Scientists have long predicted a giant 9.0 magnitude earthquake that will reverberate from the Cascadia Fault in the Pacific Northwest and quickly unleash colossal waves crashing onto the shore.
A new study , published last month in the peer-reviewed journal Earth-Science Reviews, points to such a missing piece. Researchers have revealed a previously unknown relationship between the severity of an earthquake-triggered tsunami and the so-called “outer wedge”, the area between the main seismic fault and the seabed.
Sylvain Barbot, co-author of the study, described the outer corner as the “garbage bag of subduction zones”, the place where two tectonic plates collide and can produce an earthquake, because this is where sediments accumulate.
The researchers’ findings suggest that the wider it is, the larger the maximum size of the tsunami will be.
The connection adds a new element to consider when predicting tsunamis, one that the authors say could mean increased worst-case scenario forecasts for some faults, including Cascadia.
“There are places where [le coin extérieur est] tiny, so good news,” said Barbot, an associate professor of earth sciences at the University of Southern California. “And there are places where it’s huge. -western Pacific.
For about two years, he and co-author Qiang Qiu, of the South China Sea Institute of Oceanology, studied 11 “tsunami earthquakes” that occurred across the world in the past 200 years. . These rare events involve less powerful earthquakes (the authors looked at those measuring 7.1 to 8.2 magnitudes) that produce huge tsunamis and have long puzzled scientists.
They found a correlative relationship between the maximum height of the tsunami and the outer corner. The wider it is, Barbot explained, the more faults there are, the more likely it is to move the seafloor and therefore the more extreme the tsunami can be.
“Imagine a bookshelf full of books, and you take the books and tilt them all at 45 degrees… The interface between any book is a defect. And so, in an outside corner, you have all those books , and all those flaws in between. And they can provide a pathway for the breakup to go up, instead of going left,” he explained.
From there, they used those findings to make tsunami predictions for dozens of other active subduction zones around the “Ring of Fire,” a nearly 25,000-mile path where most tremors occur. of land in the world.
Toward the top of that list was the 600-mile Cascadia Subduction Zone. It stretches from Vancouver Island, Canada to northern California and is poised for its next big earthquake. Its last Big One was in 1700, and current estimates point to a roughly 15% chance of a magnitude 9.0 earthquake within the next 50 years.
An article from the New Yorker Pulitzer Prize winner in 2015 drew attention to the subduction zone, describing its upcoming large-scale earthquake “as the worst natural disaster in North American history, outside of the earthquake of 2010 in Haiti”.
The site, according to the authors, has a fairly wide outer bevel (between 15 and 43 km). According to their research, this suggests that the earthquake-triggered tsunami could reach over 200 feet (61 meters). While there is a range of predictions for the Big One, it’s about twice as high as some of the most severe scenarios previously considered.
Compared to 30 other subduction zones analyzed by the study’s authors, Cascadia ranked fifth in terms of tsunami severity. It is behind subduction zones such as Makran (in Pakistan and Iran), Aleutians (in Alaska) and Lesser Antilles (in the Caribbean), according to the authors.
Barbot explained that the results need further validation, but that they could ultimately lead not only to changes in tsunami forecasts, but also to emergency preparedness in these regions.
“If you’re preparing for a 30-meter tsunami and a 60-meter tsunami is coming, you basically have to double the height of your evacuation zones,” he said. “You have to change where you plan to build infrastructure, like hospitals and schools. It also changes, in a more practical sense, essentially the price of insurance for real estate. It changes risk, essentially, and how it’s distributed across space.
But of course, this outer bevel is not the only variable that can influence the size of a tsunami. Many other factors come into play, including the slope of the seabed and the general topography.
Harold Tobin, director of the Pacific Northwest Seismic Network and professor of earth and space sciences at the University of Washington, warned that if this study reveals an interesting new discovery, further research is needed to take full account of it. these other variables.
He explained that it would be premature to jump to conclusions or begin to change how the Pacific Northwest or other regions prepare for tsunamis.
“What we need to do is consider the evidence this paper has given us to build better models for all of this; to refine and improve the scenarios we are preparing for,” Tobin said. “But on its own, that doesn’t mean we have to suddenly say, ‘OK, there’s double the risk of a tsunami than before.’ It just points to a possible mechanism that could mean that the risk of a tsunami could be greater than previously thought.
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Study raises risk of mega-tsunami in NW Pacific