A silent colossus lurks off the Pacific coast, threatening hundreds of miles of coastline with devastating tsunamis and earthquakes.
For decades, scientists have warned about the potential of the Cascadia Subduction Zone, a megathrust fault that runs along the coast from northern Vancouver Island to Cape Mendocino in California. When the failure — or even part of it — occurs again, it will reshape life in Oregon, Washington and Northern California.
Of particular concern are signs of massive earthquakes in the region’s geological history. Many researchers have searched for clues about the last “big problem”: an 8.7 magnitude earthquake in 1700. They pieced together the story of the event using centuries-old tsunami records, Native American oral histories, physical evidence in saltwater-drowned ghost forests. and limited maps of the fault.
But no one had mapped the structure of the fault comprehensively – until now. A study published Friday in the journal Science Advances describes data collected during a 41-day research voyage in which a ship dragged a kilometer-long cable along the fault to listen to the seafloor and gather an 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 picture of the possible impacts of a megaquake 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, more localized view of the risks to communities along the Pacific Northwest coast and could help redefine building standards for earthquakes.
“It’s like putting on glasses from a Coke bottle and then taking them off and having 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. “Before, we had a very fuzzy, low-resolution view.”
Scientists have discovered that the subduction zone is much more complex than they previously imagined: It is divided into four segments that researchers believe could rupture independently of each other or together, all at once. The segments have different rock types and varying seismic characteristics – meaning some can be more dangerous than others.
Earthquake and tsunami modelers are beginning to evaluate 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 her team, which focuses on earthquake and tsunami risk, is already using the data to inform projections.
“The accuracy and resolution are truly unprecedented. And it’s an incredible data set,” said Wang, who is also an adjunct professor at the University of Victoria in British Columbia. “It just allows us to do a better job of assessing risk and having information for building codes and zoning.”
Harold Tobin, co-author of the paper and director of the Pacific Northwest Seismic Network, said that while the data helps adjust projections, it doesn’t change the hard-to-swallow reality of living in the Pacific Northwest. .
“We have the potential for earthquakes and tsunamis as large as the largest we have ever experienced on the planet,” said Tobin, who is also a professor at the University of Washington. “Cascadia appears capable of generating magnitude 9 or a little smaller or a little larger.”
Such a powerful earthquake can cause tremors that last about five minutes and generate tsunami waves up to 25 meters high. It would damage well over half a million buildings, according to emergency planning documents.
Neither Oregon nor Washington are sufficiently prepared.
To map the subduction zone, researchers at sea performed active-source seismic imaging, a technique that sends sound to the ocean floor and then processes the echoes that return. The method is often used for oil and gas exploration.
They towed a more than 15-kilometer-long cable called a streamer behind the boat, which used 1,200 hydrophones to capture the returning echoes.
“This gives us a picture of what the subsurface looks like,” Carbotte said.
Trained marine mammal observers alerted the crew to any signs of whales or other animals; The sound generated with this type of technology can be disturbing and harm sea creatures.
Carbotte said the new research makes it clearer that the entire Cascadia fault may not rupture at once.
“The next earthquake that happens in Cascadia could be rupturing just one of these segments or it could be rupturing the entire margin,” Carbotte said, adding that several individual segments are considered capable of producing earthquakes of at least magnitude 8.
Over the past century, scientists have observed just five earthquakes of magnitude 9.0 or greater — all megathrust tremors like the one predicted for the Cascadia Subduction Zone.
Scientists have pieced together an understanding of the last Cascadia earthquake, in 1700, in part through Japanese records of an unusual orphan tsunami that was not preceded by tremors there.
“It takes an 8.7 to bring a tsunami to Japan,” Tobin said.
The people who recorded the incident in Japan could not have known that the ground had shaken an ocean in what is now the United States.
Today, the Cascadia Subduction Zone remains eerily silent. In other subduction zones, scientists often observe small earthquakes, which makes the area easier to map, according to Carbotte. That’s not the case here.
Scientists have a few theories as to why: Wang said the zone may be getting quieter as the fault accumulates stress. And now, we’re probably nearing the end.
“The recurring interval for large events in this subduction zone is on the order of 500 years,” Wang said. “It’s difficult to know exactly when this will happen, but certainly if we compare this to other subduction zones, it’s already quite late.”
This article was originally published in NBCNews. with
