Because scientists don’t know when that day will come, they’re preparing by trying to better understand the geological state of the game.
To do this, the 235-foot ship 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 echoes using a “streamer” — a 9-mile-long waterproof cable containing 1,200 specialized microphones. Much like doctors use ultrasound to see inside the body, they used the data to create a comprehensive map of underwater geology in a study published Friday in the journal Science Advances. New the resource will help scientists understand a range of earthquake and tsunami scenarios—and help policymakers design building codes that protect people.
The entire area that stretches from Northern California to Vancouver Island is threatened. But the researchers found that the geometry of the error off the coast of Washington, where the fault is flat and smooth, closer to the surface and extends further inland, it may be particularly at risk.
“I’m excited to use these results to make sure the earthquake estimates I make are as accurate as they can be,” said Erin Wirth, a seismologist with the United States Geological Survey who was not involved in the study. “I’ll be busy now.
The quiet Cascadia subduction zone is in the spotlight
For hundreds of years, the Cascadia subduction zone was quiet. But on January 26, 1700, the earth shook. Japanese historical records show that an “orphan” tsunami traveled across the Pacific without a previous earthquake. Native American oral histories describe earthquakes and sea floods. Analysis of tree rings from “ghost forests” that died when the land suddenly retreated helped scientists determine the date. Scientists estimate that a magnitude 9 earthquake struck on that day more than three centuries ago.
This fault zone is dangerous because it is a “megathrust” fault. One piece of the Earth’s crust, a tectonic plate called the Juan de Fuca plate, is subducting beneath the North American continental plate. These plates move at the rate of nail growth, but they also get stuck and create tension. The 2011 Tohoku earthquake in Japan and the 2004 Indian Ocean earthquake and tsunami occurred in subduction zones.
But to understand earthquakes, details matter. And seismologists usually learn these details by observing smaller earthquakes that rupture subduction zones. Since Cascadia has been eerily quiet in recent human history, many specifics remain unclear.
“We had models of what the fault zone looked like, but they weren’t based on any data for a large part of the reserve and small bits of data and old quality data,” said Suzanne Carbotte, a marine seismologist at the Columbia Climate School’s Lamont-Doherty Earth Observatory, who led the study . “It’s like having coke bottles in your eyes and they’ve been removed and you have the correct prescription.” Now you can see where the error zone is. And not surprisingly, the fault surface is much more complex than the picture we had before.”
Danger zone off the coast of Washington
The new study is expected to be the first of many scientific papers from the new data set, but the researchers have already made several key findings. There is a particularly flat and smooth part of the fault that spans 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 part of the fault is also shallow and closer to the surface than previous models, which could make it more dangerous, Wirth said.
The researchers also found four segments along the fault, raising the question of whether the entire fault would go at once or if the segments could rupture individually.
“That’s the tricky question, where the answer is more like: sometimes it does one thing and sometimes it does another,” said Harold Tobin, a seismologist at the University of Washington and an author of the paper. Both scenarios would represent a major natural disaster, perhaps the difference between one magnitude 9 earthquake and two magnitude 8 earthquakes. Tobin pointed to a pair of massive earthquakes in Japan in 1944 and 1946 that ruptured two different fault segments in a short period of time, and both caused a deadly tsunami.
Kelin Wang, a scientist with the Geological Survey of Canada, has already begun using the data to better understand how tsunamis can form.
“It’s such a tremendously rich data set that provides so much information in so many ways,” Wang said.