On January 16th, Japanese media such as NHK reported that the Earthquake Research Committee under the Ministry of Education, Culture, Sports, Science and Technology in Japan revised the estimated probability of a major earthquake occurring in the Nankai Trough—a deep oceanic trench off the southern coast of Japan—in the next 30 years from “70-80%” to “approximately 80%.” This adjustment, effective as of January 1st, is based on the assessment that the probability of a Nankai Trough mega-earthquake happening has increased to 75-82%.
Naoshi Hirata, the chairman of the Earthquake Research Committee and an Emeritus Professor at the University of Tokyo, emphasized that “the probability of occurrence increases by approximately 1% each year because such earthquakes happen roughly once every 100 years. However, this increase is not sudden, and people should always prepare for earthquakes.” He also clarified that this recent revision was not influenced by a recent earthquake on January 13 but due to the elapsed time since past major earthquakes.
The Nankai Trough is located in the ocean off the southern coast of Shikoku, Japan, and is known as a deep trench over 4000 meters deep, marking the boundary where the Philippine Sea Plate subducts beneath the Eurasian Plate. The movement of this geological plate boundary can eventually trigger massive earthquakes known as the Nankai Trough mega-earthquakes.
The last Nankai Trough mega-earthquake occurred in 1946 near Wakayama Prefecture, causing the collapse of 35,000 houses and resulting in 1,443 casualties. Such earthquakes are also typically accompanied by devastating tsunamis, and there are concerns about the possibility of Mount Fuji erupting concurrently, as was the case in the 1707 Hoei earthquake when volcanic ash affected even the center of Edo (Tokyo), amplifying the disaster’s impact.
Experts estimate that in a worst-case scenario, a Nankai Trough mega-earthquake could result in 320,000 casualties, destroy 2.4 million buildings, and create 9.5 million evacuees, with economic damage reaching 220.3 trillion yen (about 2,011 trillion won)—over 11 times the damage caused by the 2011 Great East Japan Earthquake.
To address these potential outcomes, the Japanese government established a basic earthquake response plan in 2014 with goals to reduce the death toll by approximately 80% and halve the number of building collapses. Local governments in earthquake-prone areas are enhancing earthquake-resistant construction and installing tsunami evacuation towers as part of these measures. They are continuously reviewing and updating the anticipated damage scale and implementing countermeasures in response to changing conditions.
Despite these efforts, accurately predicting earthquakes with modern science remains a significant challenge, and some argue that the probabilities are overstated. Nevertheless, both the Japanese government and experts are preparing for the almost certain eventuality of a mega-earthquake in the Nankai Trough region.