Japan alerts on megathrust earthquake amid rising Nankai Trough seismic activity

The government has revised its national disaster plan to aim for an 80% reduction in fatalities and to mandate earthquake-resistant buildings by 2035.

The Nankai Trough, an undersea trench off southwestern Japan, has drawn public attention following the occurrence of over 1,000 minor earthquakes across southern Japan within just 13 days. This has prompted widespread concern over the potential for a major earthquake and a powerful tsunami.

Japan is one of the most earthquake-prone countries in the world, located along volcanic belts and oceanic trenches known as the Ring of Fire. The Nankai Trough is considered one of the most dangerous earthquake zones. It stretches approximately 900 kilometres off the Pacific coast, where the Philippine Sea Plate subducts beneath the Eurasian Plate. This continuous tectonic motion causes a gradual buildup of pressure that could be released abruptly in the form of a large-scale megathrust earthquake.

These massive earthquakes tend to occur in the Nankai Trough every 90 to 200 years, and the region is now approaching the estimated timeframe for the next one. Over the past 1,300 years, at least 13 major earthquakes have been recorded along the trough. The two most recent were the Tonankai earthquake (1944) and the Nankaido earthquake (1946), both registering a magnitude of 8.1 and triggering tsunamis between 5 to 10 metres high, causing over 2,500 deaths and 4,500 injuries.

High probability of a major earthquake within 30 years

According to Japan’s Cabinet Office for disaster management, there is a 70–80% probability of a magnitude 8 to 9 earthquake occurring along the Nankai Trough within the next 30 years. The worst-case scenario could involve a magnitude 9 earthquake striking on a winter night with strong winds (8 metres per second), when many people may be unprepared to evacuate.

This scenario could result in up to 298,000 deaths, the destruction of over 2.35 million buildings, and large-scale tsunamis affecting major cities such as Osaka, Kyoto, and Hiroshima. The total economic impact is estimated at US$2 trillion, exceeding the damage caused by the 2011 Tohoku earthquake and tsunami. The effects would extend beyond Japan. Countries and territories throughout the Pacific Ocean basin, including Russia, Taiwan, the Philippines, Hawaii, and the US West Coast, could also be affected. Global supply chains could face major disruptions due to delayed exports from Japan, interrupted flights, and widespread economic fallout.

Calls for urgent risk mitigation

Professor Nobuo Fukuwa, former professor at Nagoya University and head of the seismic risk assessment team, has called for comprehensive national measures to reduce risk. He urges the government and private sector to reinforce earthquake resistance standards for all buildings and to decentralise urban populations in densely populated cities.

“My greatest regret is that we have been unable to reduce the anticipated risks. Minimising the potential damage is of utmost importance, as the consequences of a major earthquake along the Nankai Trough could be catastrophic for the future of the nation,” he stated.

Revised national disaster management plan underway

Given the unpredictability of earthquake timing and location, the Japanese government has prioritised disaster preparedness. On July 1, the central disaster management council convened at the Prime Minister’s official residence to approve an updated basic disaster management plan, which includes enhanced measures specifically targeting the anticipated Nankai earthquake.

Following a new damage assessment, 16 additional municipalities have been designated as areas requiring intensified disaster measures, raising the total to 723 municipalities in 30 prefectures. These areas are expected to experience earthquake intensities of level 6 or higher or tsunamis of at least 3 metres, particularly along the Pacific coastline, from Ibaraki to Okinawa.

The revised plan, an improvement of the 2014 version, aims to reduce casualties by 80% and structural damage by 50% over the next decade. While previous targets were not fully achieved, the government remains committed to these ambitious, life-saving goals.

Strengthened local measures and public preparedness

The plan includes extensive support for local governments in implementing safety measures, promoting the establishment of tsunami warning zones, developing and distributing hazard maps, and conducting evacuation drills by fiscal year 2030. All homes in designated high-risk areas are required to be earthquake-resistant by fiscal year 2035.

To further prevent disaster-related fatalities, the government will support the stockpiling of essential supplies such as portable toilets and bedding, and improve conditions at evacuation shelters, particularly in tsunami-prone coastal areas. Additionally, municipalities and logistics industry associations will be encouraged to form agreements to secure emergency transportation systems during disasters.