Geoscientist Tina Dura is conducting geologic detective work in the Pacific Northwest.

Troubled by the profound silence of the Cascadia Subduction Zone, a coastal fault line that runs from California to Canada, Dura and her team are looking deep into the past for clues about when the next earthquake will strike.

“The Cascadia Subduction Zone is unique in that it is very quiet. We haven’t observed any large earthquakes there in the historical or instrumental period. According to geologic records, the last earthquake at the Cascadia Subduction Zone occurred in A.D. 1700. Since over 300 years have passed, we worry about the next big event being imminent and we expect something like the Japan 2011 and Sumatra 2004 earthquakes and tsunamis to occur there,” said Dura, assistant professor of natural hazards in the Department of Geosciences, part of the Virginia Tech College of Science

“If we only rely on the time we have been on Earth and observing things, we can be totally oblivious to what kinds of earthquakes occur. That’s why geologic records are so important. Coastal marshes at the Cascadia Subduction Zone are perfect recorders for the environmental changes that occur during earthquakes and tsunamis. Using coastal marsh sedimentary records, we can gather 6,000 to 7,000 years of earthquake history at the Cascadia Subduction Zone. Cascadia is the longest recorder of earthquakes in the world,” said Dura, who is also an affiliated faculty with the Center for Coastal Studies and the Global Change Center, both part of the Fralin Life Sciences Institute.

A detailed map of the Oregon coast and the Cascadia Subduction Zone.
A map of the Cascadia Subduction Zone shows the estuaries where the Virginia Tech Coastal Hazards Lab is conducting earthquake and tsunami research. Image courtesy of Tina Dura.

The recordings are deep within the soil along the marshes of the Oregon coast. Dura and her team have been taking core samples of the earth to determine past patterns of earthquakes in the Cascadia region and the resulting tsunamis and floods. This information will determine what potential hazards await those living in this 1,300-kilometer region that spans from Cape Mendocino, California, along the Oregon and Washington coast to Vancouver Island, Canada.

Tectonic plate

Off the Pacific Northwest coast is the Juan de Fuca oceanic plate, which is sliding — also called subducting — under the North American tectonic plate, causing it to become stuck. When the tectonic plate finally releases, it will cause an earthquake. Subduction zones, where one plate slides under another, produce the largest earthquakes on Earth with magnitudes of up to 8 or 9 on the Richter scale.

Other subduction zones around the world have smaller earthquakes often and have already experienced “great” — larger than magnitude 9 — earthquakes: a 9.0 in Russia, 1952, a 9.4-9.6 in Chile in 1960, a 9.2 in Alaska in 1964, a 9.1 to 9.2 in Indonesia in 2004, and a 9.0 to 9.1 in Japan in 2011. Yet the most recent great earthquake at the Cascadia Subduction Zone was an estimated magnitude 9.0 that occurred 323 years ago.  

Dura and her team are using geologic methods to reconstruct the seismic history of the coastal region to determine past earthquake-generated coastal subsidence and tsunami inundation to help understand the impact of a future earthquake.

A woman, wearing a hard hat, use a three pronged , tall apparatus to insert deeply into the ground a metal tube for collecting core samples.
A sediment core is taken the Coquille River estuary in southern Oregon. The core contains a soil at the bottom, which represents the pre-earthquake environment; a sand bed overlying the soil, which represents tsunami inundation of the coast following an earthquake; and a gray tidal mud above the sand, which represents the post-earthquake environment following land subsidence. Photo courtesy of Tina Dura.

By taking core samples of coastal sediments in marshes, Dura’s team is able to construct a relative chronological sequence from the oldest sediment deposits to the newest. Team members not only analyze the composition of the peat, silt, mud, or sand deposits, but they also analyze the diatom microfossils to determine the environmental changes due to earthquake precipitated flooding.

Strata cores

Salt marshes along the coast preserve a stratigraphic — the dating of each strata in the core — and microfossil record of relative sea-level change related to the seismic cycle of the Cascadia Subduction Zone. Each of the deposit layers sampled can be analyzed for a date, ecology, and environment.

The diatom microfossils, which are siliceous microalgae, link communities of species to specific environments, such as high marsh, low marsh, or tidal flat. The sediment provenance also provides information about the origin of the sediment deposits. For example, the layer of coarse sand right above a layer of peat in the core sample indicates an earthquake that precipitated a flood in the region because sand originates from the ocean floor. The mud above the sand layer represents the post-earthquake lowering of the land into a new, lower intertidal environment. 

While the question is not if an earthquake will occur off the Pacific Coast but when, time is of the essence in helping that region better prepare for what is inevitable. Dura’s team has been instrumental in helping to determine how extensive flooding has been and where it has occurred in the past through their core samples. Knowing the past flood routes helps in planning for the future. 

The earthquake that occurred in 1700 had an estimated magnitude of 9.0 and caused a tsunami hours later in Japan and flooding minutes later in the Pacific Northwest. The earthquake also produced coastal subsidence that submerged coastal environments, killing forests of red cedar and Sitka spruce trees, leaving what are now known as the ghost forests. To compare the devastation possible to a modern-day earthquake, consider the same magnitude earthquake that hit the northeast coast of Japan in March 2011 killing 18,000 people, causing the meltdown of the Fukushima power plant, and costing approximately $220 billion. 

A closeup of a hand holding a knife that is inserted into collected sediment from Oregon.
Tina Dura examines a sediment core sample collected from Oregon in her lab at Derring Hall. Photo by Steven Mackay for Virginia Tech.

Preparation for the inevitable

Dura’s research is essential for preparing for the next earthquake and to help residents start the conversation about how to plan. She and her team intend to return to Oregon this October with two specific goals. The first is to take more core samples in several locations to gather information on the past 10 to 11 earthquakes rather than just the one in 1700. 

“When you have multiple earthquake events preserved at a site, you can get an idea of how often they occur and how their strength varies over time. Are they evenly spaced over time? Is the 1700 event representative of what we can expect or were there also larger or smaller earthquakes and tsunamis?” Dura said. “This is where our geologic records become very important. We can expand on that short instrumental and historical record and get a better sense of the range of earthquake and tsunami magnitudes we can expect.”

The second purpose for the return is to determine the extent of the impacts of coastal subsidence from each earthquake. By understanding how much the land went down during past earthquakes, Dura and her team can more precisely categorize the magnitude of each earthquake and prepare for how the floodplain will expand following a future Cascadia earthquake.

A man in na blue shirt closely examines some collected sediment using a magnifying glass.
Geosciences graduate student Mike Priddy examines a sediment core sample collected from Oregon in the Derring Hall lab of Assistant Professor Tina Dura. Photo by Steven Mackay for Virginia Tech.

“We are currently working on a project where we simulate Cascadia earthquake subsidence and look at the resulting expansion of coastal floodplains in California, Oregon, and Washington. An earthquake will happen, the land will subside, and it will subside over a meter probably, so all the sea-level rise that is predicted by 2100-2150, could happen tomorrow,” Dura said. “We need to be thinking about this. For coastal communities to remain resilient, stakeholders and decision makers need to prepare for land subsidence from future Cascadia earthquakes.” 

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