When Washingtonians talk about the possibility of a major earthquake in the Cascadia Subduction Zone, the conversation typically focuses on the immediate impacts: the threat of casualties, building damage and flooding.

But what could happen to the Pacific Northwest landscape, including its coastline, if a big earthquake hits? A study published earlier this week in the Proceedings of the National Academy of Sciences aims to answer that question.

The study, conducted by scientists from Virginia Tech, the University of Oregon and the U.S. Geological Survey, among others, simulated the effects of a major Cascadia Subduction Zone earthquake. With simulated quakes ranging from 7.7 to 9.2 in magnitude, it found that a strong tremor could cause the ground to sink by between half a meter and 2 meters — or between 1.6 and 6.6 feet — and dramatically change the region’s floodplain.

According to Harold Tobin, chair of seismology and geohazards at the University of Washington and director of the Pacific Northwest Seismic Network, the paper (which he wasn’t involved in) provides a lot of insight into an often overlooked effect of a major earthquake.

“[It’s] showing that when the earthquake happens, we will suddenly see a very different kind of land level and landscape,” Tobin, who is also the designated Washington State Seismologist, told McClatchy in a phone call. “Not just at the moment of the tsunami and the water washing in, but that it will be left permanently lower in many places by as much as 1 -2 meters.”

Major earthquake could cause WA ground to sink

Major earthquakes in subduction zones are often accompanied by subsidence, which is when the ground sinks by a few feet.

“We’ve known for a long time that the previous Cascadia earthquakes that we can see in the coastal deposits [were] accompanied by a sudden land level drop,” Tobin said.

According to Tobin, the phenomenon is caused by the stress that builds up between the two tectonic plates stuck together in a subduction zone.

“The analogy I like to use is if you’re an archer and you start pulling the string back with the arrow on it. You pull the string back, and you bend that bow. It’s the stored up strain in the bow that, when you let go, makes the arrow fly,” Tobin said. “So what we’re doing is we’re actually straining the crust of the North American plate. And its net effect is that the coastline in between the earthquakes is actually going up very slowly, just creeping up a millimeter per year or so. But then when the earthquake happens, it’ll all sort of un-flex.”

The study looked at three estimates of subsidence across the region: the 50th percentile estimate, which they called the “low” estimate; the top 10% (“medium”) estimate; and the maximum (“high”) estimate. The estimates varies by location.

Across all the locations that the group measured, the low estimates held that the ground would sink by 0.23 to 0.67 meters, depending on the area. The medium estimates ranged from 0.46 to 1.34 meters and the highest estimates for ranged from 0.93 to 2.67 meters.

“It’s like the low amount of subsidence, medium amount of subsidence, high amount of subsidence,” Tobin said. “It’s not low-, medium- and high-probability; it’s kind of the opposite.”

Three Washington areas were considered: Grays Harbor, Willapa Bay and the mouth of the Columbia River.

For Grays Harbor, the low estimate for how far the ground would sink in the event of a major earthquake was 0.45 meters, while the medium estimate was 0.90 meters and the high estimate was 1.80 meters. In Willapa Bay, those figures were 0.51, 1.02 and 2.05 meters, while in Columbia they were 0.67, 1.34 and 2.67 meters.

Changes to WA flood risk

According to the study, there are currently 8,120 residents and 13,370 buildings in the Pacific Northwest’s 1% floodplain, the area that would be covered in water by a once-in-100-years flooding event. If a major earthquake were to cause the ground to sink by the amount laid out in the low estimate, an additional 90 square kilometers, 2,980 people and 4,810 buildings across the region would be in the 1% floodplain. If the ground were to sink by the medium estimate, the region’s floodplain would grow by 160 square kilometers while the high estimate would put an additional 300 square kilometers in the floodplain.

That means, in the worst case-scenario, a major earthquake would put an additional 17,710 people in the Pacific Northwest in an area with flood risk.

Tobin said that he’s never seen a study look at that particular effect of a major Cascadia earthquake before.

“One thing that is really interesting, and I’ve never seen this done for this particular part of the earthquake and tsunami problem, is to cast it in terms of ‘Where is the 100-year floodplain or the 1% floodplain after the earthquake?’” Tobin said.

The study also looked at the projected floodplain for the year 2100 based on estimates of sea level change. In that scenario, an earthquake causing low subsidence would put an additional 6,880 people in the floodplain, while medium subsidence would add 10,940 people to the area and high subsidence would put 17,710 people at risk.

Why PNW earthquake study matters

According to Tobin, the study sheds light onto an important and often overlooked piece of how the Pacific Northwest would be impacted by a major earthquake.

“I think the study is certainly good scientific work,” Tobin said. “What these guys have done is a really nice job of quantifying as best they can what [subsidence levels were] the last time around as a way of predicting what the next one would look like, and then merging that with the more slow and steady, ongoing sea level rise that comes from climate change.”

While people typically think of the earthquake’s immediate impact, Tobin said that there are other, longer-term effects that need to be considered as well, and that the study starts that conversation.

“I really like their way of casting it in terms of, ‘After the earthquake happens, where are those communities left?’” Tobin said. ‘Can you expect your road to be flooded out frequently? Can you expect high tide to affect different places on the land? Whose property is going to now be more likely to flood or even be underwater periodically?”