by After a 7.1-magnitude earthquake struck southern Japan on Thursday, the country’s meteorological agency issued an ominous warning: Another, larger earthquake could be coming, and the risk will be especially high during next week.
In the first “megaquake advisory” it has ever issued, the agency said the risk of strong earthquakes and tsunamis is greater than usual in the Nankai Trough, a subduction zone that could produce 8 or 9 temblors. Residents of the area, he said, should prepare.
The message was not a prediction, but a forecast of enhanced risk – and it shows how far seismologists have come to understand the dynamics of earthquakes in the subduction zone.
Here’s what you know about the case.
Dangerous subduction zone
The Nankai Trough is an underwater subduction zone where the Eurasian Plate collides with the Philippine Sea Plate, pushing the latter under the former and into the Earth’s mantle.
Subduction zone faults build up stress, and a so-called megathrust earthquake occurs when a locked fault slips and releases that stress. “Megaquake” is a shortened version of the name. These zones have produced 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 runs off the West Coast from Vancouver Island, Canada, to Cape Mendocino, California.
The Nankai Trough fault has several sections, but if the entire margin of the fault were to slip at once, Japanese scientists believe that the trough is capable of producing an earthquake of up to magnitude 9.1.
If a major earthquake occurred near Japan, the Philippine Sea Plate would lurch, perhaps 30 to 100 feet, near the southeastern coast of the country, producing intense shaking.
The vertical displacement of the seabed would create a tsunami and push waves towards the coast of Japan. Those waves could reach nearly 100 feet in height, according to estimates by Japanese scientists published in 2020.
The history of great earthquakes
The Nankai Trough has experienced major earthquakes about every 100 to 150 years, a study showed last year. The Japan Earthquake Research Committee said in January 2022 that there was a 70% to 80% chance of a megathrust earthquake in the next 30 years.
Nankai Great Basin earthquakes tend to come in pairs, with the second often rupturing in the following two years. The most recent examples were the “twin” earthquakes on the Nankai Trough in 1944 and 1946.
The phenomenon is due to the segmented nature of the fault; when one segment slips, it can stress another.
Thursday’s magnitude-7.1 earthquake occurred on or near the subduction zone, according to the United States Geological Survey.
Harold Tobin, a University of Washington professor who studied the Nankai Trough, said the 7.1-magnitude quake occurred in a segment that shakes more often than others. Regular earthquakes can relieve stress, so the possibility of the segment itself producing a major earthquake is less of a concern. The concern is how close the earthquake is to an item that has been adding stress since the 1940s.
“It is close to the western Nankai region and that is clearly locked. That’s the reason for the caution and the concern,” Tobin said.
A forecast, not a prediction
Scientists cannot predict earthquakes, but they are developing the ability to predict times of increased risk, especially in areas with frequent shaking and good monitoring equipment, such as Japan.
Japanese authorities are asking residents to prepare, review evacuation routes and heed possible future warnings.
Although the risk of a major earthquake is higher than usual, that does not mean it will happen anytime soon. Japanese government warning guidelines indicate that the chance of a major magnitude-7 earthquake following within a week is about “one in several hundred times,” according to the study last year.
The most likely outcome is that the recent shaking will not trigger anything, although the probability of an earthquake is higher.
“It may be years before we have another earthquake at Nankai,” Tobin said.
Known danger
In 2011, an area of the ocean floor about the size of Connecticut collapsed at the same time, creating a magnitude-9.1 earthquake – the third largest earthquake recorded worldwide since 1900. That mega earthquake caused a tsunami off the east coast of Japan. More than 18,000 people died in the tsunami and earthquake, according to the US National Oceanic and Atmospheric Administration.
The following year, the Japanese government reviewed its natural disaster scenarios and found that approximately 323,000 people could die in a worst-case earthquake on the Nankai Basin, mainly due to the effects of a tsunami.
The Cascadia Subduction Zone poses a similar risk to the US West Coast, although megathrust earthquakes are expected there less frequently – every 300 to 500 years. This fault has the potential to produce a magnitude-9.1 earthquake and tsunami waves 80 feet high. Recently researchers mapped the fault in detail and found that it was divided into four parts.
This article was originally published on NBCNews.com
