‘Cascadia Subduction Zone’ — or CSZ — may just be sufficiently important to warrant its own unique three-letter acronym. For people and resources along the Pacific Northwest coast, it some day will likely make a big difference to their relative sea-level rise. 

“Cascadia Subduction Zone.”

The term doesn’t flow smoothly and readily from the lips of most editors. But that could change someday — some very unwelcome, but probably inevitable, day.

The Cascadia Subduction Zone, CSZ, is where the Juan de Fuca Plate beneath the Pacific Ocean dives under the North American Plate off the coast of the Pacific Northwest. It is straining under hundreds of years of slowly building pressure. To understand what could happen there when that pent-up strain gives way and the undersea fault line ruptures, take yourself back to the 2011 earthquake that struck Japan.

The broad details of that quake and the tsunami that followed are now familiar to many. But here’s a fact that’s still startling: In just three minutes, large swaths of Honshu’s coastline expanded eastward 13 feet and sank more than three feet. If it’s tough to visualize that kind of violence beneath you, take a look at a University of Colorado diagram and follow the arrows.

Is that worth bringing up in the context of climate change? It ends up that there’s more to the story of relative sea-level rise along the Pacific Northwest than “just” global warming and warming, and therefore expanding, seas. Put simply, the tectonic forces that periodically re-shape Japan’s coastline are similar to those aligned along the Cascadia Subduction Zone, or CSZ, off the coast of the Pacific Northwest.

“What does this have to do with climate change?” The answer, it turns out, is a lot. If you live on the coast of Honshu, where your neighborhood is now under water during high tides, or along the coast of Northern California, Oregon, Washington or British Columbia — where it’s only a matter of time before a catastrophic earthquake drops the earth from under you — rising seas in a warmer world take on new meaning.

‘A Game Changer’ for Estimating PNW Sea-Level Rise

The wrinkle in the ongoing story of our planet’s warming seas is discussed in a recent report commissioned by the National Academy of Sciences, “Sea-Level Rise for the Coasts of California, Oregon, and Washington: Past, Present, and Future.” The report estimates that a major earthquake triggered beneath the Pacific Ocean, within the CDZ, “would cause some coastal areas to immediately subside and relative sea levels to suddenly rise.” How much? More than three feet higher than what’s already projected for the region.

Daniel R. Cayan, a research meteorologist at the Scripps Institution of Oceanography in San Diego and a co-author of the recent Academy report, called the revelation a “game changer” for estimates of how high sea levels could increase off the coastline of the Pacific Northwest.

“It’s a big deal from point Mendocino northward,” said Cayan, who heads the California Nevada Applications Program and the California Climate Change Research Center.

“That is definitely something that I don’t think has received a lot of recognition. In the science community I think it’s well known, at least among specialists,” Cayan said in a recent interview. “But I don’t think that it’s been appreciated [more widely].”

A major rupture along the CSZ is expected once every 200 to 700 years, Cayan said, “but it’s one of those things that happens catastrophically, and once it happens it’s totally a game changer” for sea levels in the region.

Scientists say the last major earthquake along the Cascadia Subduction Zone occurred in 1700, probably along 620 or more miles of the zone. That quake measured between magnitude 8.7 and 9.2 and triggered a tsunami that crossed the Pacific Ocean and hit a surprised Japan, where no earthquake had been felt. Computer simulations of the quake in 1700 show the sea floor and adjacent land along the coast of the Pacific Northwest falling by more than six feet.

University of Washington researcher Brian F. Atwater and colleagues chronicled the 1700 quake in “The Orphan Tsunami of 1700 — Japanese Clues to a Parent Earthquake in North America,” (also available here) a book published in 2005 by the U.S. Geological Survey in association with the University of Washington.

Ice Melt and Moving Ground

The story of the Cascadia Subduction Zone highlights a key, but often over-looked, fact about the globe’s rising seas: sea-level rise will be uneven from place to place. Seismic features particular to a region, such as the CSZ, can play an important role. Changes in ocean and atmospheric circulation, as well as the degree to which melting glaciers deform landscapes and even alter Earth’s gravity as ice mass on land is lost to the sea, are also important factors.

Uncertainties, Year-to-Year Variability Remain

“There is more detail here, and there are more attempts to make estimates of what the change in sea level will be from ice melt,” Cayan said of the new report. “Of course, that is still highly uncertain, but at least it’s confronted head-on in this report. It’s really more thorough and comprehensive than anything that’s been done that I’m aware of. I think this is going to be a model for the way that this problem is approached elsewhere.”

When looking at the effects of rising seas on the West Coast, Cayan cautioned, there remains a lot of year-to-year variability. But rising seas are expected to be most evident during years when El Niño storms batter the California coast — particularly when those storms occur during times of high astronomical tides.

The potential for a major seismic event to re-shape the coastline just adds a new, potentially game-changing, variable.

California Sinking, Pacific Northwest Rising, Sea Levels More Complicated

Seismic and other differences between most of the California coast and points north will make a difference in regional projected sea levels, according to the Academy report.

Along the San Andreas Fault in California, the Pacific and North American Plates are sliding past one another, more or less like ships passing each other in the night; they create little vertical land motion along the coast. An important factor in land elevation in California, however, is the extraction of water and hydrocarbons, which can lower surface elevations by tens of centimeters per year if those underground reservoirs aren’t recharged. Global Positioning System (GPS) measurements show that much of the West Coast south of Cape Mendocino in Northern California is actually sinking at an average rate of about one millimeter per year.

North of Cape Mendocino, where the Cascadia Subduction Zone has shaped the coastline for eons, the Juan de Fuca Plate is slowly descending beneath the North American Plate. Unlike the ships at night with their north/south movements in most of California, picture here the North American Plate moving in a southwestern direction, over the Juan de Fuca Plate as it dives beneath the continent. The result: a rise in the coastline by about 1.5 to 3.0 millimeters per year. It’s a sudden rupture along the Cascadia Subduction Zone that can suddenly reverse that slow but strained trajectory, resulting in a sudden drop in land elevations along the coast, and as a result leading to increased relative sea-level rise.

The Academy report offers a series of projections for sea-level rise for the West Coast, given the average rise in global sea levels and the variety of other localized conditions. For the California coast south of Cape Mendocino, sea levels are projected to rise by between 1.5 inches to nearly one foot by 2030 relative to 2000; nearly five inches to about two feet by 2050; and about 16 inches to about 5-1/2 feet by 2100.

For the Washington, Oregon, and California coasts north of Cape Mendocino, sea levels by 2030 are projected to be about 1.5 inches lower to nine inches higher than 2000 levels; about one inch lower to about 19 inches higher by 2050; and four inches to more than 4.5 feet higher by 2100.

On the one hand, land uplift and gravitational and deformational effects reduce the threat of rising seas off Washington and Oregon. But that rising land is likely brought about because “interseismic strain is building in the Cascadia Subduction Zone. A great earthquake (magnitude larger than eight) … would cause some coastal areas to immediately subside and relative sea level to suddenly rise. If this occurs, relative sea level could rise an additional meter or more over projected levels.”

A Caveat about Uncertainties

The authors of the Academy report were explicit in describing the challenges and uncertainties involved in forecasting actual sea levels, despite the rigorous analysis. They wrote:

The projections of future sea-level rise have large uncertainties resulting from an incomplete understanding of the global climate system, the inability of global climate models to accurately represent all important components of the climate system at global or regional scales, a shortage of data at the temporal and spatial scales necessary to constrain the models, and the need to make assumptions about future conditions (e.g., greenhouse gas emissions, large volcanic eruptions) that drive the climate system. As the projection period lengthens, uncertainty in the projections grows. … The actual sea-level rise will almost surely fall somewhere within the wide uncertainty bounds, although the exact value cannot be specified with high confidence.

As Cayan noted, Coastal residents and resources will particularly feel the brunt of the rise in future sea levels during storms. Storm surges have exceeded sea levels projected for 2100, and the authors wrote that the integrity of rivers that carry sediment to the coast and of marshes that absorb storm surges will both be important to mitigate the effects of rising seas.

Most Media Reports Give Passing Mention to Earthquake Danger

Most media outlets on the Academy’s West Coast sea-level report gave some mentions to the influence of the Cascadia Subduction Zone.

Agence France Press, for example, mentioned the subduction zone as the cause of slowly rising coastal land, and reported far down in the story that “a major earthquake in northern California could cause a sudden sea-level rise of one meter (yard) or more.”

A Reuters story also mentioned the risks associated with a major earthquake, but far down in the story. “An earthquake of magnitude 8 or more could cause waters to rise by an additional 39 inches (1 meter),” the story reported. “This kind of strong earthquake, which could cause parts of the coast to quickly subside, is a possibility off the northern West Coast, the scientists said. Such quakes occur every few hundred to 1,000 years.”

An Associated Press story was one of the few that called attention to the inability of the many parts of the West Coast to cope with rising seas, because many marshes long ago had been replaced with coastal development, and less sediment now is reaching coastal areas because of altered rivers. Dams hold back about one-third of the sand that once washed into the sea from the Klamath River in Northern California, the story stated.

A Los Angeles Times story focused, not surprisingly, on predictions for California, reporting that sea levels along California will be slightly more than the global average and “because much of California is sinking.” Sea levels along Washington and Oregon, where “geologic processes are flexing the land upward,” will result in increases that are below the global average, the story said.

“A major earthquake in the Pacific Northwest, such as a magnitude 8, could upend that trend, causing parts of the coast to sink and suddenly raising sea levels by three feet or more,” the LA Times report continued farther down.

The Oregonian, in Portland, again not surprisingly, called attention to the potential for catastrophic Pacific Northwest sea-level rise.  In the first paragraph of its story on the Academy study, the newspaper reported:

Expect more flooding and coastal erosion as global sea levels rise more than previously predicted over the next century … Some parts of the Pacific Coast will see several inches higher in coming decades, and by the year 2100, places may need to brace for up to 4-1/2 feet rises. Throw in a major earthquake off the coast of Oregon and the land could drop further causing sea levels to rise more than 3 feet.

Ice Melt and Moving Ground

The new NAS report, “Sea-Level Rise for the Coasts of California, Oregon, and Washington: Past, Present, and Future,” provides details of how two major drivers of sea levels — ice melt and seismic activity — can influence regional sea levels. The discussions are in Chapter 4 of the report.

Melting Ice

As glaciers and ice sheets melt and lose mass, the report states, water is transferred from the continents to the ocean, deforming solid earth and perturbing Earth’s gravitational field. These changes occur differently from one region to another, resulting in variations in sea-level rise from place to place.

Modern melting of ice influences sea levels in two ways. Large masses of ice generate an additional gravitational pull that draws water closer and raises sea levels near the glacier. But as the ice melts, that gravitational pull decreases, and sea levels fall in the proximity of the glacier. And as the glaciers lose mass, the land beneath them also uplifts, further decreasing sea levels. The lower sea levels can extend a few thousand miles from the melting ice. At the same time, however, sea levels rise because melt water is entering the ocean — but the distribution of that melt water in the ocean is not even, because regional differences in land uplift and gravitational effects vary. In essence, sea levels fall near a melting glacier but rise everywhere else.

Researchers call these differences in sea level from place to place “sea-level fingerprints.” The three largest sources of melting land ice that influence sea levels along the U.S. West Coast are Alaska, Greenland, and Antarctica. Sea levels are influenced most there by the effects of melting glaciers in Alaska — with lower sea levels close to Alaska and progressively higher sea levels on down the West Coast.

Seismic Activity

The movement of tectonic plates — in particular, the influence of the Cascadia Subduction Zone, also is discussed in detail in Chapter 4 of the Academy’s report. The slow movement of the Juan de Fuca Plate under the North American Plate creates a strain that builds over hundreds of years, causing the upper plate, the North American, to bend upward. Further inland, that upward-bending at the coast actually creates a slight subsidence of land. The researchers’ description clearly details what can happen when that strain becomes too much:

In the second stage of the cycle, the plate-boundary megathrust fault slips in a great earthquake, releasing hundreds of years of accumulated strain along many hundreds of kilometers of the plate boundary. During the earthquake, the former slow vertical deformation of the upper plate is reversed: coastal areas suddenly subside as much as 2 (meters) and formerly subsiding areas landward and seaward are suddenly uplifted.

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Editor’s Note:  For an intriguing and informative read on the Cascadia Subduction Zone, see Jerry Thompson’s 2011 book Cascadia’s Fault: The Coming Earthquake and Tsunami That Could Devastate North America. Strong on the workings of science and how scientists face-off in proving or disproving one another, Thompson’s book doesn’t venture a guess on just when the next CSZ quake might hit, but says it’s a question of when and not if. “One day closer today than it was yesterday,” he quotes one expert. With no near-term likelihood of effective tsunami/quake forecasting, he cautions that adaptive measures such as construction codes are critical along these coasts. 

Bruce Lieberman, a long-time journalist, has covered climate change science, policy, and politics for nearly two decades. A newspaper reporter for 20 years, Bruce worked for The San Diego Union-Tribune...