The latest modeling of a Cascadia Subduction Zone earthquake along the West Coast suggests a more powerful tsunami and more flooding than previously expected in areas that include Skagit County.
“This result is especially significant in coastal regions that are low in elevation and protected behind structures such as dikes ... such as the Skagit Valley, where even the slightest increase in the waveform can lead to major increases in inundation extent,” the new study states. “Due to the low-lying nature of this region, a tsunami has the potential to cause several miles of inland flooding if these structures become compromised by the earthquake or the tsunami.”
The study was published by the state Department of Natural Resources’ Washington Geological Survey in collaboration with the National Oceanic and Atmospheric Administration and the University of Washington.
By analyzing all of west Skagit County, the study showed that areas including Anacortes, the Samish Flats, La Conner, Fir Island and the Swinomish Indian Tribal Community reservation would likely face more flooding following a magnitude 9 earthquake — the maximum intensity anticipated within the subduction zone — than in estimates produced in 2005 and 2018.
To create the new model, the study authors extended the earthquake scenario to the full length of the subduction zone fault line, which runs about 700 miles under the Pacific Ocean from Vancouver Island to northern California. Previous studies used a modeling scenario in which the northernmost portion of the fault didn’t rupture.
The new scenario “better represents what an earthquake offshore of Washington ... could do to our inner waterways in the Salish Sea,” lead study author Alexander Dolcimascolo of the Washington Geological Survey said.
“This northern extension ... effectively pushes more water into the Strait of Juan de Fuca and causes significantly more flooding in the low-lying areas in the interior waterways of our state,” he said. “This is especially significant in Skagit County.”
By sending larger waves at faster speeds into the area, the tsunami would put much of low-lying Blanchard, Bow, Edison, the Samish Flats, lands adjacent to the Swinomish Channel, La Conner and Fir Island under deeper water than the previous models suggested.
Water would also cover longer stretches of Highway 20 — from about Best Road to Reservation Road and along the southern edge of Fidalgo Bay.
The first tsunami wave could arrive on the west side of Whidbey Island about 90 minutes after the earthquake, with wave crests in excess of 16 feet traveling north into the Strait of Georgia and south into Puget Sound, according to the study. Other areas from Blaine to Olympia would be impacted within 4 hours.
Tsunami inundation would likely continue over 14 hours with several waves — each washing over the affected Skagit County shorelines at greater depths than previously modeled — and conditions could remain hazardous for maritime operations for more than 24 hours, according to the study.
Dolcimascolo said as the tsunami waves surge through narrow waterways, they would likely exceed speeds known to cause extreme damage to ports and other waterfront infrastructure. Those areas include the Guemes Channel and Deception Pass, according to the study.
Hans Kahl of the Skagit County Department of Emergency Management said he reviewed the study as a member of the state’s tsunami working group and the county is updating its emergency response plans.
Cities and communities throughout the county are also coordinating their own plans with the department, including mapping evacuation routes, approving the placement of emergency sirens and establishing local emergency assembly areas.
“Anacortes is pretty well mapped out already and has already implemented their signs,” Kahl said.
Two emergency sirens are operational in Anacortes, and one in Edison. The county is now preparing to install several more.
“We’re getting all kinds of new sirens in and starting to put signs up now for evacuation routes,” Kahl said. “All total, if things go as planned, we will have between 12 and 13 sirens throughout the county for tsunami awareness.”
That includes areas east around Blanchard and on Samish Island.
NOT AN EXACT SCIENCE
The study estimates that tsunami waves could reach Deception Pass State Park less than two hours from the time of the earthquake, and the inundation there could reach about 12 feet. The incoming water is also modeled to approach that area at some of the highest speeds, at about 29 mph.
Because variables including intensity of the fault line’s rupture and the height of tides at the time will influence how deep coastal flooding from a tsunami will be, the model can’t predict exact depths.
“One, it’s very dependent on the size of the earthquake and how much earth gets moved for the size of the wave; and two, does it occur during a King Tide or a high tide or another time when our waterways are already sort of at high capacity?” Kahl said. “This is just a simulation.”
What the study shows are what are likely to be average and maximum depths. Those numbers are valuable for regional hazard planning.
“We hope this information will be used to increase community resilience to tsunamis in the Puget Sound region and its adjacent waters,” the study authors wrote.
Kahl cautions that reality could be even worse that what the modeling projects.
“We just want everybody when they make plans, to plan for the greatest possible, highest tide possible,” he said. “Mother Nature doesn’t like to be outguessed, so we take our plans with a grain of salt ... We always assume the worst is going to happen.”
Dolcimascolo said the model used average high tides based on 19 years worth of data.
According to the model, water depths in the Samish Flats could average 8 feet, in the Edison area about 3 feet, and on Fir Island between less than 2 feet and up to 10 feet depending on the location of Skagit River side channels. The at-risk sections of Highway 20, both to the east of Fidalgo Island, could be under about 4 feet of water.
Kahl said the impacts to the highway will affect even those whose homes aren’t flooded on Fidalgo, Whidbey and the San Juan islands.
“That’s a lifeline to Island County through Deception Pass, and also to San Juan County through the ferries — and if Highway 20 is inundated, those counties will be impacted,” he said.
A STEP TO SUPPORT PREPAREDNESS
Much attention in recent years has been on the potential for a major Cascadia earthquake and tsunami to strike the region because according to the geological record such an earthquake is past due.
The last known occurrence of a Cascadia earthquake was in 1700, and before that, they occurred an average of every 350 years.
Knowing that an earthquake-tsunami scenario will happen again, state and local agencies have in recent years ramped up efforts to map evacuation routes, install evacuation route signs, install warning sirens and improve awareness of the possibility of such a disaster.
Kahl said it’s also up to everyone who lives and works in the region to make sure they’re prepared.
“You have a little bit of a personal obligation to be ready, such as having a grab and go bag and a family communication plan and knowing where your community assembly area is,” he said.
The county regularly hosts community events to raise awareness. The last was offered by webinar in December. The next is planned for June.