A Seattle Fault earthquake modeled by the Washington State Department of Natural Resources would send a tsunami wave 33 feet high into Elliott Bay within about three minutes of the first shaking, fast enough that there would be little time for anyone downtown to reach higher ground after feeling the ground move. The simulation is built on a real earthquake, a magnitude-7.3 rupture that struck the Seattle Fault a little over a thousand years ago and left its signature preserved in the region’s geology and in the oral history of the Salish people who lived through it. Researchers say that oral history, passed down through generations, lines up with what the geologic record shows: sudden, violent land movement followed by a fast-moving wall of water.
Alexander Dolcimascolo, a tsunami geologist at the Washington Geological Survey and the lead author of the 2022 report behind the simulation, led the team that reconstructed the ancient quake’s land movements and ran new computer models of the tsunami it triggered. Those models put the wave’s peak height at 33 feet, about 10 meters, moving as fast as 25 knots, roughly 13 meters per second, once it entered Puget Sound.
A quake a thousand years ago left the blueprint
The historic rupture that Dolcimascolo’s team modeled threw parts of Bainbridge Island upward by as much as 23 feet, about 7 meters, while dropping West Point in Seattle down roughly 3 feet, about 1 meter, in the same event. Those abrupt, opposite land movements on either side of the fault generated the tsunami: land south of the Seattle Fault rose while land to the north sank, shoving the water above it violently out of the way. Deposits from that ancient wave turned up as far north as Everett, about 30 miles from Seattle, evidence researchers used to calibrate how far the modeled wave should travel.
New simulations put the first wave in Elliott Bay in three minutes
Gabriel Lotto, a geophysicist at the Pacific Northwest Seismic Network who was not involved in the Washington Geological Survey’s work, said the Seattle Fault illustrates a hazard distinct from the Cascadia megathrust earthquake most Pacific Northwest residents have been trained to worry about. He said local, shallow faults running directly beneath population centers can cause landslides and tsunamis just as damaging locally as a major Cascadia rupture, specifically because their source sits directly beneath the ground people are standing on.
While the first wave would reach Elliott Bay within roughly three minutes of the initial shaking, the department’s modeling found that inundation across the wider Puget Sound region continued for more than three hours as water surged back and forth through the inlets and channels connecting the sound’s various bays.
The region’s last strong earthquake, the magnitude-6.8 Nisqually quake in 2001, struck about 31 miles underground on the deep, subducting boundary between the Pacific and North American plates. It shook Seattle hard but never produced a tsunami, because the section of that deep fault that slipped sat too far below the surface to shove the seafloor the way a shallow, surface-rupturing fault like the Seattle Fault can.
Some Puget Sound shoreline would simply stay underwater
The land-level changes that generate the tsunami do not reverse once the water recedes. Dolcimascolo said a new shoreline would form after the event because of the permanent land movement, meaning ground that had always been dry and habitable could end up flooded twice a day by ordinary tides. Waterfront property inside the subsidence zone north of the fault carries the risk of being lost outright once the new coastline settles in below the average high-water line, a fate the report’s modeling found would befall the Bainbridge Island ferry terminal in the scenario it simulated.
A rare scenario competes with more likely, smaller shakers
The exact scenario in the Washington Geological Survey report, a magnitude-7.3 rupture of the full Seattle Fault, carries less than a 1% chance of happening in the next 50 years. A more probable outcome is a somewhat smaller earthquake, magnitude 6.5 to 7.0, on one of the shallow crustal faults threading through Puget Sound, including the Tacoma Fault Zone to the south and the South Whidbey Island fault zone to the north, both of which the report says could also trigger local tsunamis. Current estimates put the odds of a magnitude-6.5 event on one of those faults at roughly 15% over the next 50 years, though Lotto and Dolcimascolo both note that figure could shift as monitoring networks refine what they know about the faults. Dolcimascolo and eight co-authors published the full modeling behind the scenario as Washington Geological Survey Map Series 2022-03, a 16-sheet report spanning 51 pages of technical text.
The region’s main mitigation tool is ShakeAlert, an earthquake early-warning system that can automatically shut off certain water and gas lines and push alerts to phones the moment sensors detect the first signs of shaking, buying seconds that matter when a wave could arrive in three minutes. Lotto put it simply: take the scenario seriously, he said, but not literally, since the report models one specific rupture among many the Seattle Fault and its neighboring faults could eventually produce.
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This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
