Researchers get the clearest picture yet of the fault threatening the Pacific Northwest

A silent colossus lies off the coast of the Pacific Ocean, threatening hundreds of miles of coastline with destructive tsunamis and earthquakes.

For decades, scientists have warned about the potential of the Cascadia Subduction Zone, a megathrust fault that runs offshore along the northern coast of Vancouver Island to Cape Mendocino, California. When the first fault – or even part of it – collapses, it will change life in Oregon, Washington and Northern California.

Of particular concern are the signs of massive earthquakes in the geological history of the region. Many researchers have found clues about the last “big one”: a magnitude 8.7 earthquake in 1700. They pieced together the history of the event using centuries-old records of tsunamis, Native American oral histories , physical evidence in ghost forests submerged by saltwater. and limited maps of the fault.

But no one has comprehensively mapped the fault structure — until now. A study published Friday in the journal Science Advances describes data collected during a 41-day research expedition in which a cable vessel followed a mile long along the fault to listen to the sea floor and piece together an image.

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

Their work will give modelers a closer look at the potential impact of a megathrust earthquake there — the term for an earthquake that occurs in a subduction zone, where one tectonic plate is pushed under another. It will also give planners a closer and more localized view of the risks to communities along the Pacific Northwest coast and could help redefine earthquake building standards.

“It’s like wearing coke bottle glasses and then you take the glasses off and you have the right prescription,” said Suzanne Carbotte, the paper’s lead author and a marine geophysicist and research professor at Columbia University’s Lamont-Doherty Earth Observatory. . “We had a very blurred vision before.”

The scientists discovered that the subduction zone is much more complex than they previously understood: It is divided into four segments that the researchers believe could rupture independently of each other or together all at the same time. The segments have different rock types and different seismic characteristics – meaning some may be more dangerous than others.

Earthquake and tsunami modelers are beginning to assess how the new data affects earthquake scenarios in the Pacific Northwest.

Kelin Wang, a research scientist at the Geological Survey of Canada who was not involved in the study, said his team, which focuses on earthquake hazard and tsunami risk, is already using the data to inform predictions .

“This accuracy and resolution is truly unprecedented. And it’s an amazing data set,” said Wang, who is also an assistant professor at the University of Victoria in British Columbia. “It allows us to do a better job of assessing the risk and having information for the building and zoning codes.”

Harold Tobin, co-author of the paper and director of the Pacific Northwest Seismic Network, said that while the data will help fine-tune predictions, it doesn’t change the tough-to-swallow reality of living in the Pacific Northwest. .

“We have the potential for earthquakes and tsunamis as large as the ones we’ve experienced on the planet,” said Tobin, who is also a professor at the University of Washington. “Cascadia appears to be able to generate magnitude 9 or a little less or a little more.”

A powerful tremor could cause shaking lasting about five minutes and generating tsunami waves up to 80 feet high. It would damage well over half a million buildings, according to emergency planning documents.

Oregon and Washington are not prepared enough.

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

They pulled a 9-mile-long cable, called a streamer, behind the boat, which used 1,200 hydrophones to pick up returning echoes.

“That gives us a picture of what the subsurface looks like,” Carbotte said.

ship at port dock research vessel Marcus G. Langseth (Courtesy of Harold Tobin)

ship at port dock research vessel Marcus G. Langseth (Courtesy of Harold Tobin)

Trained marine mammal observers alerted the team to any sign of whales or other animals; the sound generated by this type of technology can be disturbing and harmful to marine creatures.

Carbotte said the new research makes it clearer that the entire Cascadia fault may not rupture immediately.

“The next earthquake that occurs at Cascadia could be rupturing one of these segments or it could rupture the entire margin,” Carbotte said, adding that several individual segments are thought to be in there to produce at least magnitude-8 earthquakes.

In the past century, scientists have observed only five earthquakes of magnitude-9.0 or greater—all megathrust templars like the one projected for the Cascadia Subduction Zone.

Scientists pieced together an understanding of Cascadia’s last such quake, in 1700, in part through Japanese records of unusual orphan tsunamis that did not exist before the quake.

“It takes an 8.7 to get a tsunami all the way to Japan,” Tobin said.

The people who recorded the event in Japan did not know that the land shook the ocean, in the United States today.

Today, the Cascadia Subduction Zone remains quiet. In other subduction zones, scientists often observe small earthquakes, making it easier to map the area, according to Carbotte. That is not the case here.

Scientists have a number of theories as to why: Wang said the area may be becoming quieter as the fault stresses. And now, we are probably almost due.

“The recurrence interval for this subduction zone for major events is about 500 years,” Wang said. “It’s hard to know exactly when it’s going to happen, but certainly if you compare this to other subduction zones, it’s pretty late.”

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