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 mega-thrust fault that extends offshore along the coast from northern Vancouver Island to Cape Mendocino, California. When the fault – or even part of it – ruptures again, it will reshape life in Oregon, Washington and Northern California.
Of particular concern are the signals of massive earthquakes in the region’s geological history. Many researchers have searched for clues about the last “great earthquake”: a magnitude 8.7 earthquake in 1700. They pieced together the history of the event using centuries-old tsunami records, Native American oral histories, physical evidence in ghost forests drowned by salt water. and limited maps of the fault.
But no one had comprehensively mapped the structure of the faults – 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 cable several kilometers along the fault to listen to the seafloor and piece together an image .
The team produced a detailed map of more than 550 miles of the subduction zone, extending to the Oregon-California border.
Their work will give modelers a more precise view of the possible impacts of a mega-earthquake – the term for an earthquake that occurs in a subduction zone, where one tectonic plate is pushed beneath another. It will also provide planners with a more in-depth, localized look at risks to communities along the Pacific Northwest coast and could help redefine building standards in the event of an earthquake.
“It’s like putting glasses in a Coca-Cola bottle and then taking the glasses out and you have the right prescription,” said Suzanne Carbotte, lead author of the paper, a marine geophysicist and research professor. at the Lamont-Doherty Earth Observatory at Columbia University. “We previously had a very blurry low-resolution view.”
Scientists have discovered that the subduction zone is much more complex than previously thought: It is divided into four segments that researchers believe could rupture independently of each other or together at the same time. The segments have different rock types and varying seismic characteristics, meaning some could be more dangerous than others.
Earthquake and tsunami modelers are beginning to evaluate the impact of new data on earthquake scenarios for 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 and tsunami risks, is already using the data to inform projections .
“The precision and this resolution are truly unprecedented. And it’s an incredible 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 having information about building codes and zoning.”
Harold Tobin, co-author of the paper and director of the Pacific Northwest Seismic Network, said that while the data will help refine projections, it won’t change the hard-to-swallow reality of life in the Pacific Northwest . .
“We have the potential for earthquakes and tsunamis as large as the largest we’ve 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.”
A powerful earthquake could cause shaking that would last about five minutes and generate 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 sufficiently prepared.
To map the subduction zone, offshore researchers performed active-source seismic imaging, a technique that sends sound to the ocean floor and then processes the returning echoes. The method is often used for oil and gas exploration.
They towed a more than 9-mile-long cable, called a streamer, behind the boat, which used 1,200 hydrophones to capture the returned echoes.
“It gives us an idea of what the basement looks like,” Carbotte said.
Trained marine mammal observers alerted the crew to any signs of whales or other animals; The sound generated by this type of technology can be disruptive and harm sea creatures.
Carbotte said the new research shows more clearly that the entire Cascadia Fault may not rupture all at once.
“The next earthquake that happens in Cascadia could rupture just one of these segments or the entire margin,” Carbotte said, adding that several individual segments would be capable of producing at least magnitude 8 earthquakes.
Over the past century, scientists have observed only five earthquakes of magnitude 9.0 or greater – all mega-thrust earthquakes like the one predicted for the Cascadia subduction zone.
Scientists have pieced together their understanding of the last such earthquake in Cascadia, in 1700, in part thanks to Japanese records of an unusual orphan tsunami that was not preceded by shaking.
“It takes an 8.7 for a tsunami to reach Japan,” Tobin said.
The people who recorded the incident in Japan could not have known that the ground shook an ocean, in present-day United States.
Today, the Cascadia subduction zone remains extremely quiet. In other subduction zones, scientists often observe small earthquakes, which makes the area easier to map, according to Carbotte. That is not the case here.
Scientists have a few theories as to why: Wang said the area could become calmer as the fault accumulates stress. And now we’re probably nearing the deadline.
“The recurring interval for this subduction zone for large events is on the order of 500 years,” Wang said. “It’s hard to know exactly when this will happen, but if you compare it to other subduction zones, it’s certainly quite late.”