A silent colossus lurks off the coast of the Pacific Ocean, threatening hundreds of kilometers 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. The next time the fault — or even part of it — ruptures, 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 looking for clues to the last “big one”: an 8.7 magnitude earthquake in 1700. They pieced together the history of the event using centuries-old tsunami records, Native American oral histories, and physical evidence in the saltwater drowned ghost forests. and limited fault maps.
But no one has comprehensively mapped the structure of the disorder—until now. The study, published Friday in the journal Science Advances, describes data collected during a 41-day research cruise in which the vessel dragged a mile-long cable along the fault to listen to the sea floor and piece together a picture.
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 sharper look at the possible effects of a megashock earthquake there — the term for an earthquake that occurs in a subduction zone, where one tectonic plate slides under another. It will also give planners a closer, localized view of the risks to communities along the Pacific Northwest coast and could help redefine building standards for earthquakes.
“It’s like having Coke bottle glasses on and then you take them off and you have the correct prescription,” said Suzanne Carbotte, lead author of the paper 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 realized: It is divided into four segments, which scientists believe could tear apart independently or together all at once. Segments have different rock types and different seismic properties – meaning some may be more dangerous than others.
Earthquake and tsunami modelers are beginning to evaluate how the new data affects earthquake scenarios for the Pacific Northwest.
Kelin Wang, a 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 projections.
“The accuracy and this resolution is truly unprecedented. And it’s an amazing data set,” said Wang, who is also an adjunct professor at the University of Victoria in British Columbia. “It allows us to do a better job of risk assessment and have 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 will help fine-tune projections, it doesn’t change the hard-to-digest 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 to be capable of generating a magnitude 9 or a bit smaller or a bit larger.”
A strong earthquake could cause tremors that last about five minutes and generate tsunami waves up to 80 feet 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, scientists at sea performed active source seismic imaging, a technique that sends sound to the ocean floor and then processes the echoes that come back. The method is often used for oil and gas exploration.
Behind the boat they trailed a 9-plus mile long cable, called a streamer, which used 1,200 hydrophones to pick up the returning echoes.
“This gives us a picture of what the subsurface layer looks like,” Carbotte said.
Trained marine mammal spotters alerted the crew to any sign of whales or other animals; the sound generated by this kind 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 at once.
“The next earthquake that occurs in Cascadia could rupture just one of these segments, or it could rupture the entire rim,” Carbotte said, adding that several individual segments are believed to be capable of triggering an earthquake of at least magnitude 8.
In the past century, scientists have observed only 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 such earthquake in Cascadia in the 1700s, in part through Japanese records of an unusual orphan tsunami that did not precede the tremor.
“To get a tsunami all the way to Japan, it takes 8.7,” Tobin said.
The people who recorded the incident in Japan could not have known that the land in what is now the United States shook the ocean.
Today, the Cascadia subduction zone remains eerily quiet. In other subduction zones, scientists often observe small earthquakes, which makes it easier to map the area, Carbotte says. That’s not the case here.
Scientists have several theories about why: Wang said the zone may be quieter because the fault is accumulating stress. And now the deadline is probably approaching.
“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 this will happen, but if you compare it to other subduction zones, it’s quite late.”
This article was originally published on NBCNews.com