Japan's terrifying 'megaquake' forecast: Up to 3 lakh deaths feared. What is the Nankai Trough threat?

A "megaquake" and subsequent tsunami could cause as many as 298,000 deaths in Japan and damage costing up to $2 trillion, the government said Monday in a new estimate. The warning follows a magnitude 7.1 jolt in southern Japan which injured 14 people in 2024. The Japanese government also mentioned that it is likely to see 1.23 million evacuees or 10 per cent of its total population, in the event of the probable ‘megaquake’.

Japan had issued its first ever advisory in August 2024 for a potential "megaquake" along the Nankai Trough, a 900-kilometre undersea fault stretching from Shizuoka to Kyushu. The megaquake rules were drawn up after the devastating 2011 earthquake, tsunami and Fukushima nuclear disaster.

Tokyo’s urgency stems from the trough’s volatile tectonic activity, where the Philippine Sea Plate subducts beneath the Eurasian Plate, storing energy capable of unleashing earthquakes up to magnitude 9.1.

Why Is Tokyo Fearing a Megaquake?

The Nankai Trough has a 70-80% probability of producing a magnitude 8–9 quake within the next 30 years, according to Japanese government estimates.

Historical patterns show megaquakes recur here every 100–200 years, with the last major event in 1946.

A rupture could trigger tsunamis reaching 30–34 metres within minutes, devastating coastal prefectures like Shizuoka, Kochi, and Wakayama.

Tokyo’s updated projections—revised from a 2014 forecast of 323,000 deaths—now predict 215,000 tsunami fatalities, 73,000 from collapsing buildings, and 9,000 from fires.

What Makes the Nankai Trough So Dangerous?

The Nankai Trough’s tectonic strain, accumulated over centuries, poses a threat due to its potential for sequential megaquakes.

Research by Tohoku and Kyoto Universities warns that a magnitude 7+ quake here raises the likelihood of a follow-up tremor 100–3,600 times higher than normal within a week.

The 800-kilometre (500-mile) undersea trench runs from Shizuoka, west of Tokyo, to the southern tip of Kyushu island.

It is where the Philippine Sea oceanic tectonic plate is "subducting" -- or slowly slipping -- underneath the continental plate that Japan sits atop.

The plates become stuck as they move, storing up vast amounts of energy that is released when they break free, causing potentially massive earthquakes.

Japan: A Nation on Edge

Japan’s trauma from the 2011 Tōhoku quake—which killed 18,000 and triggered the Fukushima nuclear crisis—looms large. 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.

While advancements in early-warning systems offer some reassurance, the scale of a Nankai Trough disaster could dwarf previous calamities. As University of Tokyo seismologist Naoshi Hirata warns: “The time for preparedness is now—not when the ground begins to shake.”