Why did Japan issue its first major earthquake warning, and what does it mean?

Japan’s Meteorological Agency issued its first-ever “super-quake warning” on Thursday. The warning came after a 7.1-magnitude quake struck off the country’s southern coast. That raises the risk of a larger quake in the Nankai Trough, an underwater subduction zone that scientists believe is capable of producing quakes as strong as magnitude 9.1.

After a 7.1-magnitude earthquake struck Japan's southern islands on Thursday, the country's meteorological agency sent out an ominous warning: Another, larger quake is possible, and the risk will be especially high over the next week.

In its first-ever “major earthquake warning,” the agency said the risk of strong tremors and tsunamis was higher than usual in the Nankai subduction zone, which can produce quakes as strong as magnitude 8 or 9. Residents in the area should prepare, it said.

The message wasn't a prediction, but a forecast of increased risk – and it shows how much progress seismologists have made in understanding the dynamics of subduction zone earthquakes.

Here's what you need to know about the situation.

dangerous subduction zone

The Nankai Trough is an underwater subduction zone where the Eurasian Plate collides with the Philippine Sea Plate, forcing the latter to subduct beneath the former and into the Earth's mantle.

Subduction zone faults cause pressure to build up, and what's called a megaquake occurs when a closed fault slips and releases that pressure. “Megaquake” is just a shortened version of the name. These zones have produced some of the most powerful earthquakes in Earth's history.

The Pacific “Ring of Fire” is a group of subduction zones. In the United States, the Cascadia subduction zone stretches off the west coast from Vancouver Island, Canada, to Cape Mendocino, California.

The Nankai Basin fault has several segments, but if the entire fault margin slipped at once, Japanese scientists believe the basin could produce an earthquake of up to magnitude 9.1.

A beach in Nichinan, southwestern Japan, was closed on Friday after the country issued its first warning of a possible major earthquake. Kyodo via Reuters Connect

If a major earthquake occurred near Japan, the Philippine Sea plate would move, perhaps 30 to 100 feet, near the country's southeast coast, causing severe shaking.

The vertical displacement of the seafloor would cause a tsunami and send waves toward the coast of Japan. These waves could reach heights of nearly 100 feet, according to estimates by Japanese scientists published in 2020.

History of major earthquakes

A study last year suggested that the Nankai Trough produces large earthquakes roughly every 100 to 150 years. The Japan Earthquake Research Committee said in January 2022 that there was a 70% to 80% chance of a large quake in the next 30 years.

Large earthquakes in the Nankai Trough tend to occur in pairs, with the second quake often occurring within the following two years. The most recent examples were the “twin” earthquakes that struck the Nankai Trough in 1944 and 1946.

This phenomenon is due to the fragmented nature of the fault; when one part slips, it can put pressure on another part.

A magnitude 7.1 earthquake struck on Thursday in or near a subduction zone, the U.S. Geological Survey said.

People stand outside a building after leaving the building following an earthquake in Miyazaki on Thursday. Kyodo News via The Associated Press

The 7.1-magnitude quake struck a part of the region that shakes more frequently than others, said Harold Tobin, a professor at the University of Washington who has studied the Nankai region. Regular quakes can release stress, so the possibility that the same part could trigger a major quake is not a major concern. The concern is that the quake is close to a part that has been building stress since the 1940s.

“It's adjacent to the western Nankai area and it's clearly closed off. That's why there's alarm and concern,” Tobin said.

Prediction, not prediction

Scientists cannot predict earthquakes, but they are working to develop the ability to predict times of increased danger, especially in areas that experience frequent shaking and have good monitoring equipment, such as Japan.

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