Cascadia Fault – Mega Earthquake Looms Off Pacific Northwest

The Cascadia Fault, aka the Cascadia subduction zone, runs for seven hundred miles off the coast of the Pacific Northwest, beginning at the northern end of California,near Cape Mendocino, then running the entire length of Oregon and Washington, and continuing to at least the middle of Vancouver Island, Canada. The “Cascadia” part of its name comes from the Cascade Range, a chain of volcanic mountains that follow the same course a hundred or so miles inland. The “subduction zone” part refers to a region of the planet where one tectonic plate is sliding underneath (subducting) another. Tectonic plates are those slabs of mantle and crust that, in their epochs-long drift, rearrange the earth’s continents and oceans. Most of the time, their movement is slow, harmless, and all but undetectable. Occasionally, at the borders where they meet, it is not.

The Juan de Fuca plate is slipping steadily beneath the northwestern portion of North America.

The New Yorker magazine wrote an article about the Cascadia Fault.

new yorker

In this eye-opening paper they state, “In the Pacific Northwest, the area of impact will cover some hundred and forty thousand square miles, including Seattle, Tacoma, Portland, Eugene, Salem (the capital city of Oregon), Olympia (the capital of Washington), and some seven million people. When the next full-margin rupture happens, that region will suffer the worst natural disaster in the history of North America.

Below is a seismic hazard map for Seattle.


Roughly three thousand people died in San Francisco’s 1906 earthquake. Almost two thousand died in Hurricane Katrina. Almost three hundred died in Hurricane Sandy. FEMA projects that nearly thirteen thousand people will die in the Cascadia earthquake and tsunami. Another twenty-seven thousand will be injured, and the agency expects that it will need to provide shelter for a million displaced people, and food and water for another two and a half million.”


The Huffington Post recently published an article entitled Assessing the Probability of a Massive Cascadia Earthquake. Below is a copy of that article. Even though the article is about the Cascadia Fault, there are some

Watching the waves spill onto the beach in Clatsop County, Oregon, it’s easy to forget that the nearby houses are doomed. The Pacific waters are frigid blue. The beaches run inland from white sand to tough, salt-sprayed grass. It’s too peaceful to envision disaster.

Yet, at a tectonic convergence far out in the water, the geologic process that will invariably devastate Clatsop’s coastline is sedately underway.

The convergence is called the Cascadia subduction zone, a 700-mile-long grind of hot and locked crust, only recently understood to be the United States’ most dangerous fault line. It’s where North America’s continental shelf meets the Juan de Fuca tectonic plate, which has, for centuries, compressed the shelf eastward and upward, a few millimeters every year.

Think of a spring slowly being wound. For years, the spring winds and winds, its potential energy mounting. Then, suddenly, it recoils.

A recent article in the New Yorker titled “The Really Big One“ chronicles this recoil and its consequences in stomach-churning detail. First, a single, isolated jolt called a compressional wave will ripple inland. Dogs will start barking. People will sit up in bed. Then, thirty seconds later, there will be an earthquake. A really big one.

FEMA currently estimates that 13,000 people will die in a Cascadia subduction-zone event and millions of others, mostly in Portland and Seattle, will be displaced or lose access to power, running water, and functioning sewer systems. Bridges will sway and snap, houses will slip from their foundations, and when the shaking is over, Cascadia will have jolted 30 feet west and 6 feet down.


When the earthquake gives way, the slow distortion side (ie the Washington, Oregon side) will flatten, and lower the west coast by up to six feet.

A tsunami will arrive on the Northwest’s coastline 15 minutes later, the series of waves towering between 20 and 100 feet tall, depending on the magnitude of the earthquake.

powerful earthquake tsunami

That tsunami will wipe Clatsop’s coast clean. Below, while it isn’t Clatsop, is a tsunami simulation of how just one coastal community (Ocean Shores) in Washington state will have no time to evacuate. On top of that, the land around Ocean Shores is lower than the expected tsunami height.

ocean hores

There won’t be any prior notice. As Sandi Doughton, science writer for the Seattle Times, stated in a recent Reddit Q&A, “Every attempt to predict earthquakes has fizzled.” Instead, the problem will trade much more heavily in disaster preparedness and good old-fashioned risk assessment. So, if you live on or plan to visit the Pacific Northwest’s beaches (where most casualties are projected to occur), what are the chances you’ll be there when the really big one comes? How should you think about (or not bother to think about) the risk?

Risking the really big one today, tomorrow, and 20,000 days later

Recent earthquake research from Oregon State University professor Chris Goldfinger puts the chances of a magnitude-eight earthquake in Cascadia at one in three over the next 50 years. An earthquake of that power would, at best, unfold like San Francisco’s famous 1906 earthquake, which killed roughly 3,000 people. Goldfinger puts the chances of a much more severe earthquake—a magnitude-nine event—at one in ten for the same time period. That would look something like Japan’s 2011 Tohoku earthquake, which killed over 15,000 people in the most earthquake-prepared country on the planet.

So, one in three and one in ten over the next 50 years.

To simplify (optimistically), let’s say there’s a one-in-three chance of any sort of catastrophic subduction event in the next half century. That’s a one-in-three chance of the really big one over the next 18,262 days, or—neglecting that the probability of this earthquake increases over time (which it does, slightly, up to a point)—a 0.002% chance on any given day.

Put another way, if you flip 54,786 coins, you’ll get heads 54,785 times. You’ll only get tsunami once.

But how does this compare with another risky behavior, say, driving, a gold standard in ad-hoc risk comparison?

In the United States, a single vehicular trip bears a roughly one in ten million chance of resulting in a fatality. Assuming you make two trips each day, the choice to drive for a year means accepting an annual 1-in-13,750 chance of a fatal accident.

If that’s our benchmark, it’s important to note that pretty much no one bothers to think about driving risk. For most people, driving is less a choice than a circumstance. And if people don’t think about driving risk, they should think at least four times less about a tsunami crushing them on their annual day at the beach (especially given that safe evacuation is an anticipated and growing possibility).

Choose to stay at the beach for a month or longer and things get a bit dicier, though perhaps not in a practical sense. A month-long beach sojourn—which, we should say, is in the cards for very few Americans—increases your chances to about 1 in 2,000, or 0.05%. It’s still not a large chance, but at seven times greater than our back-of-the-envelope driving risk, to some it’s perhaps no longer negligible.

Tolerating risk and preparing for the inevitable

The real news to anyone outside the Northwest might be that the New Yorker’s article didn’t actually reveal anything new to Northwesterners.

With relish and anxiety, I rattled off the article’s highlights to a companion from Portland, and she replied simply: “I know.”

“It’s just something we live with.”

Coastal communities and inland metropolises have been aware of their precarious positions for years and have, at the very least, started along paths toward building disaster-proof infrastructure and establishing adequate emergency-response systems. In some places, there’s still everything to do. But the ball is rolling.

For now, Pacific Northwesterners (and other American taxpayers, who will help foot the emergency-relief bill) are lucky. There’s time to prepare. And, thankfully, those preparations will unfold on a human rather than geologic timescale, with significant progress just a decade or two away. Meanwhile, the tectonic mechanism will proceed without taking notice, a winding spring doing as a winding spring does.

Another earthquake research article speculates an additional theory with a fault that is just under the Olympic Mountains.

Olympics megaquake

A mega-thrust earthquake would be different from those that shake the Northwest occasionally. A mega-thrust quake occurs right on the boundary of two tectonic plates, while other earthquakes occur along cracks in the plate. Vidale likened what’s going on beneath the Earth’s crust to a bunch of blocks jostling around. Where the smaller blocks collide, you can have more standard-type quakes. Where the biggest blocks, the tectonic plates, collide, you have a mega-thrust earthquake.

Since the deep tremors were first detected 15 years ago, scientists have been trying to determine what was causing them along the Cascadia subduction zone. Eventually, they concluded that the tremors reflected the slippage of the Juan de Fuca plate under the North American plate.

“It’s a burst of noise that can go on for up to 24 hours over a period of several weeks,” said Herb Dragert, a geophysicist with the Geological Survey of Canada in Victoria, British Columbia, who was among those who first developed the theory.

What’s unique about the deep tremors, which occur in an area stretching roughly from Olympia, Wash., to Canada’s Vancouver Island, is that they reappear about every 15 months. While tremors have been detected elsewhere along the Cascadia subduction zone, none is as regular or as prolonged as those in the Puget Sound basin, Dragert said.

“Every 15 months it’s like tightening the guitar string a little more,” Dragert said. “You don’t know whether it will take it beyond the break zone.”

According to the timetable, episodic tremor and slip should be going on just about now. Instead, it came last spring, catching scientists by surprise. Malone said that some tremors were detected southwest of Olympia last week, but that it was too soon to determine whether they were part of a new episode or just isolated ones.

If all the energy associated with tremors over two weeks were released in 10 seconds, Vidale said, it would equal a 7.0 earthquake.

Can the Cascadia Fault cause Mt. Rainier or Mt. St. Helens to erupt?

Many people speculate that the primary danger with Mount Rainier, during or after a 9.3 earthquake, isn’t an eruption, but rather that the mountain isn’t “solid.”  There will be massive landslides from the steep slopes, avalanches from the snow pack and Mt. Rainier has an interior like soft clay which has been formed by the active hydrothermal system underneath and freezing/thawing expansion/contraction of its’ 25 glaciers on top. Thus, the major threat is from the collapse of a portion of the mountain itself into lahars — which are huge mudslides that wipes out everything in its’ path.


Lahar pathways from events heading on Mount Rainier – map showing three major events from last 10,000 years. Lahar map for Mt. Rainier courtesy of the USGS.


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