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Scary headlines about the Pacific Northwest sinking into the sea are circulating online, with warnings that a major earthquake in the notorious Cascadia subduction zone could be worse than expected.

What’s behind this new alarm? Fortunately, research has not uncovered a new risk that Seattle will become the Lost City of Atlantis. Instead, scientists have examined the combined effects of two well-known phenomena: sea-level rise from climate change and the likely consequences of a major earthquake in the region. It was already known that sea level along the coast of northern California, Oregon, and Washington State is estimated to rise by 1.3 to 2.9 feet by 2100 because of a warming climate. It was also well known that a magnitude 8 or higher earthquake in the area could cause the coastline to slump by up to 6.5 feet. What the new study, published on April 28 in the Proceedings of the National Academy of Sciences USA, adds is an understanding of how much additional land would end up being at a high risk of flooding because of these two combined threats.

Why Does Sea-Level Rise Matter?

While the East Coast is already seeing the effects of beach erosion and rising sea levels, the Pacific Northwest (PNW) has been protected by its geology, so far. The coast that extends from northern California to Vancouver Island in British Columbia sits on a subduction zone where the Juan de Fuca, Explorer, and Gorda oceanic plates slip beneath the North American continental plate—parts of the ever-moving slabs of rock that make up our planet. Right now, the fault system is quiet, meaning it has not had a large earthquake for more than 300 years, and the coast is gradually rising by a few fractions of an inch each year. This geological uplift, a consequence of the interactions of the tectonic plates, outpaces sea-level rise in many areas, so the PNW has been relatively shielded from effects such as extreme flooding events or coastal erosion.

But over the past 7,000 years or so, at least 11 major earthquakes have struck the Cascadia region where these faults reside. The last of these temblors occurred in 1700, and geologists can still see evidence of it causing the coastline to drop between 1.6 and 6.5 feet in the blink of an eye. “We have these really rooted organic soils that are suddenly overlain by this really clean tidal mud, indicating they were suddenly dropped down and were buried and basically converted to tidal flats,” says Tina Dura, a coastal geologist at Virginia Tech and first author of the new study.

What no one had really studied, Dura says, was the combined effect of this sudden subsidence and the slower inundation caused by sea-level rise.

How Much of Cascadia Will Become Prone to Floods?

Dura and her colleagues looked at earthquake scenarios that would yield different ranges of subsidence, from 1.6 to 6.5 feet. They also compared such an earthquake’s effects at today’s sea levels with those at sea levels forecasted for 2100. By that time, sea-level rise is expected to outpace the geological uplift of the PNW and may reach as much as 2.9 feet.

The team found that if an earthquake that caused more than six feet of subsidence were to happen today, the 100-year floodplain in estuaries in Cascadia would expand by 115 square miles. Were this quake to happen in the year 2100, with the additional pressures of sea-level rise, those estuaries would expand by 145 square miles. That would be triple the flood-prone area that is seen today.

How Worried Should We Be?

In the event of a magnitude 8 quake in Cascadia, subsidence would not be the first issue on anyone’s mind. A large undersea quake could cause a devastating tsunami that would immediately threaten lives and structures. The new study is focused on areas within about six feet of elevation of the current 100-year floodplain, Dura says, and the earthquake-driven tsunami could be more than 30 feet high.

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