Alarming headlines are making the rounds online, suggesting that the Pacific Northwest could dramatically sink due to a major earthquake in the infamous Cascadia subduction zone, with potentially dire consequences.

What’s triggering these recent concerns? Thankfully, there’s no newfound risk of Seattle turning into a modern-day Atlantis. Instead, researchers have been focusing on how two well-understood threats—rising sea levels due to climate change and the significant impact of a major earthquake—might combine. It’s already known that sea levels along the coasts of Northern California, Oregon, and Washington are projected to rise between 1.3 and 2.9 feet by the year 2100 due to a warming climate. Additionally, it’s a well-established fact that a magnitude 8 or greater earthquake could cause the coastal land to drop by as much as 6.5 feet. A recent study, published on April 28 in the Proceedings of the National Academy of Sciences USA, reveals the extent to which additional land could be at serious risk of flooding from these dual threats.

Understanding the Impact of Rising Sea Levels

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While the East Coast frequently deals with beach erosion and rising sea levels, the Pacific Northwest has been somewhat protected by its unique geological features—until now. Stretching from Northern California to Vancouver Island in British Columbia, this region sits atop a subduction zone where the Juan de Fuca, Explorer, and Gorda oceanic plates slide beneath the North American continental plate. These massive tectonic plates have been relatively quiet for over 300 years, with the coast gradually rising a few millimeters each year. This geological uplift has so far helped the area stave off severe flooding and significant coastal erosion.

However, the Cascadia region has experienced at least 11 major earthquakes over the past 7,000 years. The most recent one in 1700 caused the coastline to suddenly drop between 1.6 and 6.5 feet. Tina Dura, a coastal geologist at Virginia Tech and the lead author of the recent study, noted, “We see deeply rooted organic soils abruptly covered by clean tidal mud, indicating a sudden drop and transformation into tidal flats.”

Until now, the simultaneous impact of this rapid subsidence and the gradual sea-level rise had not been thoroughly investigated, according to Dura.

The Flood Risk Increase in Cascadia

Dura and her team examined various earthquake scenarios, with subsidence ranging from 1.6 to 6.5 feet. They compared the effects of such earthquakes at current sea levels with those predicted for 2100. By then, rising sea levels are expected to surpass the geological uplift of the Pacific Northwest, potentially reaching up to 2.9 feet. The researchers discovered that if an earthquake causing over six feet of subsidence occurred today, it would increase the 100-year floodplain in Cascadia’s estuaries by 115 square miles. If such an earthquake were to strike in 2100, amidst higher sea levels, the flood-prone area could expand by 145 square miles, tripling the current flood-prone regions.

Assessing the Level of Concern

In the event of a magnitude 8 earthquake in Cascadia, the immediate concern would be the devastating tsunami it could trigger, posing an immediate threat to life and property. The study focuses on areas within about six feet of the current 100-year floodplain, but the potential tsunami could exceed 30 feet in height. This scenario is reminiscent of the 2004 Sumatra earthquake and tsunami and the 2011 Tōhoku earthquake and tsunami, both of which resulted in massive loss of life and significant subsidence but were primarily noted for their initial catastrophic impacts.

However, understanding the potential for subsidence is crucial for future planning, Dura explains. City planners should consider avoiding the development of essential infrastructure like electrical substations or wastewater treatment facilities in areas that may become new floodplains. Additionally, the flooding of critical infrastructure such as low-lying bridges, roads, and airports following the earthquake could hinder rescue and evacuation efforts. Many residents might discover that their properties, previously considered safe from flooding, are now susceptible to regular inundation, especially in developed areas like Seaside and Gearhart in Oregon, and Grays Harbor County in Washington, according to Dura.

“The tsunami is an event that unfolds in minutes to hours after the earthquake,” Dura notes, “but once the tsunami recedes, we are left with a permanently altered landscape and an expanded floodplain.”