Geologists don’t have a crystal ball that can foresee future earthquakes, but seismic specialists in the Japanese government believe that the odds of a big one are creeping upwards. According to Japan’s Earthquake Research Committee, there’s an 80 percent chance that a megaquake could occur in the Nankai Trough within the next 30 years. That’s slightly up from the 70 to 80 percent chance that was put out last year. 

“It’s been 79 years since the last quake, and the possibility of another quake occurring is rising every year at a pace of by about one percent,” an official of the Earthquake Research Committee’s secretariat told AFP.

The Nankai Trough carves out around 900 kilometers (559 miles) along the south of Japan’s biggest island of Honshu. It’s a subduction zone located where the Philippine Sea Plate is being subducted beneath the Eurasian Plate, making it a hotbed of seismic activity.

It’s notorious for its megathrust earthquakes, powerful seismic events that occur where one tectonic plate is forced beneath another, releasing colossal quantities of energy.

If the Nankai Trough experiences a major shake-up, it could prove to be devastating – a risk the Japanese Government takes very seriously. Nankai megathrust earthquakes are considered a top priority of Japan’s disaster management strategy and the government has introduced schemes to prepare for a potential blowout. 

One of the most recent Nankai earthquakes earned the grim distinction of being one of the deadliest in the world. In 1946, an 8.1 magnitude earthquake stemming from the Nankai claimed the lives of 1,362 people. It destroyed or damaged over 36,000 houses in southern Honshu and Shikoku, while a further 2,100 properties were washed away by 6-meter-high (20-foot) tsunami waves created by the quake. 

Forecasting these events isn’t easy, though. Per the USGS, scientists cannot predict earthquakes, noting that no one has ever predicted a major earthquake. However, they note that it is possible to “calculate the probability that a significant earthquake will occur in a specific area within a certain number of years.” 

In other words, it’s not like a weather forecast; it’s more like knowing the odds in a card game – you can’t predict your next card, but you can estimate the likelihood of a certain hand based on the deck.

The new threat level issued by Japan’s Earthquake Research Committee is similar to estimates published in the journal Scientific Reports back in 2023, which found there’s a 70 to 80 percent chance that the Nankai Trough Megathrust Earthquake will occur in the next three decades. 

The researchers also projected that a Nankai megathrust earthquake could be devastating, potentially causing more extensive damage than the 2011 Great East Japan Earthquake with fatalities possibly exceeding 320,000.

However, once again, they stressed that their forecasts were far from set in stone.

“Statistically speaking, Nankai Trough earthquakes are likely to occur once every 90–150 years based on past earthquakes,” Professor Yoshioka Shoichi, a professor at the Research Center for Urban Safety and Security, said in a 2023 statement.

“Robust earthquake predictions must accurately extrapolate three factors: the location of occurrence, the time of occurrence, and the magnitude of the earthquake. Among the three, it is particularly difficult to predict the timing of an earthquake,” he added.