Cascadia Subduction Zone Earthquakes Could Be More Severe Than Previously Anticipated

As an earthquake ruptures along the Cascadia subduction zone fault, much of the US West Coast will experience intense shaking for five minutes, with tsunamis potentially generating waves up to 100 feet crashing towards the shore. However, this is only the onset of anticipated devastation.

Even if coastal communities in Northern California, Oregon, and Washington withstand the initial earthquake, recent research indicates that flooding could inundate many of these susceptible regions. This is due to an expected drop of 6½ feet in the entire coastal land when the earthquake strikes, according to a new study published Monday in the Proceedings of the National Academy of Sciences (PNA).

Researchers examined earthquake and flood models to provide some of the most comprehensive predictions about how Cascadia earthquakes can lower or subside coastal land, potentially affecting over twice as many people, structures, and roadways as currently established. The exacerbating effects of climate change are projected to raise sea levels, compounding the issue over time.

“The repercussions of these hazards will linger for decades or even centuries following the earthquake,” stated Tina Dura, the study’s lead author. “Tsunamis will strike and have a considerable impact. Don’t misunderstand me; however, a lasting change in flood frequency… that is a critical concern.”

Dura explained that geological fossil evidence suggests that previous Cascadia earthquakes resulted in a significant drop in land level, transforming once dry areas into tidal mud flats along the Pacific Northwest’s estuaries.

An assistant professor of geoscience at Virginia Tech, Dura noted: “This is how we have a harbor…and where we’ve established towns, yet that land will plummet by a maximum of two meters.”

The Cascadia subduction zone fault offshore of North America presents an imminent threat, capable of generating magnitude 9.0 earthquakes. Such events are expected to occur on average every 450-500 years, with the last major quake dating back to 1700.

The national seismic hazard model indicates a 15% chance of an earthquake measuring 8.0 or higher occurring along the zone within the next 50 years.

When the fault ruptures, experts assert that it could lead to the most catastrophic natural disaster in the nation’s history. Simulations from 2022 predict that the Cascadia earthquake could damage around 620,000 buildings in the Pacific Northwest, including 100 hospitals and 2,000 schools, resulting in over 100,000 injuries and approximately 14,000 fatalities.

Recent findings emphasize that coastal planners must seriously consider not only the immediate threats of strong shaking and tsunami waves but also the long-term impacts of land reshaping and rapid subsidence of the coastline itself.

“There will be the flooding itself, as well as enduring changes in land elevation along the coast, greatly affecting community planning,” remarked Harold Tobin, director of the Pacific Coast Earthquake Network and professor at the University of Washington. “Where will schools and hospitals be built? Where will transportation networks be established? A long-term perspective is vital.”

Following the earthquake, Dura’s research predicts that towns along the Pacific Northwest coastline, such as Seaside, Oregon, Westport, Washington, and Aberdeen, Washington, will likely experience frequent flooding, at least once every century.

The study also highlights that climate change-induced sea level rise will accelerate, aggravating the consequences of post-earthquake flooding in the future.

Global average sea levels have risen by approximately 8-9 inches since 1880, according to the National Oceanic and Atmospheric Administration (NOAA). Dramatic acceleration in sea level rise is anticipated in the coming decades due to global warming, with NOAA estimating an increase of 10-12 inches.

The impact of sea level rise varies depending on location and can significantly affect the coastline.

In places like Chesapeake Bay, Virginia, land is gradually sinking, a process termed subsidence, while portions of the Pacific Northwest are experiencing uplift due to continental movements. This uplift can offset some of the sea level rise.

The uplift is attributed to the stress build-up within the plates forming the Cascadia subduction zone. In this zone, the Juan de Fuca plate is forced beneath the North American plates, causing a slight upward movement of land.

Currently, the subduction zone faults remain inactive, accumulating stress. When the fault eventually ruptures, the released plate bow leads to rapid land-level subsidence, effectively negating the uplift for centuries.

“It all transpires in a matter of minutes, resulting in meter-level drops,” stated Dura. “The land continues to shift, and as I mentioned, this has ramifications that will last for decades and centuries. Consequently, the critical areas of the floodplain are significantly impacted.”