TOKYO — More than a year after Japan issued its first Nankai Trough megaquake advisory, warning that an offshore magnitude 7.1 tremor had briefly pushed the weekly risk of a catastrophic quake to about 1 percent, officials and researchers say the country is still struggling to live with a danger that cannot be predicted but cannot be ignored. The updated assessments show how a low-probability disaster could overwhelm coastal communities and the national economy if it strikes on Dec. 10, 2025.
Advisory revives fears of a Nankai Trough megaquake.
On Aug. 8, 2024, a magnitude 7.1 quake off Miyazaki in southwestern Japan triggered the new “Nankai Trough Earthquake Extra Information” bulletin, the first time the Japan Meteorological Agency (JMA) had activated its megaquake protocol. Officials stressed that the alert was not a prediction. Still, it raised their estimated chance of a major Nankai Trough rupture in the following week from roughly 0.1 percent to about 1 percent. It urged residents in 29 prefectures to review evacuation plans, secure furniture, and consider voluntary evacuation for children, older people, and people with disabilities.
Trains on the Tokaido Shinkansen slowed, Prime Minister Fumio Kishida scrapped an overseas trip, and supermarkets rationed bottled water as hotel bookings and summer festivals from Shikoku to Kansai recorded thousands of cancellations before the advisory expired a week later with no abnormal seismic activity detected. More recently, a separate megaquake advisory issued after a magnitude 7.5 quake off Aomori in Japan’s northeast has kept the word “megaquake” in the headlines, even though that alert concerns the Japan and Kuril trenches rather than the Nankai Trough.
Tiny odds, staggering stakes
Even as the JMA framed the 1 percent weekly risk as slight, new government studies released this year underline why officials keep returning to the prospect of a Nankai Trough megaquake. A March Cabinet Office report, summarized by Reuters, estimated that a magnitude 9-class rupture along the 900-kilometer trough and the resulting tsunami could kill up to 298,000 people, destroy well over a million buildings, and cause economic losses of about $1.8 trillion, roughly half of Japan’s annual output.
The government’s Earthquake Research Committee has also been revising how it explains the long-term risk. In January, it lifted the 30-year probability of a Nankai Trough megaquake to “around 80 percent,” according to reporting in The Japan Times, and later reviews using new statistical methods widened that range to roughly 60 to 90 percent or higher while an alternative model produced a lower 20 to 50 percent estimate, figures highlighted in a Science Japan explainer on the competing forecasts. For people living above the subduction zone, the message is simple: the precise odds may be debated, but the chance of a life-altering Nankai Trough megaquake within a generation is uncomfortably high.
Decades of warnings and evolving estimates
For residents of Shikoku, Kansai, and the Tokai coast, the numbers are unsettling but not new. More than a decade ago, a government panel told ClaimsJournal that a worst-case Nankai scenario could kill more than 320,000 people and cause about ¥220 trillion in damage, figures that were widely cited in the early 2010s. A 2016 interactive feature by the Financial Times portrayed the Nankai Trough as Japan’s likely “next big quake,” using a 323,000-death scenario to illustrate how a tsunami racing into low-lying coastal plains could overwhelm sea walls and evacuation routes.
Those estimates have since been folded into official planning documents, including a 2023 disaster-management handbook prepared with the U.N. Office for Disaster Risk Reduction, which repeats the roughly 323,000-death worst case while stressing that better early warning, evacuation drills, and stronger buildings could cut casualties by as much as 80 percent. This year’s Japanese studies broadly confirm that order of magnitude even as they update the science behind tsunami modeling and building-collapse forecasts, reinforcing the sense that the core threat from a Nankai Trough megaquake has not diminished, only been more finely quantified.
Living with a Nankai Trough megaquake risk
The challenge for officials now is to turn frightening but abstract probabilities into sustained preparedness without sparking panic every time a major tremor hits. The first Nankai Trough megaquake advisory in 2024 prompted confusion over whether people should flee immediately, a communication gap dissected in an August 2024 Science News analysis of the alert. Since then, national and local governments have tweaked their messaging, emphasizing that such advisories are meant to trigger checks on evacuation routes, emergency stockpiles, and care plans for people who may need help moving, rather than mass flight.
Experts say the real story behind the latest advisory and the new probability ranges is not the headline-grabbing number — whether 1 percent this week or 80 percent over 30 years — but the steady accumulation of evidence that the region is overdue for another significant rupture. For now, the Nankai Trough remains quiet, yet the country’s own projections suggest that a Nankai Trough megaquake would be one of the most consequential disasters in modern Japanese history. How seriously people treat that low-probability, high-impact threat between alerts may determine how many lives are saved when the ground finally shifts.
