A new report warns that Seattle waterfront and other low-lying areas could be inundated by a tsunami wave within minutes of a Seattle Fault earthquake.
By Laura Fattaruso, Simpson Strong Tie Fellow (@labtalk_laura)
Citation: Fattaruso, L., 2022, Faults underneath Seattle could trigger 33-foot tsunami wave, Temblor, http://doi.org/10.32858/temblor.267
Just over a thousand years ago, a fault running east-west under Puget Sound ruptured, throwing parts of Bainbridge Island skyward as much as 23 feet (seven meters), and dropping West Point, Seattle, down three feet (one meter). These abrupt land surface changes were accompanied by an approximately magnitude-7.3 earthquake that triggered a tsunami wave, which left deposits throughout Puget Sound, as far as Everett, about 30 miles (50 kilometers) north of Seattle.
New computer simulations reveal that the first wave probably hit within just three minutes of the earthquake. The research, released in a report by the Washington State Department of Natural Resources (DNR), highlights areas at risk for a future event.
“Right under your feet”
Researchers used records of the historic earthquake, found in the natural landscape, to re-create the land surface changes and resulting tsunami wave. Land south of the fault moved up, whereas land north of the rupture moved down. The resulting displacement of water in the Puget Sound produced a tsunami wave 33 feet (10 meters) high, moving as fast as 25 knots (13 meters/second). Though the first wave would hit Elliot Bay in downtown Seattle within about 3 minutes of the first ground shaking, inundation throughout different parts of Puget Sound continued for more than three hours. The oral history of the native Salish people in the region affirms the geologic observations of the catastrophic event.
“Everyone thinks about the big one — the Cascadia megathrust rupture, but there are local sources of hazard in the Puget Sound region that can cause cascading issues like landslides and tsunamis,” says Gabriel Lotto, a geophysicist at the Pacific Northwest Seismic Network who was not involved with the new research. “Those can be just as damaging locally as a really big one, because the source is right under your feet.”
The last strong earthquake in the region — the magnitude-6.8 Nisqually earthquake in 2001 — struck 31 miles (50 kilometers) underground, beneath the southern Puget Sound. That quake occurred on the Pacific tectonic plate, which is subducting beneath the North American plate. Although it produced significant shaking and damage in Seattle, the section of the fault that slipped in the quake was so deep that it did not trigger a tsunami. Earthquakes on shallow, surface-rupturing faults such as those running through Puget Sound, are a different story.
The Seattle Fault — the source of the shock modeled in the DNR report — is made up of a series of roughly parallel fault traces that run east-west through the metro’s southern suburbs and dissect Mercer and Bainbridge islands.
An earthquake on the Seattle Fault would not just have a short-term impact, says Alexander Dolcimascolo, a tsunami geologist at the Washington State Department of Natural Resources and lead author of the report. “Once the floodwaters recede, there’ll be a new shoreline due to land level changes.” A new coast could see land that was one always dry and habitable, be flooded twice a day due to tides, he says.
“If you have waterfront property in that subsidence zone, there’s a chance that your home could be lost,” Dolcimascolo says. The Bainbridge Island Ferry Terminal, for example, dropped below the average high-water line during the earthquake modeled in the report.
The scenario, though alarming, has a low probability — less than a one percent chance — of happening in the next 50 years. What is more probable, though, is a somewhat smaller earthquake, between a magnitude 6.5 and 7.0, on any one of the shallow crustal faults in the Puget Sound. The study simulated the historic earthquake on the Seattle Fault, but the Tacoma Fault Zone to the south, and the South Whidbey Island fault zone in the north, could also trigger shaking and tsunami waves in the Puget Sound.
Current estimates give a 15% chance of a magnitude-6.5 event on one of these faults in the next 50 years, but as our knowledge about these faults improves, that probability could change. The exact hazard faced by any one region will depend on many factors — the magnitude of the earthquake, its timing relative to the tides and currents, the distance from the epicenter, the intensity of land surface changes. The scenario in this report is one of many possibilities. “Take it seriously, but not literally,” says Lotto.
The ShakeAlert Earthquake Early Warning system offers one way to mitigate some risks. The system can automatically respond to the first signs of shaking by shutting off certain water and gas lines, and delivers automated alerts to people in affected areas.
Bill Steele of the Pacific Northwest Seismic Network, who was not involved with the report, says he hopes this study will motivate wider use of the ShakeAlert system. “This study is a reminder that we can always be doing more to prepare for hazards,” he says. “Looking at scenarios like this reminds us that we don’t have time, when there’s only a few minutes before a wave arrives on the coast, to be getting humans to make the right decisions and push the right buttons. It’s just not enough time.”
Laura Fattaruso is Temblor’s Simpson Strong Tie Fellow. They are a Ph.D. candidate at U Mass Amherst, where they study how rocks break to better understand earthquake processes (laurafattaruso.com). Simpson Strong Tie is sponsoring a science writing fellow to cover important earthquake news across the U.S.
Dolcimascolo, Alexander; Eungard, D. W.; Allen, Corina; LeVeque, R. J.; Adams, L. M.; Arcas, Diego; Titov, V. V.; González, F. I.; Moore, Christopher, 2022, Tsunami inundation, current speeds, and arrival times simulated from a large Seattle Fault earthquake scenario for Puget Sound and other parts of the Salish Sea: Washington Geological Survey Map Series 2022-03, 16 sheets, scale 1:48,000, 51 p. text.
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