Japan Reports Increased Probability of Megaquake: Over 80% in 30 Years

The probability of a “megaquake” occurring in Japan within the next 30 years has risen to over 80 percent, according to findings from the government’s earthquake investigation panel.

A megaquake is defined as an earthquake with a magnitude of 8 or greater, capable of causing devastating destruction and likely to trigger tsunamis.

The Nankai Trough, an 800-kilometer-long undersea trench near Japan’s Pacific coast, is identified as the most likely site for such an event. The panel now estimates the probability of a megaquake in this area to exceed 80 percent, an increase from the previous 70–80 percent.

Each year, the Headquarters for Earthquake Research Promotion updates the probabilities of earthquakes occurring along Japan’s active faults and seabed areas using data as of January 1.

“This probability is a number indicating that it would be no surprise if an earthquake were to happen at any time,” Naoshi Hirata, head of the expert panel and professor emeritus at the University of Tokyo, said during a press conference. “We’d like to ask people to continue to be prepared.”

The panel also reported an increased likelihood of a megaquake in other areas, including the Japan Trench and Chishima Trench. There is now a 20 percent chance of an 8.6-magnitude quake off the coast of Tokachi.

The potential earthquake would result from the interaction of two tectonic plates: the Philippine Sea Plate is slowly subducting beneath the continental plate on which Japan sits. Over time, stress builds as these plates get stuck against each other. When they finally slip, the energy released could cause a megaquake.

Historical data shows megaquakes in the Nankai Trough have occurred every 100 to 200 years over the past 1,400 years. The most recent was in 1946, which destroyed 36,000 houses in southern Honshu. In August 2024, the Japan Meteorological Agency issued its first megaquake advisory since the 2011 Tohoku earthquake and tsunami, warning of a heightened risk of a major earthquake along the Nankai Trough.

The panel’s report also noted the repeated occurrence of significant earthquakes in regions like the Noto Peninsula in Ishikawa Prefecture over recent years, describing this as “something Japan has never seen before.”

Japan’s location on the “Ring of Fire,” a zone of intense seismic and volcanic activity, makes it highly earthquake-prone. “Japan sits on the boundaries of four tectonic plates, which makes it one of the most earthquake-prone areas in the world,” said Shoichi Yoshioka, a professor at Kobe University, in a CNN interview last year.

“About 10 percent of the world’s earthquakes of magnitude 6 or higher occur in or around Japan, so the risk is much higher than in places like Europe or the eastern United States, where earthquakes are rare,” he added.

The report, released on Wednesday, identified high-probability regions for megaquakes over the next 30 years, including the coast off Nemuro in Hokkaido along the Chishima Trench and Miyagi Prefecture along the Japan Trench. The likelihood for these areas is estimated at around 80 percent, with Miyagi’s probability increasing from 70–90 percent last year to 80–90 percent now.

Historical events underscore the potential devastation of megaquakes. On December 26, 1707, all segments of the Nankai Trough ruptured simultaneously, causing the Hoei Earthquake, Japan’s second-most powerful on record, and triggering Mount Fuji’s last eruption. After World War II, the Nankai Trough saw two megaquakes, in 1944 and 1946.

Not all experts agree on the likelihood of an imminent Nankai Trough megaquake. Robert Geller, a seismologist and professor emeritus at the University of Tokyo, has criticized the practice of issuing regular warnings about a Nankai Trough earthquake, calling it a “made-up construct” and a “purely hypothetical scenario” in a CNN report last year.

He argued that earthquakes are inherently unpredictable, saying, “It’s pointless to forecast future quakes based on past ones.” This view contrasts with the conventional scientific theory of the “stick-slip” process, which holds that stress builds along faults and is eventually released in earthquakes.