Because scientists don’t know when that day will arrive, they prepare by trying to better understand the geological state of affairs.
To do that, a 70-meter-long ship sailed 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 a ‘streamer’ – a 15 kilometer long waterproof cable with 1,200 specialized microphones. Just as doctors use ultrasound to see inside the body, they used the data to piece together a comprehensive map of underwater geology in a study published Friday in the journal Science Advances. The new This tool will help scientists understand the range of earthquake and tsunami scenarios – and help policymakers come up with building codes that keep people safe.
The entire area, stretching from Northern California to Vancouver Island, is at risk. But the scientists discovered that the geometry of the fault off the coast of Washington, where the fault is flat and smooth, closer to the surface and extends further onshore, may be at particular risk.
“I’m glad I can use these results to make sure the shock estimates I make are as accurate as possible,” said Erin Wirth, a seismologist with the United States Geological Survey who was not involved in the study. “I must be busy now.”
A quiescent Cascadia subduction zone comes into view
For hundreds of years, the Cascadia subduction zone was silent. But on January 26, 1700, the earth shook. Evidence from Japanese history shows that an “orphan” tsunami swept across the Pacific Ocean, without a preceding earthquake. Oral histories of the Native Americans describe earthquakes and sea floods. Analysis of tree rings from “ghost forests” that died when the land abruptly sank helped scientists pinpoint the date. Scientists estimate that on that day, more than three centuries ago, a magnitude 9 earthquake occurred.
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 diving down beneath the North American continental plate. These plates move at the speed at which the fingernails grow, but can also become stuck and build up 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 typically learn those details by observing smaller earthquakes that burst from subduction zones. Because Cascadia has been eerily quiet in recent human history, many details remain vague.
“We had models for what the fault zone looked like, but they were based on missing data for much of the margin, and on small pieces of data and old quality data,” said Suzanne Carbotte, a marine seismologist at Lamont school of the Columbia Climate School. Doherty Earth Observatory, who led the research. “It’s like you had Coke bottles in front of your eyes, and they’re removed, and you have the right prescription. Now you can see where the fault zone is located. And not surprisingly, the fracture surface is much more complex than the picture we had before.”
A danger zone off the coast of Washington
The new study is expected to be the first of many scientific papers from the new dataset, but scientists have already made a few important findings. There is a particularly flat and smooth section of the fault, which extends from Washington State to southern Vancouver Island. In other similar fault systems around the world, those areas often produce the largest and most destructive earthquakes. That part of the fault is also shallow and closer to the surface than previous models, which could make it more dangerous, Wirth said.
Scientists have also found four segments along the fault, raising questions about whether the entire fault will disappear at once, or if segments can rupture individually.
“That’s the messy question, where the answer comes down to: sometimes it does one thing, and sometimes it does another,” says Harold Tobin, a seismologist at the University of Washington and author of the paper. Either scenario would constitute 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, in which two different segments of a fault quickly ruptured, both causing deadly tsunamis.
Kelin Wang, a researcher at the Geological Survey of Canada, has already started using the data to better understand how tsunamis can form.
“It’s such an overwhelmingly rich data set that offers so much information, in many directions,” Wang said.