Mostly, when people talk about a potential tsunami, the targets are usually communities right on the Pacific Ocean. But this study suggests that a tsunami could accompany a magnitude 7 or greater earthquake in the more inland Seattle area, leaving residents in some areas under three minutes to find higher ground.
The study also indicated that the inundation in the Seattle area from such a tsunami would be about 20 feet and continue for more than three hours.
This potential quake would originate from the Seattle fault and has a probability of about 5 to 7 percent of happening in the next 50 years. An earthquake originating from the Cascadia Subduction Zone would have a 15 to 25 percent chance of occurring within 50 years.
Scientists and emergency managers acknowledge the low odds of the Seattle fault rupture and ensuing tsunami, but they also know that the possibility can’t be ignored and the area’s residents have to be prepared. The Seattle fault crosses Puget Sound and downtown Seattle from east to west.
“Most people in Puget Sound felt they were kind of safe, I think, feeling that the Pacific Coast was more the tsunami threat zone from Cascadia, which is another source offshore,” said Alex Dolcimascolo, lead author on the study. “It’s kind of unlikely, but it’s a possibility, and that’s the whole gist of the study.”
The study was composed of modeling using the last similar earthquake on the fault, which occurred 1,100 years ago and registered about a magnitude 7 or 7.5. The model was developed by scientists at the University of Washington and the resulting data was garnered by including that long-ago quake data along with current data such as elevation and topography data.
Researchers know there have been smaller events, but DNR was interested in a scenario that suggested a possibly catastrophic result.
“We wanted to look at this one end of the spectrum for emergency preparedness reasons so people can be aware of potentially that maximum scenario that could occur,” Dolcimascolo said.
He said the shaking could possibly last for 30 seconds, followed by the tsunami waves, which would give people on shore about three minutes to get to higher ground. People in buildings would probably have time only to go to the highest part of the building for safety.
“In terms of the tsunami, the waterfront area in Seattle and places nearby could expect potentially an average of 20 feet of flooding,” Dolcimascolo said. “That’s pretty significant and can cause a lot of destruction.”
DNR works closely with local emergency managers to ensure that the information is passed on to residents in a way that they can use to keep themselves prepared, however unlikely the scenario.
“I guess the surprising things for me were just how widespread the inundation was going to be in Puget Sound and then the wave arrival times were certainly faster than I thought,” said Maximilian Dixon, the hazards and outreach program supervisor for the Washington Emergency Management Division.
Dixon said mitigation efforts have been ongoing and he believes emergency managers finally have the ear of the populace in terms of taking such a scenario seriously.
“I think we’ve turned the corner as far as explaining the science and evidence of these types of events and the modeling, and we’ve built the trust of the public,” Dixon said. “The challenge is to get folks to carve out time from their busy lives to pay attention to the messaging and be prepared.”
The state hosts the Great Washington ShakeOut every October, a reminder about earthquake safety and tsunami safety strategy.
The state also creates pedestrian evacuation walk maps to educate the public on where to go in the event of an earthquake and tsunami. The maps include expected wave arrival times and inundation depths. They also calculate walking speed, using the measures the United States Department of Transportation uses for crosswalks, and calculate terrain and slope to provide arrival time at the higher ground destinations.
“Residents should practice thinking about what they are going to do, what’s the closest, safest way to get to safety and how they get there,” Dixon said. “That’s the key education outreach we do, including tons of video, massive media campaign presentations and road shows, webinars, and Facebook Live events.”
