Japanese authorities have issued a rare warning that a “megaquake” could strike the north coast of the country’s main island within the next week, following a magnitude 7.5 tremor that hit the same area on Monday.

Japan Meteorological Agency (JMA) officials stated that Monday’s quake, which occurred at 11:15 p.m. off the Pacific coast of Aomori prefecture at a depth of 54 km, has raised the probability of another earthquake of comparable or greater magnitude in the same region in the coming week.

This marks the first time the agency has issued a warning for a megaquake (magnitude 8.0 or higher) since the forecasting system was established in December 2022.

At a press conference on Tuesday, Tsukasa Morikubo, a Cabinet official for disaster management, said a megaquake could follow Monday’s tremor along the Japan Trench and the Kuril Trench off Hokkaido.

“It is unclear whether a large-scale earthquake will occur,” Morikubo stated. “But everyone should heed the call to take precautions to protect their own lives.”

The Monday earthquake was strong enough to generate tsunamis up to 2 feet high, injure at least 30 individuals, and lead to evacuation orders for approximately 90,000 residents. Social media videos captured the intense shaking. Aftermath photos revealed cracked roads and structural damage to buildings in Aomori, with effects felt by some residents in Tokyo over 350 miles away.

Given the increased risk of a more powerful subsequent quake, Prime Minister Sanae Takaichi cautioned residents to “pay close attention to updates” from the JMA and local authorities. “In addition to reviewing your regular earthquake preparedness—such as securing furniture—please ensure that you are ready to evacuate immediately if you feel any shaking,” Takaichi advised.

The issuance of a “megaquake” warning is not unprecedented: following a magnitude 7.1 quake in southern Japan in August 2024, the government issued an alert for a megaquake from the nearby Nankai Trough. No major related earthquake occurred, and life returned to normal for citizens a week later.

However, despite Japan’s frequent experience with tremors—its position on the circum-Pacific “Ring of Fire” results in earthquakes, both noticeable and minor, approximately every three minutes—the level of concern, especially for megaquakes and the so-called “Big One,” remains elevated due to the potential risk.

Here is essential information about megaquakes in Japan.

What is a “megaquake”?

Earthquakes registering a magnitude of 8.0 or higher are classified as megaquakes. These events typically happen when one tectonic plate slides beneath another at a subduction zone. Japan is encircled by numerous such zones, which are hotspots for volcanic and seismic activity.

These powerful quakes can induce intense shaking over broad regions, with their epicenters spanning distances of 100 to 200 km.

Japan has a historical record of significant earthquakes, including the 1923 Kanto earthquake (magnitude 7.9), the 1944 Tonankai earthquake (magnitude 7.9), the 1946 Nankai earthquake (magnitude 8.0), and the 2003 Tokachi-oki earthquake (magnitude 8.0). Major quakes can also originate from active inland faults, like the magnitude 8.0 1891 Nobi earthquake.

One of the most notable megaquakes, however, was the 2011 Great East Japan Earthquake. Its force was immense, resulting in a devastating tsunami, the loss of over 20,000 lives, and significant damage to the Fukushima Daiichi nuclear power plant.

Earlier this year, a government panel reported that there is an 80% probability of a megaquake occurring in the Nankai Trough region in the south within the next three decades. A worst-case scenario for such an event could lead to as many as 298,000 fatalities.

Regarding a mega-quake in the Japan Trench and Kuril Trench to the east, estimates indicate that a magnitude-9.0 quake striking the east coast could result in up to 199,000 deaths.

Why did the latest quake trigger megaquake concerns?

The JMA explained that, based on historical global data for large earthquakes, the likelihood of a magnitude-8.0 or higher quake happening in the same area within a week after a magnitude-7.0 or higher event is significantly elevated compared to normal conditions.

According to NHK, a megaquake in this region could generate tsunamis affecting Japan’s Pacific coastline from Hokkaido to Chiba Prefecture.

The Headquarters for Earthquake Research Promotion estimates a 4% to 20% probability of a magnitude-8.0 or higher quake occurring along the northern Sanriku coast within the next 30 years. The last recorded event of this scale was the magnitude 8.3 Tokachi quake in 1968.

Considering the area’s history of powerful seismic activity, Yoshihiro Hiramatsu, an earthquake expert and professor at Kanazawa University, suggested that Monday’s tremor might have altered stress conditions in the ground, potentially creating circumstances conducive to a larger earthquake.

Lower-magnitude quakes have sometimes acted as foreshocks to more powerful ones. For instance, a magnitude-7.3 earthquake occurred off the coast of Hokkaido and Sanriku two days before the catastrophic 2011 quake.

However, a megaquake alert does not automatically mean people should evacuate, as the JMA emphasizes that its forecasts contain “a very high degree of uncertainty.”

Authorities instead recommend that residents focus on thorough preparation, which includes identifying evacuation routes, assembling emergency supplies, and ensuring access to backup food, water, and portable toilets.