000 people.

A report from Japan's government has put the expected economic damage at 270.3 trillion yen, or nearly half the country's total gross domestic product, up sharply from the previous estimate of 214.2 trillion yen.

The new estimate accounts for inflationary pressures and updated terrain and ground data with expanded anticipated flood areas, the Cabinet Office report shows.

Japan is one of the world's most earthquake-prone countries, and the government sees about an 80 per cent chance of a magnitude 8 to 9 earthquake along a tremulous seabed zone known as the Nankai Trough.

Under the worst-case scenario, based on a potential magnitude 9 earthquake in the area, Japan is likely to see 1.23 million evacuees or 10 per cent of its total population.

As many as 298,000 people could die from tsunamis and building collapses if the quake occurs late at night in winter, the report showed.

The trough is off Japan's southwest Pacific coast and runs for approximately 900 kilometres, where the Philippine Sea Plate is subducting under the Eurasian Plate.

The accumulating tectonic strains could result in a megaquake roughly once in 100 to 150 years.

In 2024, Japan issued its first-ever megaquake advisory that there was a "relatively higher chance" of a quake as powerful as magnitude 9 in the trough, after a magnitude-7.1 quake occurred at the edge of the trough.

A magnitude 9 quake in 2011 that triggered a devastating tsunami and the triple reactor meltdowns at a nuclear power plant in northeast Japan killed more than 15,000 people.