A Megaquake Warning Was Issued In Japan, But What Is A Megaquake?

7.4M Quake Hits Japan, Tsunami Sirens Blare

There’s a small chance a megaquake could hit Japan in the following week. But what even is that?

Here's our explainer, drawing from scientific measures, historical quakes and how the unique geological features of Japan relate to the current situation.

What Happened In Japan?

To start, here’s some background on the current situation.

A powerful 7.4 magnitude earthquake struck off the coast of Japan on Monday. Buildings swayed, and at least 11 towns were warned to evacuate and seek higher ground. There was another 5.6 aftershock less than an hour after the main quake.

(MORE: Powerful Earthquake Strikes Off Japan's Coast)

Tsunami warnings were also issued, and two small tsunamis of about 2 feet high were recorded, according to Japan's meteorological agency.

The Cabinet Office and the Japan Meteorological Agency then said there was a 1% chance for a megaquake to take place in the wake of Monday's temblor. While 1% is still a relatively small chance, it is a relatively big jump compared to the typical 0.1% chance.

How Are Earthquakes Measured And What Makes One "Mega?"

To understand a megaquake, you need to know how the strength of an earthquake is categorized, which is in “magnitude,” and it’s not simple.

This is a measure of a combination of factors, including shaking intensity, the physical size of the earthquake by the length of the fault on which it occurs, energy release and the amplitude of seismic waves recorded on seismometers.

The reality is, measuring earthquakes is a very complicated process that can be a bit inconsistent and arbitrary. It’s also changed a lot over time. For example, the more commonly known Richter Scale has mostly been retired and replaced by a more modern measure, Moment Magnitude.

But regardless, the higher the magnitude, the stronger the quake. One between 7.0 and 7.9 is considered a major earthquake that can cause serious damage; about 10 to 15 of these occur in a year. An earthquake of 8.0 or greater happens only once a year or two, and can “totally destroy communities near the epicenter,” according to Michigan Technological University.

What Could A Megaquake In Japan Look Like?

For Japan’s Headquarters for Earthquake Research Promotion, they consider anything of a magnitude 8.0 or higher to be a megaquake.

If this were to occur, it wouldn’t be Japan’s first. Because the country is in an area where several continental and oceanic plates meet, earthquakes are common. If one happens below or close to the ocean, it can trigger a tsunami — a giant wave that can cause massive destruction once it makes it on shore.

(MORE: Powerful Earthquake Strikes Off Japan's Coast, Evacuations Ordered As Tsunami Warning Issued)

Japan has a long history of significant earthquakes and has issued these warnings before.

The first megaquake warning in Japan was issued in August 2024 after a magnitude 7.1 earthquake shook southern Japan.

In December 2025, a megaquake warning was issued after a magnitude 7.5 tremor struck off the northern coast. The government said a megaquake could cause up to a 98-foot tsunami and kill as many as 199,000 people. No megaquake occurred.

The government is not necessarily forecasting a megaquake. Instead the warning is a reminder to residents to be prepared for such a disaster.

The strongest earthquake ever recorded in Japan at a magnitude of 9.0 happened 15 years ago in 2011, known as the “Great East Japan Earthquake.” The quake and particularly the resulting 23-foot-high tsunamis killed nearly 20,000 people and caused a nuclear reactor accident at a power plant in Fukushima.

The largest earthquake ever recorded across the globe was a magnitude 9.5 in 1960 in Chile along a fault that is almost 1,000 miles long.

So, scientists say something of this scale could happen again in Japan — a “megaquake” — but that chance is still very slim. The government has encouraged residents in 182 towns along the northeastern coasts to raise their preparedness, but continue with their daily lives.