A SLICE OF CASCADIA: This is a cross-section of the Cascadia Subduction Zone where the Juan de Fuca Plate is being pulled by gravity under the North American Plate.
Next time around, waves could be twice as high, and hit within minutes
On clear days, when the wind is light and the storms have passed, the views from the bluffs of Pebble Beach evoke a feeling of serenity.
The waves lull. The surfers wait.
Placid waters disappear into the haze on the horizon.
Some who live here endearingly call this spectacle Lake Pacific. It’s peaceful. Beautiful. Calm.
But underneath our feet, and far below the pillows where we rest our heads, is a violently churning conveyor belt, endlessly recycling the earth’s crust in an area known as the Cascadia Subduction Zone.In a way, it’s the North Coast’s bogeyman, a menace that lurks out of sight.
Scientists agree that one day the Cascadia will rupture. It will shake the ground furiously. Roads will break. Bridges will collapse. Within a matter of minutes — not hours like in 1964 — the ocean will surge higher than we’ve ever seen here.
“It’s a credible threat,” Humboldt State University Geology Professor Lori Dengler said. “It’s certainly something that is well within the possibility of occurring within our lifetime, so we need to pay attention to it in terms of preparation.”
Dengler is the West Coast’s premier tsunami expert, and she’s considered an authority when it comes to large-scale earthquakes.
Despite the Doomsday notions associated with a major Cascadia Subduction Zone earthquake, Dengler said the survival rate will be high.
“There’s just this common misconception that we’re all going to die,” Dengler said. “That’s absolutely not true.”
The Great Alaskan Earthquake of 1964 — the one that generated a tsunami that killed 11 people in Del Norte County — proved that point. Even though the 9.2 magnitude subduction zone earthquake was the second-largest ever recorded, only nine people died as a direct result of the ground shaking.
Most of the 115 deaths in Alaska that night came from the subsequent tsunamis that hit the towns along the Gulf of Alaska shortly after the quake. These tsunamis were caused by both the movement of the subduction zone and underwater landslides.
“I actually think the ’64 Alaska earthquake is a very optimistic story for us,” Dengler said, because it gives us an example of what our own subduction zone event will be like.
“It’s the exact same story.”
Some of the plentiful tsunami debris was piled up and burned in Crescent City. (Photos by Maris Ward courtesy of the Maris Ward family)
THE BIGGEST QUAKES ON THE PLANET
The Cascadia Subduction Zone is a nearly 700-mile fault that starts at Cape Mendocino and runs north to Vancouver Island, British Columbia. It’s where the Gorda and Juan de Fuca oceanic tectonic plates are being pulled under the continental North American plate. The fault breaks the surface of the sea floor about 25 miles off our coast.
As the oceanic plates gradually slide underneath the continental plate, they become stuck. The plates don’t stop pushing against one another, but instead start to build up in pressure.
“Gravity keeps pulling it,” Dengler said. “It’s being pulled down by the rate your fingernails grow.”
Over the course of hundreds of years, the stalemate between the oceanic and continental plates change the landscape. The sea floor dips from the pressure while land on the continental plate can rumple, going up in some places and down in others. Geologists are just now starting to measure these changes in land level in our area.
“Finally you get this sudden snap,” Dengler said. “It’s kind of like a spring.”
Large subduction zone earthquakes are the biggest on the planet, reaching around magnitude 9.0. The sudden movement under the ocean thrusts massive amounts of water upward, creating a tsunami that can speed across an entire ocean and wreak havoc on distant shores.
Both the 1964 tsunami that hit Crescent City and the 2004 Indian Ocean event that killed more than 200,000 people were results of these types of quakes.
Map shows the 1964 tidal wave inundation zone and the anticipated bigger reach of a tsunami resulting from a large Cascadia Subduction Zone earthquake.
Like major earthquakes, tsunamis are rare. Because of this there’s a lack of instrumental data — when compared to other sciences — that is available for understanding them. This means the field of research is constantly evolving.
“I think it’s really useful to put this in the perspective of an unfolding story. We’ve made great strides, but we’re still, in the tsunami world, relatively youthful,” Dengler said. “We’re constantly changing and revising our ideas.”
This was apparent in 1992 when a 7.2 magnitude earthquake hit Cape Mendocino about 30 miles southwest of Eureka and generated a tsunami that hit Humboldt Bay within 20 minutes. This surge also reached Crescent City, but because it was at low tide, both towns were spared significant damage.
According to Redwood National and State Parks Geologist Vicki Ozaki, this event brought renewed attention to the Cascadia Subduction Zone.
“In 1992, that first light bulb goes on where we can have those locally generated tsunamis,” said Ozaki, who is also a co-chair of the Redwood Coast Tsunami Work Group. “Prior to that there was more work on the earthquake side of it.”
Until then, scientists had no real experience with near-source tsunamis along the North Coast, and much of California’s hazard mitigation was focused on distant source, or tele-tsunamis, resulting from far-away earthquakes. In fact, before 1992, not many seismologists believed the Cascadia Subduction Zone was even capable of large earthquakes.
After Cape Mendocino, the culture changed, Ozaki said. Researchers realized the zone was not benign, and they turned elsewhere to try to understand the threat that it poses locally. To do this they needed to look at recent subduction zone earthquakes.
Specifically, they studied the 1960 Great Chilean Earthquake, and the infamous 1964 Alaska event.
“Here we have two subduction zones we can look at,” Ozaki said. “We can see what they experienced there and how we can apply it to our own coastline.”
It wasn’t until the mid-’90s that our own subduction zone story was revealed. When it finally was, it predated recorded history in our area.
Researchers discovered — through a combination of American Indian oral histories, ancient Japanese writings about tsunamis and core samples taken from local marshes — that the last time the Cascadia Subduction Zone ruptured was Jan. 26, 1700. The magnitude of this earthquake is estimated to be around 9.0, and it is considered by the U.S. Geological Survey to be the largest known earthquake in the lower 48 states.
When it happens again, Rick Wilson, an engineering geologist with the California Geological Survey, estimates the earthquake will be similar in size and the shaking will last a number of minutes.
“If the event were to occur, it could be a magnitude 9 earthquake similar to what we saw in Sumatra (in 2004) and similar to what we saw in Alaska (in 1964),” he said. Tsunamis caused by the massive displacement of water would hit our coast quickly.
“The travel time, we’re not talking a matter of hours, we’re talking a matter of minutes, which does not give the local population much time,” Wilson said.
The surges will be large, too, much bigger than anyone in Crescent City has seen before.
“It’ll basically be close to twice the size of what happened in 1964,” Wilson said.
That year, Crescent City was hit by four surges. The last and biggest was estimated at 22 feet above the mean low tide level. This means it was nearly 16 feet above the tide at the time, which was near its peak.
Wilson’s agency, in conjunction with the Redwood Coast Tsunami Work Group and California Emergency Management Agency, is working on new inundation maps for the entire state that show each community’s worst-case scenario for a tsunami. For Crescent City, this would be what he calls an “extreme Cascadia event” during high tide.
“Although our inundation maps are interesting scientifically,” he said, “really the key is the evacuation routes that the cities and counties develop using them.”
The Cascadia Subduction Zone can give emergency planners fits. If a major event happens, many of the sophisticated warning devices that have been installed for far-field events, such as remotely operated sirens, might become useless. The waves are coming too fast and the earthquake could disable sirens before they sound.
That’s why most emergency personnel charged with educating the public say to pay attention to natural indicators, such as significant ground shaking, the ocean receding, or a rapid rise in the water’s elevation. Should any of these things happen, the message is clear — head inland or to high ground.
“The challenge is basically getting people to understand the threat and resolve to take action,” said Troy Nicolini, warning and coordination meteorologist for the National Weather Service in Eureka and co-chair of the Redwood Coast Tsunami Workgroup.
Unfortunately, he said, not much is known about exactly how a Cascadia event would play out.
“For the Cascadia, there’s just so many uncertainties,” Nicolini said. “For me, the questions I need to answer are when the wave is going to get here and how big it’s going to be.”
Knowing the answers could be the difference between life and death when choosing an evacuation route, he said.
For instance, if people can get to a height of 30 feet in five minutes or a higher elevation in 25 minutes but they have to cross low-lying land to get there, which is the better option?
“They might have a certain height that’s pretty close at hand, but if it’s not high enough that’s pretty bad,” Nicolini said, immediately countering with the other scenario. “The problem is, if they’re at lower ground for maybe five minutes they may get hit.”
So when can we expect the next Cascadia Subduction Zone event? Again, that’s hard to tell.
Lori Dengler said the average time between Cascadia events is 500-600 years. But she also said that’s misleading because the actual time between two known events has never quite lived up to that average.
“They can be as close together as 200 years or as far apart as 1,000 years,” Dengler said. “The bottom line is that these faults do not work like clockwork, the earthquakes are irregular.”
The earth could shake at any moment, and that’s why she said it’s important to always be prepared for the worst. Not only will this get you ready for lesser events, such as floods and earthquakes, but it will help you survive the big one.
“In a Cascadia event, Crescent City won’t be the only news story,” Dengler said. “Crescent City will be on the back page.”
Disaster will strike many population centers along the West Coast when our fault line ruptures, she said. The destruction will be widespread, and Del Norte County will likely be even more isolated than it already is because the bridges and highways that connect us to the world could be wiped out. This also means we could be stranded for days, left without supplies and incoming aide.
But even as Dengler describes this apocalyptic scenario, she’s quick to point out that it’s not all bad.
“It’s not this horrible catastrophe that’s going to wipe us out,” she said. “We’re going to live and have to deal with the aftermath.”
Though she added with a laugh, “It’s certainly an event that I hope I will never have to experience in person.”
Debris is strewn along U.S. Highway 101 after the 1964 tsunami. (Photos by Maris Ward courtesy of the Maris Ward family)
Water, about 1 gallon per person per day, for drinking, hygiene and cooking
Food, packaged, canned, no-cook, as well as baby food and food for special diets
Can opener, non electric
Blankets or sleeping bags
Food and water for pets
Portable radio, including NOAA weather radio
Alternate cooking source and matches
Crescent wrench for utility shut off