A silent colossus lurks off the coast of the Pacific Ocean, threatening hundreds of miles of coastline with tsunamis and devastating 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, California. When the failure — or even part of it — is next, it will change life in Oregon, Washington and Northern California.
Of particular concern are the signals of massive earthquakes in the geologic history of the region. Many researchers have been chasing clues to the last “big one”: an 8.7 magnitude earthquake in 1700. They have pieced together the story of the event using centuries-old records of tsunamis, Native American oral histories, physical evidence in ghostly forests submerged in salt water and limited damage maps.
But no one had 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 voyage in which a vessel ran a long cable along the fault to listen to the seafloor and assemble an image.
The team completed a detailed map of more than 550 miles from the subduction zone, to the border between Oregon and California.
Their work will give modelers a clearer picture of the possible effects of a megathrust earthquake there – the term for an earthquake that occurs in a subduction zone where one tectonic plate is pushed beneath another. It will also give planners a closer, localized view of the risks to communities along the Pacific Northwest coast and could help redefine earthquake standards.
“It’s like wearing glasses like a Coke bottle and then taking the glasses off and having the right prescription,” said Suzanne Carbott, lead author of the report and a marine geophysicist and research professor at Columbia University’s Lamont-Doherty Earth Observatory. “Before, we had a very blurry, low-resolution view.”
Scientists have discovered that the subduction zone is much more complex than previously understood: It is divided into four segments, which the researchers say can tear apart independently or together all at once. 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 evaluate how the new data affects earthquake scenarios in the Pacific Northwest.
Kellyn Wang, a researcher 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.
“The accuracy and this 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 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 fine-tune forecasts, it doesn’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 slightly smaller or slightly larger.”
An earthquake this strong can cause shaking lasting about five minutes and generate tsunami waves up to 80 feet high. That would damage more than 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 down to the ocean floor and then processes the echoes that return. The method is often used for oil and gas exploration.
They towed over 9 miles of cable, called a streamer, behind the boat, which used 1,200 hydrophones to pick up the returning echo.
“It gives us a picture of what the subsurface looks like,” Carbott said.
Trained marine mammal spotters alerted the crew to any signs of whales or other animals; the sound generated with this kind of technology can be destructive and harm sea creatures. Carbott said the new research makes it clearer that the entire Cascadia Fault may not collapse at once.
“The next earthquake that occurs in Cascadia could rupture just one of these segments, or it could rupture the entire margin,” Carbott said, adding that several separate segments are believed to be capable of producing earthquakes of at least magnitude 8.
In the past century, scientists have observed only five earthquakes of magnitude 9.0 or greater—all megashocks like the one predicted for the Cascadia subduction zone.
Scientists have gleaned an understanding of the last such earthquake in Cascadia in 1700, in part through Japanese records of an unusual orphan tsunami that was not preceded by a tremor there.
“It takes 8.7 to get a tsunami all the way to Japan,” Tobin said.
The people who recorded the incident in Japan could not have known that the ground shook an entire ocean away in what is now the United States.
Today, the Cascadia Subduction Zone remains eerily quiet. In other subduction zones, scientists often observe small earthquakes, making the area easier to map, according to Carbot. Not so here.
Scientists have several theories as to why: Wang said the zone may be getting quieter as the fault builds up stress. And now we are probably approaching the deadline.
“The recurrence interval for this subduction zone for major events is on the order of 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.”
This article was originally published on NBCNews.com
