Is a bigger quake coming? What Japan's experts are actually saying

A magnitude 7.5 earthquake in north-eastern Japan prompted authorities to re-issue cautionary statements about the possibility of a future "megaquake" linked to the Nankai Trough.

Officials said there is a chance that "a large-scale earthquake with a magnitude of 8 or higher could occur as a follow-up earthquake," though they stressed this is a possibility rather than a forecast, estimating the likelihood at about one in 100, says the BBC.

What is the Nankai Trough?

The Nankai Trough is a major seismic zone off Japan's Pacific coast, stretching from Suruga Bay in central Japan to the Hyuganada Sea near Kyushu. The region is prone to "megathrust" earthquakes caused by the subduction of one tectonic plate beneath another. Geologists describe it as the "original definition of the 'Big One'."

The Nankai Trough is a significant submarine trench off Japan's southwest Pacific coast, formed by the Philippine Sea Plate sliding (subducting) under the Eurasian Plate, creating immense tectonic stress and making it prone to powerful megathrust earthquakes and devastating tsunamis, with high probabilities of future major seismic events. Screenshot: Google Maps

How likely is a major quake?

Japan's earthquake investigation panel reported in September that the probability of a Nankai Trough megaquake occurring within the next 30 years is between 60% and 90%. Historically, large quakes in this zone occur about every century, often in pairs. The most recent pair struck in 1944 and 1946. A full-length rupture in 1707 caused the second-largest earthquake ever recorded in the country.

What could the impact be?

Government assessments outline severe potential consequences in a worst-case scenario if the quake were to hit densely populated southern areas:

• Fatalities: Around 300,000 deaths
• Economic losses: Trillions of dollars in damage
• Tsunami: Waves exceeding 20 metres (66 ft) that could affect parts of Tokyo and other prefectures

Officials say these figures reflect an absolute worst-case model rather than a prediction.

How are authorities responding?

Following the recent quake, residents in seven prefectures from Hokkaido to Chiba were urged to remain alert. The Japan Meteorological Agency (JMA) used a special warning system—created after the 2011 magnitude 9.0 earthquake—for the first time in August 2024.

The JMA said the likelihood of another major earthquake is "higher than normal, but this is not an indication that a major earthquake will definitely occur." While no evacuation orders were issued, authorities encouraged people to secure furniture, identify evacuation routes and prepare emergency kits with food, water and portable toilets.

Is it possible to predict these events?

There is ongoing debate among scientists about interpreting seismic activity as a precursor to larger events.

Robert Geller, professor emeritus of seismology at the University of Tokyo, said recent warnings had "almost nothing to do with science" and were "not a useful piece of information." He noted that earthquakes are a "clustered phenomenon," and it is not possible to know in advance whether a tremor is a foreshock or an aftershock.

Geologists Kyle Bradley and Judith A. Hubbard point out that only about 5% of earthquakes are typically foreshocks. However, they also note that the 2011 magnitude 9.0 earthquake was preceded by a 7.2 magnitude foreshock that drew little attention at the time.

Why does this matter now?

Japan's high population density, extensive coastline and complex seismic environment make preparedness a central part of disaster planning. The renewed warnings reflect heightened caution but not an indication that a major quake is imminent.