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The estimated death toll in the event of a massive earthquake in the Nankai Trough was lowered to about 298,000 from the previous estimate of up to 323,000 a decade ago in a report released Monday by a working group of the government’s Central Disaster Management Council. On the other hand, the estimate of an area predicted to be flooded by an ensuing tsunami was 30% larger than in the previous estimate.
The report also estimated between 26,000 and 52,000 people would die indirectly from the disaster based on its initial calculation on disaster-related deaths. The economic damage would be about ¥292 trillion.
A massive earthquake of magnitude 8 or 9 has been predicted along the Nankai Trough, which extends from off Shizuoka Prefecture to Miyazaki Prefecture. A 7.9-magnitude earthquake occurred on the eastern side of the trough in 1944, and an 8.0-magnitude quake occurred on the western side in 1946.
In January 2025, the government’s Earthquake Research Committee raised the probability of a magnitude 8 or 9 megaquake occurring in the Nankai Trough within the next 30 years from “70% to 80%” to “around 80%.”
The Central Disaster Management Council has envisaged the largest possible earthquake and tsunami that could occur along the trough and compiled its first report in 2012-13 after the 2011 Great East Japan Earthquake. Since then, the council refined topographical data to revise its initial estimates by reflecting the current state of earthquake-proofing houses, tsunami evacuation facilities and coastal levees.
As a result, an area to be inundated by a tsunami of 3 meters high or more — which would carry people away, endangering their lives — reaching the coast from Fukushima and Okinawa prefectures would be 115,150 hectares, up 30% from the previous estimate. The latest report estimated the highest tsunami of 34 meters would hit Kuroshio and Tosashimizu in Kochi Prefecture.
The death toll of 298,000 is estimated in a situation where a tsunami occurs late at night in winter when many people are at home, with 20% of them being able to evacuate immediately from the tsunami, inflicting large damage in the heavily populated Tokai region. The report estimated 215,000 would die from a tsunami, or 70% of the estimated death toll.
Up to 12.3 million would evacuate, 950,000 would be injured, and 2.35 million houses — about the same from the previous 2.38 million — would be destroyed according to the latest report.
The economic damage was estimated to be ¥292 trillion, a significant increase from the previous ¥237 trillion, due to higher prices. The breakdown was ¥225 trillion for assets and ¥67 trillion for the impact on economic activity.
Based on past quake trends, the report for the first time calculated the possibility of a coseismic slip, where the eastern and western parts of the expected focal area experience an 8.0-magnitude earthquake separately with a time lag. This could kill 176,000 people.
The report also pointed out the aging and declining population “could make the situation even more severe,” and concluded, “All entities must mobilize their full resources to tackle disaster prevention.”
Based on this report, the government will review its 2014 Basic Plan that includes disaster mitigation targets by this summer.
