Japan Widens Nankai Megaquake Probability to 60 to 90 Percent, or Higher

A wider estimate keeps preparedness in focus

Japan's Earthquake Research Committee has revised the probability of a magnitude 8 to 9 earthquake along the Nankai Trough within the next 30 years to around 60 to 90 percent, or higher. Its technical report puts the upper bound at about 94.5 percent. A separate calculation gives a range of 20 to 50 percent, leaving officials with two scientifically defensible estimates and the same urgent task: preparing communities for devastating shaking and tsunamis.

The government panel widened its estimate after reviewing historical records of coastal uplift and how its models handle uncertainty. Officials said the revision reflects changes in calculation, rather than a sudden increase in the likelihood of a major earthquake this year.

The Nankai Trough runs offshore from central Honshu, past the Kii Peninsula and Shikoku, to eastern Kyushu. Along this boundary, the Philippine Sea Plate dives beneath Japan. Great earthquakes tend to recur every century or two, with intervals described as roughly 90 to 150 years. The most recent pair were the 1944 Tonankai and 1946 Nankai earthquakes.

Naoshi Hirata, honorary professor at the University of Tokyo and head of the committee, said readiness matters more than trying to identify an exact year. He explained that the timing remains uncertain:

An actual earthquake is an unpredictable natural phenomenon. We can't clearly say when it will happen. It may be more than 30 years away but may come within a year.

The committee intends to stress the higher probability range in public messaging because communities facing the Pacific could experience life threatening tsunamis and strong shaking. Residents, schools, businesses and local governments are being urged to maintain preparations despite the uncertainty over timing.

Why old coastal measurements matter

Muroto Port in Kochi Prefecture sits at the center of one reason for the revision. Historical measurements there have long helped researchers estimate how the coast rose and fell between major earthquakes. Researchers found errors and changes at the measurement site that skewed earlier calculations.

Coastal uplift records act as a proxy, an indirect way of studying movement along the plate boundary. For decades, probability estimates relied on the dates of past ruptures and uplift observed at a handful of coastal locations. The small dataset remains useful, but its limitations make a narrow probability estimate difficult to justify.

The committee also reconsidered the assumption that strain builds at a steady pace. Strain is the deformation that develops as the plates move and stress accumulates along their boundary. The pace can vary, which affects calculations of when the fault may rupture.

The revised method explicitly includes uncertainty from old measurements and variation in stress loading from year to year. It combines a view that accounts for the time since the last earthquake with statistical treatments of those uncertainties. The result is a broader probability band instead of a single, apparently precise figure.

Why the panel has two probability ranges

The lower estimate of 20 to 50 percent uses average intervals between large earthquakes without relying on uplift data. Committee members said the limited sample of observed megaquakes means neither method is clearly superior.

The decision to stress the higher range reflects the scale of the danger facing coastal communities. Presenting both calculations also creates a communication challenge: people need to understand the uncertainty while knowing what actions to take.

Naoya Sekiya, a University of Tokyo scholar who studies disaster information, said explanations of probability need to be accompanied by practical guidance:

When experts communicate such information, they need to present what disaster-preparedness steps can be taken.

The committee's central message remains steady. The revised calculations clarify uncertainty while keeping attention on evacuation planning, safer buildings and preparations at home and work.

How the estimates and warnings developed

Japan's Nankai probability estimates have risen over the past decade as time has passed without another major rupture. Roughly eight decades have elapsed since the 1946 earthquake, a key input in models based on recurrence.

The sequence of earlier earthquakes, probability updates and recent warnings shows how the present assessment developed:

• 1707: The Hoei earthquake, the largest known along the Nankai zone, sent waves tens of meters high into parts of Shikoku, the Kii Peninsula and Kyushu.
• 1854: Two major ruptures struck the region within days.
• 1944 and 1946: The Tonankai and Nankai earthquakes struck in succession.
• 2013: The committee put the probability of a major Nankai earthquake within 30 years at 60 to 70 percent.
• 2019: Japan adopted the advisory system later used to warn of temporarily increased megaquake risk.
• August 2024: A magnitude 7.1 earthquake in the Hyuga Sea off Kyushu triggered the first use of that advisory.
• Early 2025: The committee's probability estimate stood at about 80 percent.
• March 2025: A national assessment projected up to about 298,000 deaths and economic losses near 292 trillion yen in a worst case event.
• Early July 2025: A preparedness plan outlined further work on coastal defenses, evacuation facilities, drills and essential services.
• By September 28, 2025: The revised assessment gave a range of around 60 to 90 percent, or higher, alongside the separate 20 to 50 percent calculation.

The figures have generally increased by less than one percentage point each year as time since 1946 grows. Natural earthquake cycles remain irregular: stress can be released suddenly after decades of expectation, or remain stored longer than anticipated.

A probability covering 30 years describes the chance across that entire period. It gives planners a window for assessing risk while the exact date remains unknown. The wider range puts uncertainty into the calculation so authorities can test whether evacuation systems, infrastructure and public services could withstand a major event.

The scale of the damage Japan is planning for

The national estimate released in March projected up to about 298,000 deaths and economic losses on the order of 292 trillion yen in a worst case earthquake and tsunami. Government planning documents describe a magnitude 8 to 9 rupture producing widespread shaking, extensive coastal flooding and towering waves, with tsunamis exceeding 20 meters in some areas.

Shizuoka, Aichi, Mie, Wakayama, Tokushima, Kochi and Miyazaki are among the prefectures facing the greatest tsunami danger. Nagoya and Osaka could suffer heavy damage to homes, ports and supply chains. Disruption could spread from damaged coastal facilities to industries and infrastructure farther away.

Government scenarios guide decisions about land use, evacuation planning, seawalls, shelters and early warning. Their purpose is to help authorities decide where people can escape, what facilities need protection and where investment could reduce losses.

The history of the trough means engineers prepare for more than one pattern of rupture. A single massive earthquake could break several segments of the boundary. Alternatively, one segment could fail and place additional stress on another, producing a sequence of large earthquakes like those recorded in 1854 and in 1944 and 1946.

Why evacuation time can be measured in minutes

Low lying districts close to the rupture could see the first major tsunami wave within minutes of strong shaking. That short window makes the location of escape routes and evacuation buildings a practical concern for everyday life along the coast.

Towns across the Kii Peninsula and Shikoku have built vertical evacuation towers and marked routes to high ground. Vertical evacuation gives people a place to climb when reaching higher land would take too long.

Sirens, smartphone alerts and local radio can help people respond. Recognizing natural warning signs, including prolonged shaking or a sudden retreat of seawater, can provide the fastest trigger to leave the shore and head to safety.

Japan's earthquake early warning system can provide a few seconds before strong shaking begins. That may allow someone to get under a sturdy table, activate an emergency stop or open platform doors. Local tsunami alerts follow quickly when a large offshore rupture is detected.

What the 2024 advisory changed

The magnitude 7.1 Hyuga Sea earthquake in August 2024 prompted the Japan Meteorological Agency to convene experts and issue the first megaquake advisory under the system adopted in 2019. The agency urged extra caution for a week, then lifted the advisory when signs of escalation did not persist.

The warning identified a temporary increase in risk that called for vigilance. Scientists still could not specify whether or when a much larger earthquake would occur.

Transport delays, event cancellations and a rush to buy some staples followed. Some store shelves emptied as residents stocked up. The episode exposed confusion over the advisory's meaning and showed how quickly concern could affect travel, public events and household purchasing.

The panel's revised communication approach seeks to connect clearer probability language with practical steps. For residents, the useful response is preparation that can be maintained over time, including knowing evacuation routes and keeping supplies, rather than relying on a forecast of the exact day.

Preparing homes, workplaces and essential services

The preparedness plan released in early July called for faster construction of sea embankments and tsunami evacuation buildings, more frequent drills and mutual aid agreements across prefectures. It also outlined better backup power for hospitals and data centers, helping critical services continue when normal electricity supplies fail.

National and local governments are working to shorten evacuation times while protecting essential infrastructure. Port authorities and manufacturers are revising continuity plans, which set out how operations can continue or restart after disruption, to limit damage to shipping and production.

Households and workplaces have their own immediate tasks. Local hazard maps identify areas that could flood and show evacuation sites. Emergency supplies should include water, food, flashlights, medicines and copies of key documents.

Furniture and appliances can be anchored to reduce injuries during shaking. Families can practice the journey to high ground and agree on how to reconnect if they become separated. Recommended preparations include:

• Identify the nearest high ground or vertical evacuation site and two routes to reach it.
• Keep at least three days of drinking water and food that can be stored without refrigeration for each person, with more supplies in remote areas.
• Anchor bookcases, televisions and gas appliances, and secure cabinet latches.
• Prepare a small bag with a flashlight, portable radio, batteries, whistle and basic first aid supplies.
• Agree on a family meeting plan and a contact outside the affected area.

Schools, businesses and families face the same basic timing problem near the coast: evacuation may need to begin quickly. Practicing a route in advance can make the response more straightforward when shaking and disruption make decisions harder.

Effects could reach beyond Japan

A very large Nankai rupture could send tsunami energy across the Pacific. Modeling indicates waves could travel toward Taiwan, the Philippines and areas farther south. Greater distance increases travel time, giving national meteorological services more time to issue warnings.

The Hong Kong Observatory notes that a distant Nankai earthquake would leave Hong Kong without earthquake shaking, while a very large tsunami could produce sea level anomalies several hours later. For a shallow rupture of around magnitude 9, wave energy could reach southeastern Hong Kong waters about six hours after the earthquake.

The effects there would depend on the local seabed and harbor design. International monitoring therefore includes the possibility of minor tsunami effects in distant regions, alongside the much more immediate danger to communities close to Japan's rupture zone.

Regional trade could also be disrupted. Maritime routes across the Philippine Sea and approaches to Tokyo Bay, Osaka Bay and the Seto Inland Sea could face temporary closures. Damage to ports would affect the movement of goods, while interruptions at factories could spread through supply chains.

LNG import terminals and refineries along Japan's Pacific coast could also sustain damage. Disruption to those energy facilities could ripple through regional markets, adding to the effects on shipping and manufacturing.

Japan's revised probability range leaves the central task unchanged: preparing for a major earthquake whose timing remains uncertain. Clear explanations, functioning evacuation routes, protected essential services and supplies already in place give communities concrete ways to act on that risk.

At a Glance

• The Earthquake Research Committee puts the chance of a magnitude 8 to 9 Nankai earthquake within 30 years at around 60 to 90 percent, or higher.
• The technical upper bound is about 94.5 percent. A method excluding uplift data gives 20 to 50 percent.
• The revision accounts for errors in Muroto Port records and variation in how strain accumulates.
• The panel plans to stress the higher range in preparedness messaging.
• The last major earthquakes along the system were the 1944 Tonankai and 1946 Nankai events.
• The March 2025 worst case assessment projected up to about 298,000 deaths and losses near 292 trillion yen.
• Some coastal districts could face tsunami waves within minutes, with waves exceeding 20 meters in some areas.
• The August 2024 advisory urged extra caution for a week after a magnitude 7.1 earthquake off Kyushu.
• Government preparations include embankments, evacuation buildings, drills, mutual aid and backup power.
• Households are urged to know escape routes, secure furniture and keep several days of essential supplies.
• A very large rupture could affect distant waters and disrupt regional shipping, manufacturing and energy markets.