TOKYO - Japan's economy could lose as much as $1.81 trillion in the event of a long-anticipated megaquake off its Pacific coast, which could trigger devastating tsunamis, the collapse of hundreds of buildings and potentially killing about 300,000 people, a government report said on Monday.

The expected economic damage of 270.3 trillion yen, or nearly half of the country's total gross domestic product (GDP), was up sharply from the previous estimate of 214.2 trillion yen as the new estimate accounted for inflationary pressures and updated terrain and ground data which have expanded anticipated flood areas, the Cabinet Office report showed.

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

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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 percent 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 km (600 miles), 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.

Last year, 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.

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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.