Why are we tsunami prone?

Written by Adam Spencer and Nick Grube March 23, 2014 09:40 pm

 Seafloor and harbor shapes worsen hazard 

As a small coastal town, hundreds of miles from major urban centers, Crescent City is off the radar of your average American, but in a room full of tsunami experts, everyone knows this place.

After all, 37 tsunamis have been recorded along our shores since 1933, and in the past 150 years no other community in the contiguous U.S. has sustained more damage from them than Crescent City.

 

This distinction attracts many tsunami experts to the area, in particular Humboldt State University Geology Professor Lori Dengler.

“Crescent City is an exciting place if you’re interested in tsunamis,” Dengler said.

She is one of the founding members of the Redwood Coast Tsunami Work Group, an interagency association that works to reduce earthquake and tsunami hazards in Northern California. She is considered by her peers to be one of the preeminent authorities on the West Coast when it comes to earthquakes and tsunamis.

Much of Dengler’s research focuses on the North Coast, including the events of March 27-28, 1964, when an Alaskan earthquake generated a tsunami that raced across the Pacific Ocean and swamped Del Norte.  

In the wake of the March 11, 2011, Tohoku earthquake and tsunami that killed more than 19,000 and caused nearly $300 billion in damage to Japan and also wrecked Crescent City Harbor, Dengler repeatedly visited destruction in both Japan and in Del Norte, furthering her knowledge about North Coast tsunamis.

She has studied why Crescent City continues to be susceptible, and much has been learned about tsunamis over the past 50 years since that fateful starlit night in 1964, bringing insight into the Tsunami Capital’s precarious situation.

For instance, one of the biggest lessons from the Good Friday Tsunami is that the first waves that hit Crescent City are not going to be the biggest, and the surges will be spread over a number of hours.  

Much of HSU Geology Professor Lori Dengler’s research focuses on North Coast tsunamis.  Triplicate file photo
Much of HSU Geology Professor Lori Dengler’s research focuses on North Coast tsunamis. Triplicate file photo

“I can always say it will never be the first, second or third waves in Crescent City,” Dengler said. “It will always be later.”

The major reasons for this are the shape of the coastline and the location of the harbor. Together they trap a tsunami’s energy, creating a perfect environment for waves to build up in height.

Jutting out into the sea as the second most western point in California only strengthens Crescent City’s reputation as a tsunami magnet. 

“Unlike Cape Mendocino (the farthest west point in California), Crescent City is low-lying and the only relatively populated exposed community north of Mendocino County,” said Dengler, adding that the spits at Humboldt Bay protect the populated areas in Humboldt County.

‘FICKLE FINGER OF FATE’ MAKES US A MAGNET

The shape of the Pacific sea floor offshore of the North Coast amplifies tsunami energy and channels it toward the coast.

There’s what Dengler calls a “Fickle Finger of Fate” that points at the North Coast. This is the Mendocino Fracture Zone, an underwater ridge that extends more than 2,500 miles due west from Cape Mendocino.

This scar is easily visible in satellite images, including Google Maps, and Dengler said the sea floor directly north of the fracture zone is more shallow than it is to the south.

“Fishermen say going south across that boundary is like going into the deep end because the water drops so abruptly,” she said. “We actually get a slightly larger tsunami just north of Cape Mendocino rather then just south of it.”

A tsunami’s speed is a direct function of how deep the ocean is. The greater the depth, the faster the wave. As the tsunami hits shallow water approaching a coastline, friction causes the wave to slow down. And it doesn’t just tap on the brakes — both feet are down.

“You still can’t outrun it,” Dengler said, “but it’s going a lot slower.”

A tsunami that had been traveling almost as fast as a commercial jetliner in the deep ocean slows down to 25-30 mph when it reaches shore. Although slower, this wave is now much higher.  The change in speed “causes the water to bunch up because that fast-moving water behind it catches up to it,” Dengler said.

The shallower ocean depth north of the Mendocino Fracture Zone means a tsunami’s amplitude will be larger than in other parts of California.

“That actually acts as a wave guide. It concentrates the wave energy,” Dengler said. “We just basically have a bigger incoming wave from almost any other place in the Pacific along the Northern California Coast.”

Further funneling the impact of a tsunami toward Crescent City is the shape of our portion of the continental shelf.

California’s northern coastline resembles a backward C-shape from Crescent City down to Cape Mendocino. Inside of this is another curve between Point St. George in Del Norte and Patrick’s Point in Humboldt.

“Our shelf seems to be kind of bowl-shaped,” Dengler said. “Tsunami energy hitting this coast excites secondary oscillations in the very large bowl — rattling around for days when a large tsunami strikes.”

That’s why the later surges are always bigger than the first. Energy from the first surges are still bouncing around in the series of bowl-shaped basins, allowing later surges to combine forces.  The 2011 tsunami that destroyed Crescent City Harbor could be clearly seen on the tide gauge for at least six days, Dengler said.

Another multiplying factor locally is the position of the harbor in the natural bend of the beach that gives Crescent City its name.

“It’s this crescent-shaped bay that again tends to amplify the tsunami energy,” Dengler said. “The natural geometry of the harbor seems to welcome tsunamis from all directions.”

 

A DEADLY VISITOR ROUNDS THE BEND

In 1964, even though the Good Friday Tsunami came from the north, the waves never broke over the jetty, and instead came into the harbor from the opposite direction.

“The tsunami attack sort of came from the southwest,” Dengler said. “It funnels from the south.”

Deeper water offshore allows a tsunami to travel faster farther away from the coastline. As the tsunami approached Crescent City and its harbor the water started to slow down from the shallower depths, but fast waves farther out at sea actually bent the tsunami inland.

“That causes it to literally swing around into the harbor,” Dengler said.

Once there, the energy from the tsunami got trapped inside and started to bounce around. This rebounding, which also occurred on a larger scale outside the harbor from Point St. George down to Patrick’s Point and even farther to Cape Mendocino, further increased the size of the tsunami’s waves.

“It actually excites oscillations,” Dengler said. “Crescent City rings like a bell.”

Like dropping a rock into a pond, you get more than just one wave in a tsunami. These waves slosh back and forth along the North Coast and within Crescent City’s harbor. This energy can stick around for hours and even days, building upon itself and increasing the size of waves.

‘THE BATHTUB EFFECT’

Troy Nicolini of the National Weather Service in Eureka describes this motion as the “bathtub effect,” and he said it’s particularly pronounced inside Crescent City’s harbor.

“You can imagine that when a wave comes in there it bounces off the concrete walls. It just bounces and keeps most of its energy,” Nicolini said. “It stays in there long enough for the next wave to come in and now they’re both bouncing around.”

He said this is like putting two kids in a bathtub together. When they both start splashing around, the water resonates off the sides of the tub, becoming more turbulent and eventually winding up on the floor.

“I have worked with tsunami modelers who have shown about a six-fold increase in the water speed inside the boat basin compared to the larger outer bay,” Dengler said. “In addition, the relatively solid walls of the boat basin means that energy is reflected and the resulting chaos is what you saw in 2011.”

This doesn’t happen in Humboldt Bay because it “shallows out” and the wave doesn’t have anything to reflect off of, Nicolini said.

“It’s the verticalness of the walls inside the Crescent City Harbor,” he said. “When those waves come in they bounce around in there before they die.”

Forcing the tsunami through the narrow entrance of the inner boat basin also boosts the energy of a surge. The architects of Crescent City Harbor’s new inner boat basin, completed in 2014, tried to mitigate this with a ‘wave attenuator‘ — a 16-foot wide, 8-foot deep dock, a hanging underwater wall of concrete — intended to block the energy of an incoming tsunami.

The $33 million dollar inner boat basin is considered the first tsunami-resistant harbor in the U.S., and in Crescent City, that’s money well spent.