How tiny fossils in Oregon's coastal marshes can help us prepare for big earthquakes

If all goes according to plan, the core sampler's long metal rod will cut through the salt marsh soil like a knife through warm butter.

Mike Brady and David Bruce take their places in the waist-high grass.

“ready?” Bruce asks as they grab the T-handle. “One. Two. Three!”

The two graduate students push down with all their weight.

The rod disappears into the ground, sinking through layers of sediment that accumulated in Oregon's Salmon River estuary hundreds, even thousands of years ago.

“The Oregon coast is one of the best places in the world, with the longest record of marshland, making it a great candidate for studying ancient tsunamis,” says Brady, a doctoral student at Virginia Tech.

Virginia Tech researchers David Bruce, Tina Dora, and Mike Brady (left to right) took this selfie with a core specimen on the Oregon coast. The gray sand bar in the middle is evidence of the 1700 Cascadia earthquake.

Courtesy of Tina Dora/OPB

This estuary north of Lincoln has seen its fair share of Cascadia Subduction Zone earthquakes and the massive tsunamis they often cause.

The core that seismologists pull from the ground not only reveals the swamp's recent geological history in sedimentary layers. It also contains the fossilized remains of millions of tiny microorganisms. By bringing what they can learn from both together, researchers hope to rewrite our understanding of how tsunamis affect the Pacific Northwest coast.

Tina Dora, head of the Coastal Hazards Laboratory at Virginia Tech, is leading the research. Her lab is part of the Cascadia Region Earthquake Science Center (CRESCENT), which includes researchers from the University of Oregon, Oregon State, Portland State, and other institutions around the country.

She kneels on the spongy floor and opens the 3-foot-long sediment plug that her students have just dug out of the ground.

“Oh, my God!” Where layers of clay, sand and soil are exposed, Brady says.

“This is what it should look like,” Dora says, then laughs after looking at Bridie's face. “Mike's going to pass out.”

“

“We may be underestimating the extent of that [inland] “The tsunami waves have inundated the coast.”

— Tina Dora, Director of the Coastal Hazards Lab at Virginia Tech

Right in the middle, between layers of muddy marsh sediment, is a two-inch-thick strip of beach sand. It was washed away more than a mile away by a tsunami from the last Cascadia earthquake in 1700.

Looking at this clear evidence of a massive tsunami, Priddy quips in the voice of a posh Southern woman: “You've got the fumes! You've got the fumes!” 'Tsunami fumes!'

The heart tells them that a tsunami wave submerged where they stand more than 300 years ago – but how far inland did it get before heading back out to sea?

Cascadia earthquake threat

The Cascadia subduction fault runs north-south about 100 miles off the Pacific Northwest coast. Here the Juan de Fuca tectonic plate is constantly pushing against the North American plate, with the Juan de Fuca being forced to subduct – or subduct – beneath the continent.

Researchers working on the Oregon coast are using buried microfossils to find out how far tsunamis in the Cascadia subduction zone traveled inland. They believe that our current estimates used in tsunami hazard mapping are likely too low.

Todd Sonnefleth/OPB

But the plates do not slide past each other easily. They lock together where they meet, causing the top plate to bend upward. At some point, the stress of making a mistake becomes too much. The plates collide with each other, causing an earthquake along the Cascadia fault line and one or more tsunamis. On the coast, within just a few minutes, the land sinks or subsides, raising relative sea level by six feet or more. Moments later, the tsunami waves roll onto the shore, swallowing everything in its path. The last time this happened was about 300 years ago.

Now, seismologists working on the Oregon coast are trying to figure out how far inland Cascadia tsunamis could travel so coastal communities can prepare for the next big disaster.

“Tsunami waves are very dangerous,” Brady says. “It can happen very quickly. It's important for people to know where they can go.” [get] Outside the tsunami flood zone.

Bridi and Dora suspect that our current estimates of how far a tsunami can travel inland are too low, and they are looking for evidence to support their hunch in local salt marshes.

The layer of beach sand they found about a mile inland under the Salmon River Swamp is their first clue.

“Moving this sand and dumping it into this swamp requires a big wave and a constant push of water,” Dora says.

Following this logic, previous researchers headed inland and took core samples until the sand layer disappeared. They concluded that this was where the tsunami stopped. Our tsunami inundation maps were drawn, in part, based on these data.

But it's not that simple.

“We know from observations of recent subduction zone earthquakes that tsunamis inundate areas much further inland than they carry sediment,” Dora says. “We know from observations in Japan, for example, that the flooding reached more kilometers inland than where there is sedimentary evidence.”

In the massive 2011 earthquake and tsunami in Japan, about 20,000 people were killed. The same could happen in the Pacific Northwest.

“We may be underestimating the extent of that [inland] “The tsunami has inundated the coast,” says Dora.

Diatoms are single-celled algae with hard exoskeletons made of silica. These diatoms were found in core samples collected on the Oregon coast.

Courtesy of David Bruce/Virginia Tech/OPB

Fossils hold the answers

To get a more accurate picture of how far inland tsunamis can reach in Oregon, Brady is taking a closer look at the cores using a powerful microscope.

“I appreciate the things we can't see with the naked eye,” he says.

What sets these seismologists apart from other seismologists is that they look for tiny fossilized algae called diatoms.

“Diatoms are single-celled, photosynthetic algae that are ubiquitous in all aquatic environments,” Dora says. “They're in all the water, like they're in tap water too. They're in my pool. They're in your aquarium.”

Diatoms have complex shells called valves.

“The main type of features and structures are based on silica, which is very resistant to wear, corrosion and chemicals [interactions]“They are very robust, single-celled algae,” says Bruce, a Ph.D. candidate at Virginia Tech who also uses diatoms in his research.

When they die, diatoms can remain uninterrupted in sediments for thousands of years.

But despite their toughness, they are not strong enough to survive a tsunami.

Dora had seen layers of scattered, broken-up diatoms in the sediment before and didn't really know what to do with them.

“It's messy, and the diatoms are broken,” she says. “What I'm starting to realize is that this is most likely a tsunami — the chaotic inundation caused by the tsunami bringing in fine sediments.”

Diatoms are smaller and lighter than grains of sand, so they ride the tsunami wave inward.

“Looking at broken diatoms as an indicator — you know, they've been crushed on their way into the bog — makes a lot of sense,” says Sarah Woodruff, a professor of physical geography at Durham University in the UK. Woodruff used diatoms to study land subsidence and sea level change in Alaska, but he was not involved in the team's research.

A core sample from the Oregon coast shows a mixture of sand and other sediments. Researchers at Virginia Tech use cores and the microfossils they contain to calculate earthquake subsidence and tsunami inundation.

Jess Burns/OPB

By looking under a microscope at these layers of shattered fossils in core samples collected farther inland where the sand layer disappears, Brady will get a better idea of ​​how far the tsunami floodwaters traveled.

What seismologists find could give coastal communities the information they need to provide more accurate tsunami evacuation maps. It can also inform where these communities are built.

“If they're going to build a new school, or a new shopping center, or a new retirement community, they'll know better where to put those things — and not put them in a tsunami danger zone,” Priddy says.

Ultimately, these beautiful little fossils may help us protect the future of the Pacific Northwest Coast.

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