Earthquakes and flood zones: Hidden risks in the Pacific Northwest

It’s not just the next major earthquake that the Pacific Northwest needs to worry about. A powerful earthquake coupled with rising sea levels could spell disaster for thousands of people in northern California, Oregon, and Washington. And this threat may be much closer than most realize.

Recent research from scientists at Virginia Tech paints a concerning picture. Their study reveals how a large earthquake along the Cascadia subduction zone could cause the ground to sink by as much as 6.5 feet (2 meters).

This land drop would dramatically expand coastal flood zones, adding up to 116 square miles (300 square kilometers) to areas already at risk of annual flooding. These zones, known as the 1-percent coastal floodplain, represent regions with a one-in-one-hundred chance of flooding each year.

“The expansion of the coastal floodplain following a Cascadia subduction zone earthquake has not been previously quantified, and the impacts to land use could significantly increase the timeline to recovery,” said Tina Dura, lead author of the study and assistant professor of geosciences in the College of Science.

Flood risks grow with sinking land

The researchers focused on southern Washington, northern Oregon, and northern California – areas where both population density and infrastructure vulnerability are high.

To understand what might happen, Dura’s team created thousands of earthquake models to simulate how much the land could sink during the next big quake.

The researchers used geospatial tools to estimate how this subsidence would expand the floodplain across 24 estuaries and communities along the Cascadia zone.

Because no one knows when the next major earthquake will strike, they modeled two scenarios: one in which the earthquake hits today, and another where it occurs in the year 2100, by which time sea levels may be up to 3 feet (0.9 meters) higher due to the impact of climate change.

More people and infrastructure at risk

If an earthquake struck today, an estimated 14,350 more people would find themselves living in a floodplain. Over 22,000 additional buildings and nearly 800 miles (1,290 kilometers) of roads would also be at risk.

The expanded flood zone from an earthquake could reach five airports and 18 essential public facilities, including hospitals, schools, and fire stations.

In addition, eight wastewater treatment plants, one electric substation, and 57 contaminant-prone facilities – like gas stations and waste centers – would fall within the danger zone.

As sea levels continue to rise, these risks become even more severe. By 2100, the number of people, buildings, and roadways exposed to flooding could more than triple.

“Today, and more so in 2100, as background sea levels rise, the immediate effect of earthquake-driven subsidence will be a delay in response and recovery from the earthquake due to compromised assets. Long-term effects could render many coastal communities uninhabitable,” said Dura, who is an affiliate with the Global Change Center.

Earthquakes and shifting flood zones

Low-lying land along the coast, much of it currently used for farming and grazing, could be lost to tidal flooding. When seawater floods inland, it leaves behind salt. This salt ruins soil, making it impossible to farm.

Natural ecosystems will also be hit hard. Estuaries, wetlands, dunes, and beaches that protect inland areas from storm surges could be eroded or lost. These natural buffers also help slow down waves and protect property against damage.

“The loss of intertidal wetlands directly impacts ecosystem services such as water filtration, habitat for fisheries and shorebirds, and carbon storage capacity,” said Dura.

“Intertidal wetlands function as natural carbon sinks, and their erosion or conversion to tidal flats significantly reduces their ability to sequester carbon.”

Seismic history of the Pacific Northwest

The Cascadia subduction zone lies within the “Ring of Fire,” the volatile boundary where the Pacific Plate collides with other tectonic plates. It’s one of the most seismically active areas on the planet. But it’s been eerily quiet for centuries. The last great earthquake – defined as magnitude 8.0 or higher – struck the region in January 1700.

Dura and her team are piecing together the seismic history of this area. As part of the Paleoseismology Working Group within the Cascadia Region Earthquake Science Center (CRESCENT), they are studying soil and sediment records that span the last 6,000 to 7,000 years.

Their findings show that the region has experienced at least 11 great earthquakes over that time, occurring roughly every 200 to 800 years. The last one caused widespread land subsidence, with coastal areas sinking by up to 6.5 feet.

“Cascadia is a unique place. It’s not super heavily populated, but most estuaries have a community in them, and they’re all right in the zone of subsidence,” said Dura.

“This is honestly where I think the subsidence could have bigger impacts than it has during other recent large earthquakes around the world.”

Flood zones and earthquake risks

What’s happening in Cascadia could happen anywhere that a subduction zone exists. Similar geological settings are found off the shores of Alaska, Japan, Chile, Indonesia, and other places.

When two tectonic plates get stuck and pressure builds, a huge quake eventually releases that energy. Offshore, the land may rise, triggering a tsunami.

On land, the ground drops, causing flooding even before the tsunami hits. In just 30 minutes, coastal communities can go from earthquake shaking to underwater chaos.

The devastation isn’t limited to the day of the quake. Some effects, like sinking land and increased flooding, can last for decades or even centuries.

Examples from around the world offer a grim preview: the 1960 earthquake in Chile drowned forests and farmland, turning them into salt marshes. The 1964 quake in Alaska forced entire communities to relocate to higher ground.

The 2004 Sumatra earthquake devastated waterfront aquaculture and reshaped coastal areas. Japan’s 2011 earthquake disrupted ports, caused extensive coastal erosion, and contributed to a nuclear crisis.

Implications beyond the Pacific Northwest

“Given the global prevalence of subduction zones, these insights hold relevance beyond Cascadia, informing hazard assessments and mitigation strategies for tectonically active regions worldwide,” Dura stressed.

The next great Cascadia earthquake is not just about the shaking. It’s about what comes after – the rising waters, the sinking land, and the long road to recovery.

With sea levels creeping up and the seismic clock ticking, understanding and preparing for these layered risks could mean the difference between recovery and retreat.

The full study was published in the journal Proceedings of the National Academy of Sciences.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–