Japan’s First “Major Earthquake Warning” and Its Significance

After a magnitude 7.1 earthquake struck Japan’s southern islands on Thursday, the country’s Meteorological Agency issued an ominous warning: More powerful quakes are possible, with the risk especially high over the coming week.

The Japan Meteorological Agency issues its first-ever “major earthquake warning,” warning that the Nankai Trough is a subduction zone where a magnitude 8-9 earthquake is likely to occur, and that there is a higher-than-usual risk of strong shaking and tsunamis. It urges people in the area to prepare.

The message is not a prophecy but an outlook for increasing risk, and it shows how far seismologists have come in understanding the dynamics of subduction-zone earthquakes.

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

Dangerous subduction zones

The Nankai Trough is an underwater subduction zone where the Eurasian plate collides with the Philippine Sea plate, causing the latter to subduct beneath the Eurasian plate and sink into the Earth’s mantle.

Faults in subduction zones build up stress, and when the locked faults slip and release that stress, so-called megaquakes occur. “Mechatronics” is the shortened form of the name. These zones have produced some of the most powerful earthquakes in Earth’s history.

The Pacific “Ring of Fire” is a collection of subduction zones. In the United States, the Cascadia Subduction Zone off the West Coast stretches from Vancouver Island in Canada to Cape Mendocino in California.

The Nankai Trough fault is divided into several segments, but if the entire edge of the fault slips at once, Japanese scientists say the trough will It could cause an earthquake of up to magnitude 9.1.

If a major earthquake were to occur off the coast of Japan, the Philippine Sea Plate would likely shake 30 to 100 feet near the country’s southeast coast, causing violent shaking.

Vertical displacement of the ocean floor could trigger a tsunami, sending waves crashing onto the coast of Japan that Japanese scientists estimate could reach heights of nearly 100 feet. Published in 2020.

History of major earthquakes

Large earthquakes occur in the Nankai Trough approximately every 100 to 150 years. In last year’s surveyJapan’s Earthquake Research Committee announced in January 2022 that there is a 70 to 80 percent chance of a major earthquake occurring in the next 30 years.

Great Nankai Trough earthquakes tend to occur in two separate events, with the second often occurring within two years of the previous one, recent examples being the “twin” earthquakes that occurred in the Nankai Trough in 1944 and 1946.

This phenomenon is due to the segmented nature of the fault, where a shift in one segment can put strain on other segments.

Thursday A magnitude 7.1 earthquake occurred at or near a subduction zone.According to the U.S. Geological Survey.

Harold Tobin, a professor at the University of Washington who studies the Nankai Trough, said the magnitude 7.1 quake occurred in a segment that shakes more frequently than other earthquakes. The regular quakes reduce stress, so there’s less concern that the segment itself could cause a big one. What’s concerning is the quake’s proximity to a segment that’s been building up stress since the 1940s.

“This one is adjacent to the Southwest Sea region and is obviously sealed off, so that’s a reason for caution and concern,” Tobin said.

Predictions, not predictions

Scientists cannot predict earthquakes, but they are developing the ability to forecast when danger is heightened, especially in areas like Japan where tremors occur frequently and monitoring is well established.