The Japanese government has issued an urgent alert over the severe threat of a catastrophic megaquake predicted to strike the Nankai Trough, located off the country’s Pacific coast. Experts estimate there is an 80% chance of a magnitude nine earthquake occurring in the region within the next 30 years.
According to newly released government projections, such a disaster could result in up to 300,000 deaths in a worst-case scenario. Of these, an estimated 215,000 would be caused by towering tsunami waves, expected to exceed 30 metres (98 feet) in certain areas.
The Nankai Trough, a deep-sea trench marking the boundary between two tectonic plates, typically experiences megaquakes every 100 to 200 years. The last major revision of disaster projections was in 2013, but with the rising risk, the Japanese Cabinet Office has updated its impact estimates.
These figures paint a grim picture: approximately 2.35 million buildings could be destroyed due to ground collapse, flooding, and fires, while economic losses may reach £1.44 trillion (214.2 trillion yen). Additionally, about 12.3 million people—or roughly 10% of Japan’s population—would be displaced within just a week of the disaster.
Japan’s geographical position on the Pacific Ring of Fire makes it particularly vulnerable to seismic activity. This volatile region is responsible for about 81% of the world’s largest earthquakes due to the interaction of multiple tectonic plates.
The Nankai Trough, where the Philippine Sea Plate subducts beneath the Eurasian Plate, is one of the most dangerous zones. Spanning roughly 600 miles (900 kilometres), it has the potential to produce extraordinarily powerful and deadly tremors.
The threat became more tangible last year when a magnitude 7.1 earthquake struck southwestern Japan, prompting the government to issue its first advisory specifically warning of a potential Nankai Trough megaquake.
Using updated modelling, experts have simulated the potential scale and impact of a magnitude nine tremor, including the expected tsunami and flood damage. Although Japan has implemented stricter building codes and coastal defences over the past decade, the new findings show only minimal reduction in projected casualties.
Impact Scenario: What Could Happen?
Should a megaquake occur in the Nankai Trough, affected areas could have as little as two minutes to prepare. Massive waves, reaching up to 10 metres, are expected to strike Tokyo and 12 other prefectures along the southeastern coast. In Kochi Prefecture, towns like Kuroshio and Tosashimizu face the terrifying prospect of being hit by a wave as high as 34 metres (112 feet).
The tsunami would flood around 444 square miles (1,151.5 square kilometres) of land with water depths of 30cm or more. In total, close to 300,000 people could lose their lives: 215,000 from the tsunami, 73,000 due to building collapses, and 8,700 in fires.
Beyond the immediate impact, the aftermath could prove just as deadly. The report predicts between 26,000 and 52,000 people may die from post-disaster causes such as exposure to the elements, lack of medical care, or the outbreak of infectious diseases in flooded areas and evacuation centres. These numbers are over 13 times higher than post-disaster deaths recorded after the 2011 Tohoku earthquake.
In that disaster, a magnitude nine quake struck beneath the North Pacific, triggering tsunami waves up to 40 metres (132 feet) high. It killed 15,500 people and caused a nuclear crisis at the Fukushima power plant, forcing mass evacuations and contaminating the environment.
However, experts warn that a Nankai megaquake could eclipse even the 2011 Tohoku earthquake in terms of destruction. In a scenario where the quake strikes during winter at night—when evacuation would be especially difficult—fatalities could be significantly higher.
The study also reveals that 10 of Japan’s 47 prefectures could experience level seven tremors, the highest on Japan’s seismic intensity scale, while 24 others may be hit with intensity level six.
Given Japan’s vulnerability and the high probability of such an event, authorities urge continued investment in preparedness, infrastructure resilience, and public awareness to mitigate the potentially unprecedented human and economic toll of a Nankai Trough megaquake.
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