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Scientists don’t know when that day will come, so they’re preparing by trying to better understand the geological situation.
To do so, the 235-foot-long vessel cruised along the coasts of Oregon, Washington and British Columbia for 41 days in the summer of 2021, sending sound waves deep into the ocean and recording the echoes with nine-mile-long waterproof cables called “streamers.” The streamers contain 1,200 specialized microphones. In the same way that doctors use ultrasound to see inside the body, the researchers used the data to create a comprehensive map of the ocean floor geology. The study was published Friday in the journal Science Advances. The resource helps scientists understand a range of earthquake and tsunami scenarios and also helps policymakers develop building codes that protect people.
The entire region is at risk, but scientists have not yet determined whether the fault line is correct. Areas off the coast of Washington state may be particularly at risk because the faults are flat, smooth, close to the surface and extend onshore.
“I’m looking forward to using these results to make sure that the shaking estimates that I’m making are as accurate as possible,” said Erin Wirth, a seismologist with the U.S. Geological Survey who was not involved in the study. “Now I’m going to be busy.”
Quiet Cascadia Subduction Zone Comes into Focus
The Cascadia Subduction Zone was quiet for hundreds of years. But on January 26, 1700, an earthquake struck. Japanese historical records show that an “orphan” tsunami swept across the Pacific Ocean without warning of the earthquake. Native American oral traditions tell of earthquakes and seawater inundation. Scientists were able to determine the date by analyzing tree rings from “ghost forests” that died from the sudden subsidence. Scientists estimate that a magnitude 9 earthquake occurred on that day more than three centuries ago.
What makes this fault zone so dangerous is that it’s a “mega fault.” A tectonic plate called the Juan de Fuca plate, part of the Earth’s crust, is subducting beneath the North American continental plate. These plates move at the speed of a fingernail, but they can also freeze together and build up stress. The 2011 Tohoku earthquake in Japan and the 2004 Indian Ocean earthquake and tsunami occurred at subduction zones.
But details matter to understand earthquakes, and seismologists typically get those details by observing the smaller quakes that occur in subduction zones. Because Cascadia has been eerily quiet in recent human history, many details remain unknown.
“We had models of what fault zones looked like, but those models were based on little data and old quality data, with no data for most of the boundaries of the fault zones,” said study leader Suzanne Calbot, a marine seismologist at Columbia University’s Lamont-Doherty Earth Observatory in the Department of Climatology and leader of the study. “It’s like having a Coke bottle over your eyes, and then you take it off and you have the right prescription. Now we know where the fault zones are, and of course the fault planes are a lot more complicated than what we thought the picture was before.”
Danger zone off the coast of Washington
The new study is expected to be the first of many scientific papers to be published from the new dataset, but scientists have already made some key discoveries. The fault has a particularly flat, smooth section that stretches from Washington state to southern Vancouver Island. In other similar fault systems around the world, these areas often produce the largest and most destructive earthquakes. This section of the fault is also shallower and closer to the surface than previous models suggest, which could make it more dangerous, Wirth said.
Scientists also found four segments along the fault, raising questions about whether the entire fault would collapse at once or whether each segment would break separately.
“That’s a tricky question, and the answer is: sometimes it’s this way, and sometimes it’s the other way,” said Harold Tobin, a seismologist at the University of Washington and author of the study. Either scenario would result in a massive natural disaster, the equivalent of the difference between one magnitude-9 earthquake and two magnitude-8 earthquakes. Tobin cited two major earthquakes in Japan in 1944 and 1946, when two different parts of a fault line ruptured in a short space of time, and both triggered deadly tsunamis.
Kelin Wang, a research scientist with the Geological Survey of Canada, has already begun using the data to better understand how tsunamis form.
“This is a very rich dataset that provides a lot of information across many dimensions,” Wang said.
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