A long-anticipated mega-earthquake off the Pacific coast could cost Japan’s economy about $1.81 trillion, triggering devastating tsunamis, collapsing hundreds of buildings and killing as many as 300,000 people, a government report said on Monday.

The Cabinet Office report showed that the expected economic damage of 270.3 trillion yen, or about half of the country’s total gross domestic product (GDP), has risen sharply from a previous estimate of 214.2 trillion yen, as the new estimate expands the expected flood areas due to inflationary pressures and updated land and terrain data.

Japan is one of the world’s most earthquake-prone countries, and the government sees an 80% chance of a magnitude 8 to 9 earthquake along a seismically active seafloor region known as the Nankai Trench.

Based on a possible magnitude 9 earthquake in the area, under the worst-case scenario, Japan could evacuate 1.23 million people, or 1% of its total population. The report said that if the earthquake were to strike at night in winter, up to 298,000 people could die from tsunamis and building collapses.

The trench is located off Japan’s southwestern Pacific coast and runs for approximately 900 kilometers (600 miles) along the Philippine Sea Plate, where it is subducted under the Eurasian Plate. The cumulative seismicity could cause a megaquake roughly once every 100 to 150 years.

Last year, Japan issued its first mega-earthquake advisory, saying there was a “relatively high chance” of a magnitude 9 earthquake at the edge of the trench, after a magnitude 7.1 earthquake struck the trench.

A magnitude 9 earthquake in 2011 triggered a devastating tsunami and a meltdown of three reactors at a nuclear power plant in northeastern Japan, killing more than 15,000 people.