The risk of a ‘megaquake’ is higher in Japan after a powerful earthquake

After a 7.7 magnitude earthquake struck the coast of Japan and triggered tsunami warnings, there is a high risk of a “megaquake” following in its wake.

By Robin George Andrews edited by Andrea Thompson

At 4:53 p.m. local time on Monday, a magnitude 7.7 earthquake struck off the northeast coast of Japan’s largest island, Honshu, where the Pacific tectonic plate plunges beneath the North American plate at the deep Japan Trench. Immediately, the Japan Meteorological Agency (JMA) issued a tsunami warning. alert. Although small tsunami waves quickly reached various sections of the coast, there were no reports of injuries, deaths, or significant damage to homes or infrastructure.

However, the danger is not necessarily over. Following the quake, a JMA spokesperson said Media and affected residents said that “the probability of another huge earthquake occurring is relatively higher than normal.” Specifically, there is a high risk of a “megasquake” – with a magnitude of 8.0 or greater – in the coming days.

The probability of an imminent mega-earthquake is very low, around one in 100. “This 1% probability is still low in absolute terms, but it is 10 times higher than normal, which is significant from a risk management perspective,” explains Amilcar Carrera – Cevallosan independent earthquake specialist.

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This concern comes from the fact that a magnitude 8.0 earthquake is considerably more powerful than a magnitude 7.7 earthquake, so if a mega-earthquake were to occur, it could produce a much more extreme tsunami. “It is important that people who may be at risk understand that another major earthquake is possible, even if it is unlikely,” says Wendy Bohonseismic geologist.

But aren’t aftershocks generally much weaker than the initial earthquake? Well, it’s a little more complicated than that.

Every time a large earthquake occurs, it releases a lot of pent-up stress that can cause faults around the ruptured one to slip. In other words, “earthquakes make other earthquakes more likely,” Bohon explains. This usually takes the form of aftershocks. The strongest earthquake in a sequence is (in retrospect) called a mainshock, and aftershocks are almost always less powerful than the mainshock.

So far, this appears to be happening in Japan, where a myriad of aftershocks have rocked the island, including several events with magnitudes greater than 5.0.

It is rare for an aftershock to be larger than the original mainshock. If that were to happen here, it would mean that Monday’s magnitude 7.7 earthquake (which the U.S. Geological Survey recorded as magnitude 7.4) was actually a pre-shock of the “real” mainshock. The JMA calculated that, based on stress changes around the Japan Trench, there is now a 1% chance that a magnitude 8.0 earthquake will occur in the coming hours or days.

As a result, the government issued a “mega-earthquake advisory”. Such notices are a relatively new concept for Japan. The first was issued in August 2024, when a 7.1 magnitude earthquake shook southern Japan. They are not intended as forecasts but as alerts to people nearby: these people should be aware of the high risk, know where their evacuation routes are, and prepare their bags of emergency supplies.

The most likely scenario is that Monday’s 7.7 magnitude earthquake was the main event and the seismic party is dying down. “There have been many other times when large earthquakes occurred in Japan that were not followed by larger events,” says Bohon. But over the coming days, millions of people along the coast will be ready to flee in case the odds are truly stacked against them.