A clearer picture of Cascadia emerges from modern mapping

Information gathered during a 2021 marine research expedition offers insight into the possible behavior of the Pacific Northwest’s most enigmatic fault.
 

By Rebecca Owen, Science Writer (@beccapox)
 

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

On January 26, 1700, a magnitude 9.0 megathrust earthquake originating from the Cascadia Subduction Zone rocked the Pacific Northwest. The subsequent tsunami and coastal inundation from this historic event were immortalized. They’re mentioned in traditional stories shared by generations of local Indigenous people, in records of an orphan tsunami traveling across the Pacific Ocean to Japan without local people feeling an earthquake, and in the dead and submerged trees (or “ghost forests”) that haunt the Pacific Northwest’s coastline today. Since that event more than three hundred years ago, the Cascadia Subduction Zone has been very quiet. Its relative lack of movement — even small quakes are scarce — is perplexing scientists who study its behavior.
 

 

The Cascadia Subduction Zone, where the Juan de Fuca plate descends below the North American plate, stretches 700 miles from Cape Mendocino in Northern California to Vancouver Island, British Columbia. People living in coastal communities along the rugged and rocky Pacific Northwest coastline know about the possible dangers that exist as the plates underneath them slowly shift—as are residents in the major metropolitan areas of Portland, Seattle, and Vancouver, BC. A similar earthquake and tsunami to the 1700 event would affect millions of people from California to Canada, likely leading to significant fatalities, displacement, and damage — especially to infrastructure — that will affect and alter the region for years to come.

A new study, published in Science Advances, reports findings from the first-of-its-kind marine research expedition to map nearly the entire Cascadia Subduction Zone from the Oregon border north to British Columbia. The study provides high-resolution imaging of the fault zone that can help researchers, policymakers, and residents understand more about this sleeping giant.
 

 

Outdated information provides an incomplete picture

Previous studies indicate that the Cascadia Subduction Zone has unleashed a major earthquake every 300 to 500 years. If that pattern continues, the Pacific Northwest may be due to experience a deadly, destructive earthquake and tsunami, one which some call “the Really Big One.” But missing details about the fault’s structure and mechanics limit our understanding of what the eventual megathrust quake might look like.

Much of what we know about the Cascadia Subduction Zone’s fault system and rupture history comes from models produced in the 1980s and ‘90s, says Harold Tobin, a seismologist at the University of Washington and one of the study’s authors. Though geophysical imaging from that period established the general architecture of the subduction zone, these dated surveys only allowed for a basic interpretation. “There has never been one survey that really covered the whole thing from north to south,” Tobin says.

Earlier models that reconstructed the 1700 Cascadia Subduction Zone earthquake using the “vintage” data showed several areas of possible slip along the fault, says Suzanne Carbotte, a marine geophysicist at Columbia University’s Lamont-Doherty Earth Observatory and lead author of the study. Some of those areas seemed to have lesser or greater slip, known as segmentation, she explains. Turbidite deposits form when an event like an earthquake, storm or tsunami triggers an underwater landslide that flows down the steep continental slope, depositing a distinct sequence of sediment layers. Previous studies analyzed the turbidite deposits for their ages, and those results roughly corroborated the models’ interpretation of where the Cascadia Subduction Zone ruptured in 1700.

But there is only so much information that researchers can glean from an earthquake that happened 300 years ago.

“The Cascadia subduction zone is very quiet. There are very few small earthquakes,” Carbotte says. “In most subduction zones, there are lots of small earthquakes going off all the time.” Those small, relatively harmless quakes can help scientists pin down a fault’s location and geometry. Cascadia is missing that critical information.

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

 

Setting sail along Cascadia

In the summer of 2021, researchers embarked on a 41-day expedition off the Oregon, Washington, and British Columbia coast. They were aboard the Marcus G. Langseth, a 235-foot research vessel. Their goals were to map and image the subduction fault to better constrain its geometry and to capture images of its possible segmentation.

Because different segments might have different magnitudes of slip and rupture, certain communities throughout the Pacific Northwest could face greater shaking risks during future earthquakes, Carbotte says. “If there’s actual geologic structure that’s contributing to this segmentation, then we can make predictions that the segmentation is likely to be contributing to the next earthquake,” she says.

As the Langseth sailed, it dragged a 9-foot cable equipped with air guns and an approximately 7.5-mile seismic streamer of 1,200 hydrophones. The air guns emitted pulses that bounced off the seafloor while the hydrophones recorded these echoes. “We’re using sound to image the subsurface of the Earth,” Carbotte says. The method is akin to taking an ultrasound of the seabed, providing more detailed and higher-resolution images than the previous surveys. The hydrophones provide data that help scientists determine the location, shape, and angles of the fault zone — critical information for scientists looking to model how the fault will behave when it eventually ruptures.

“The first big takeaway from this from this study is a better view of what the fault zone actually looks like and evidence for actual segmentation of the plate boundary fault, places where it looks like it changes depth, or it’s broken by cross faults,” Tobin says. The second important findings, he says, are the depth of the plate and the angle as it dips into the earth. In some areas along its 700-mile span, Cascadia is close to horizontal, the new study found — much flatter than previously thought, Tobin says.
 

 

Cascadia comes into focus

From the new 3-D imaging, a fuller picture of the megathrust fault emerged to reveal at least four segments. The segments are separated from one another by vertical faults. “These are segment boundaries,” Carbotte says. And each segment has a distinct geometry, including how steeply the plate segments dive into the mantle.

One segment stretches from Northern California to Southern Oregon; the next segment travels up the Central Oregon coast; and a third ends by the mouth of the Columbia River on the Oregon-Washington border. The fourth segment runs parallel to the Washington coast toward British Columbia.

Along the large swath of territory encompassed by the fourth segment, the Juan De Fuca plate slides below the North American plate at a very shallow angle. The part of the fault capable of rupturing during a large earthquake is limited to a specific temperature range — 150 to 350 degrees Celsius. This temperature range correlates to a specific depth, about 20 kilometers below sea level, says Tobin. “That’s the key to why the flat, shallow dip angle of the fault off Washington can host bigger ruptures: there’s just a lot more fault surface area between zero and twenty kilometers, compared to a steeper dip angle.”
 

 

To the south, the segments of the fault along the Oregon coast are rougher and more broken-up. Plus, they plunge below the North American plate at steeper angles. “It’s harder for the slip to spread over a wide area, and it turns out it’s also harder for the whole large area to get truly stuck and build up the largest stresses,” says Tobin. In comparison, the smoothness of the Washington segment might mean that once the fault begins to slip, that movement will spread over a larger area of the fault.

These new findings might be particularly troubling for communities in the region between Washington and Vancouver Island because they may experience a much more severe earthquake and tsunami. If that occurs, it will mean more significant shaking and potential damage for the populated metro areas of Seattle and Vancouver. “The active earthquake-generating portion of the megathrust is likely to extend farther on shore [t]here,” Carbotte says.

The researchers now need to explore the critical question of whether the four segments of the Cascadia Subduction Zone might rupture individually or all at once. It’s unclear if all four segments ruptured simultaneously during the earthquake in 1700, but each segment can be dangerous on its own. The segment running from Washington into British Columbia, for instance, “is perfectly capable of having a magnitude 9 amount of slip all by itself,” Tobin says. “As we learned from the Tohoku earthquake in 2011, you can pack magnitude 9 into a more compact area if there’s a very large amount of slip.”

As researchers continue to learn about the potential dangers of each fault segment using the 2021 Langseth data, they can learn more about the entire Cascadia Subduction Zone and its history. Already, researchers are exploring possibly earthquake- or tsunami-generated landslides on the Southern Oregon end of the Cascadia Subduction Zone and methane seeps along the margin that may be related to the fault, Carbotte says.

A more complete picture of the Cascadia Subduction Zone can help researchers build better models that will improve shaking estimates and tsunami inundation maps for Pacific Northwest coastlines. Taking these actions will ultimately improve the safety of coastal towns and heavily populated inland regions.

“Having that information included in building codes, or bridge codes, or used for designing dams and infrastructure is very important,” says John Cassidy, a seismologist from the University of Victoria who was not involved in the study. “It’s about understanding the earthquake hazards.”
 

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