Researchers got a clear picture of the fault threatening the Pacific Northwest

A silent giant lurks off the Pacific coast, threatening hundreds of miles of coastline with devastating tsunamis and earthquakes.

For decades, scientists have warned of the possibility of the Cascadia Subduction Zone, a massive fault that runs offshore along the coast from northern Vancouver Island to Cape Mendocino, California. When the rift — or even part of it — occurs next, it will reshape life in Oregon, Washington and northern California.

Of particular concern are references to large earthquakes in the geological history of the region. Many researchers have chased evidence of the last “big earthquake”: an 8.7-magnitude quake in 1700. They pieced together a history of the event using centuries-old records from tsunamis, Native American oral histories, and physical evidence in ghost forests flooded with salt water. and limited error maps.

But no one has comprehensively mapped the structure of the fault, until now. A study published Friday in the journal Science Advances describes data collected during a 41-day research cruise in which a ship tracked a miles-long cable along the fault to listen to the seafloor and piece together the image.

The team completed a detailed map of more than 550 miles of the subduction zone, all the way to the Oregon-California border.

Their work will give modelers a clearer view of the potential effects of a strong earthquake there, the term given to an earthquake that occurs in a subduction zone, where one tectonic plate pushes under another. It will also provide planners with a closer, local look at the risks to communities along the Pacific Northwest Coast and could help redefine seismic building standards.

“It's like wearing Coca-Cola bottle glasses and then you take the glasses off and you have the correct prescription,” said Susan Carbut, the paper's lead author, a marine geophysicist and research professor at Columbia University's Lamont-Doherty Earth Observatory. “We had a fuzzy, low-resolution view before.”

Scientists found that the subduction zone is much more complex than they previously understood: it is divided into four parts that researchers believe could rupture independently of each other or together simultaneously. These parts contain different types of rocks and different seismic characteristics, which means that some may be more dangerous than others.

Earthquake and tsunami modelers are beginning to evaluate how new data will affect seismic scenarios in the Pacific Northwest.

Kellen Wang, a research scientist at the Geological Survey of Canada who was not involved in the study, said his team, which focuses on earthquake and tsunami hazards, is already using the data to guide forecasts.

“The accuracy and resolution is really unprecedented. “It's an amazing data set,” said Wang, who is also an assistant professor at the University of Victoria in British Columbia. “It just allows us to do a better job of assessing risk and getting information about building codes and zoning.”

Although the data will help improve forecasts, it does not change the reality of living in the Pacific Northwest, which is difficult to accept, said Harold Tobin, study co-author and director of the Pacific Northwest Seismic Network. .

“We have the potential for earthquakes and tsunamis on the scale of the largest earthquakes we've ever seen on this planet,” said Tobin, who is also a professor at the University of Washington. “The Cascadia appears to be capable of generating a force of 9 or a little smaller or a little larger.”

An earthquake of this magnitude can cause shaking that lasts about five minutes and generates tsunami waves up to 80 feet high. It would damage more than half a million buildings, according to emergency planning documents.

Neither Oregon nor Washington is adequately prepared.

To map the subduction zone, researchers at sea conducted active source seismic imaging, a technique that sends sound to the ocean floor and then processes the echoes that return. This method is often used for oil and gas exploration.

They towed a cable more than 9 miles long, called a streamer, behind the boat, which used 1,200 hydrophones to pick up the returning echoes.

“This gives us a picture of what the underground surface looks like,” Carbot said.

Research vessel Marcus Langseth docks in Seattle after a 41-day expedition along the Pacific Coast. The ship allowed researchers to map the Cascadia subduction zone. Courtesy of Harold Tobin

Trained marine mammal monitors alerted the crew to any sign of whales or other animals; The sound produced by this type of technology can be annoying and harmful to marine organisms. The new research further demonstrates that the entire Cascadia Fault may not explode at once, Carbot said.

“The next earthquake that happens in Cascadia could rupture just one of these segments or it could rupture the entire rim,” Carbot said, adding that many individual segments are believed to be capable of producing earthquakes of at least a magnitude 8.

Over the past century, scientists have observed only five earthquakes of magnitude 9.0 or higher, all of them as strong as those expected in the Cascadia subduction zone.

Scientists have pieced together an understanding of the last Cascadia earthquake, which occurred in 1700, in part through Japanese records of unusual, never-preceded tsunami waves there.

“It takes a magnitude of 8.7 for a tsunami to reach all the way to Japan,” Tobin said.

The people who recorded the event in Japan could not have known that the ground had shaken an ocean away, in present-day United States.

Today, the Cascadia subduction zone remains eerily quiet. In other subduction zones, scientists often observe small earthquakes frequently, which makes mapping the area easier, according to Carbot. This is not the case here.

Scientists have a few theories about why: Wang said the area may become quieter as pressure builds on the fault. Now, we may be closer to the deadline.

“The recurring interval of this subduction zone in major events is about 500 years,” Wang said. “It's hard to know exactly when that will happen, but certainly if you compare this to other subduction zones, it's too late.”

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