A clearer picture of Cascadia emerges from modern maps.

Information collected during a 2021 research cruise provides insight into the potential behavior of the most enigmatic rift in the Northwest Pacific Ocean.

By Rebecca Owen, Science Writer (@beccapox)

Citation: Owen, R., 2024, A clearer picture of Cascadia emerges from modern cartography, Temblor, http://doi.org/10.32858/temblor.347

On January 26, 1700, a magnitude 9.0 earthquake struck the Pacific Northwest. The ensuing tsunami and coastal flooding are immortalized in history. They are mentioned in traditional stories passed down through generations of local indigenous people, in records of a single tsunami that crossed the Pacific to Japan without the locals noticing the quake, and in the dead and submerged trees (or “ghost forests”) that haunt the Pacific Northwest coast today. Since that event more than three hundred years ago, the Cascadia subduction zone has been remarkably quiet. Its relative lack of movement—even small earthquakes are rare—puzzles scientists who study its behavior.

The Cascadia subduction zone, where the Juan de Fuca plate descends beneath the North American plate, stretches 700 miles from Cape Mendocino in northern California to Vancouver Island in British Columbia. People living in coastal communities along the rugged, rocky Pacific coast know the potential dangers that arise as the plates slowly shift beneath them—as do residents of the major metropolitan areas of Portland, Seattle and Vancouver, British Columbia. An earthquake and tsunami similar to the 1700 event would affect millions of people from California to Canada, likely causing significant deaths, displacement and damage—particularly to infrastructure—that would impact and change the region for years to come.

A new study published in Science Advances reports the results of the first-ever oceanic research expedition to map nearly the entire Cascadia subregion from the Oregon border north to British Columbia. The study provides high-resolution images of the rift zone that could help researchers, policymakers and residents understand more about this sleeping giant.

Map showing the location of the Cascadia subduction zone. Image credit: Temblor, CC BY-NC-ND 4.0

Outdated information provides an incomplete picture.

Previous studies suggest that the Cascadia subduction zone triggers a major earthquake every 300 to 500 years. If this pattern continues, the Pacific Northwest could experience a devastating earthquake and tsunami, what some are calling a “really big earthquake.” But missing details about the fault’s structure and mechanics limit our understanding of how a major earthquake might eventually occur.

Much of what we know about the fault system and fault history of the Cascadia subduction zone comes from models produced in the 1980s and 1990s, says Harold Tobin, a seismologist at the University of Washington and one of the study’s authors. Although geophysical imaging from that period established the general structure of the subduction zone, those early surveys allowed only a basic interpretation. “There’s never been a single survey that really covered everything from north to south,” Tobin says.

Previous models that reconstructed the 1700 Cascadia subduction zone earthquake using “vintage” data showed several zones of potential slip along the fault, says Susan Carbot, a marine geophysicist at Columbia University’s Lamont-Doherty Earth Observatory and lead author of the study. Some of these zones appear to have less or more slip, known as fragmentation, she says. Turbidite deposits form when an event such as an earthquake, storm, or tsunami causes an underwater landslide to flow down a steep continental slope, depositing a distinct series of sediment layers. Previous studies have analyzed turbidite deposits for their ages, and those results have roughly confirmed the models’ interpretation of where the Cascadia subduction zone ruptured in 1700.

But the information researchers can extract from an earthquake that occurred 300 years ago is very limited.

“The Cascadia subduction zone is very quiet,” Carbot says. “There are very few small earthquakes. In most subduction zones, you have lots of small earthquakes all the time.” These small, relatively harmless earthquakes can help scientists pinpoint the fault’s location and geometry. But Cascadia lacks this crucial information.

With limited and outdated data, scientists needed better mapping and imaging of the subduction zone to understand the potential scope of the next event. “That’s why this survey was conceived,” Tobin says. “Let’s take a modern approach.”

Sailing along the Cascadia

In the summer of 2021, researchers embarked on a 41-day expedition off the coasts of Oregon, Washington, and British Columbia aboard the 235-foot research vessel Markus J. Langseth. Their goal was to map and image the subduction fault to better determine its geometry and capture images of its potential divisions.

Some communities in the Pacific Northwest may face greater shaking risk during future earthquakes, because different parts of the region may experience varying degrees of slippage and rupture, Carbot says. “If there’s a real geological structure contributing to that division, we can predict that that division is more likely to contribute to the next earthquake,” she says.

As the Langseth sailed, it was towing a 9-foot cable equipped with air cannons and a seismometer that was about 7.5 miles long and consisted of 1,200 hydrophones. The air cannons sent out pulses that bounced off the seafloor, while the hydrophones recorded those echoes. “We’re using sound to image the interior of the Earth,” Carbot says. The method is similar to taking ultrasound pictures of the seafloor, providing more detailed and higher-resolution images than previous surveys. The hydrophones provide data that helps scientists pinpoint the location, shape and angles of the fault zone—invaluable information for scientists looking to model how the fault will behave when it eventually breaks.

“The first thing to take away from this study is a better view of what the fault zone actually looks like and evidence of the actual division of the plate boundary fault, and where it appears to be changing depth, or broken by cross-sectional faults,” Tobin says. The second important finding, he says, is the depth and angle of the plate as it sinks into the Earth. In some areas along its 700-mile stretch, the new study finds that Cascadia is close to horizontal—flattener than previously thought, Tobin says.

Cascadia comes into focus.

With the new 3D imaging, a more complete picture of the massive fault has emerged, revealing at least four segments. The segments are separated by vertical faults. “These are the segment boundaries,” says Carbot. Each segment has a distinct geometry, including how steep the plate segments are in the mantle.

One section runs from northern California to southern Oregon; another runs along the central Oregon coast; a third ends at the mouth of the Columbia River on the Oregon-Washington border; and a fourth runs parallel to the Washington coast toward British Columbia.

Along the large area of ​​land that the fourth plate encloses, the Juan de Fuca plate slides beneath the North American plate at a very shallow angle. The part of the fault that can rupture in a large earthquake is limited to a specific temperature range—150 to 350 degrees Celsius. That temperature range is associated with a specific depth, about 20 kilometers below sea level, Tobin says. “That’s the key to why the shallow, flat dip angle of the fault off Washington is able to host larger ruptures: there’s a lot more fault surface area between zero and 20 kilometers than there is at a steeper dip angle.”

This cartoon shows the difference between the shallow subduction of the Juan de Fuca plate under Washington versus the steeper subduction under Oregon. The y-axis shows depth in kilometers. The red-brown color of the subducting plates shows the area that could break. In the example above for Washington, this area is much larger in area due to the shallow angle of the dip. The plate is flatter here, so there is more area in the area that is vulnerable to breaking. Copyright: Harold Tobin

To the south, the parts of the fault along the Oregon coast are much rougher and more fragmented. They also dive under the North American plate at steeper angles. “It’s hard to get slippage over a large area, and it turns out it’s hard to get a large area to break down and accumulate stress,” Tobin says. By contrast, the smoothness of the Washington section could mean that once slippage starts, it will spread over a larger area of ​​the fault.

These new findings could be particularly disturbing for communities between Washington and Vancouver Island, where a more powerful earthquake and tsunami could occur. If that happens, it would mean more shaking and potential damage to the populated urban areas of Seattle and Vancouver. “The active, seismogenic portion of the massive thrust would likely extend farther offshore,” said Dr. [t]“Here,” says Carbot.

Researchers now need to explore the crucial question of whether the four segments of the Cascadia subduction zone might break individually or all at once. It’s unclear whether all four segments broke simultaneously during the 1700 earthquake, but each segment could be dangerous on its own. For example, “the segment from Washington to British Columbia is perfectly capable of handling a magnitude-9 slip on its own,” Tobin says. “As we learned from the 2011 Tohoku earthquake, you can stack a magnitude-9 on a much tighter area if there’s a lot of slip.”

As researchers continue to learn about the potential hazards of each segment of the rift using Langseth’s 2021 data, they can learn more about the entire Cascadia subregion and its history. Carbotte says researchers are already exploring potential landslides triggered by earthquakes or tsunamis at the southern end of the Cascadia subregion and methane seeps along the margin that may be linked to the rift.

Having a more complete picture of the Cascadia subduction zone could help researchers build better models that improve earthquake estimates and tsunami inundation maps off the coasts of the Pacific Northwest. Taking these actions could ultimately improve the safety of coastal cities and densely populated inland areas.

“Incorporating this information into building codes, bridge codes, or using it in the design of dams and infrastructure is critical,” says John Cassidy, a seismologist at the University of Victoria who was not involved in the study. “It’s about understanding earthquake risk.”

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