PORT ANGELES — When the Cascadia Subduction Zone earthquake rocks the North Olympic Peninsula, the temblor’s tsunami waves will roll across La Push in about 10 minutes, surging to Port Angeles in an hour and to Port Townsend 30 minutes later, according to a state inundation study.
“The earthquake shaking can last 3 to 6 minutes, so if you’re protecting yourself for 3 to 6 minutes while the shaking is happening, and then a tsunami arrives 4 to 6 minutes later, that’s not any time at all, really, to evacuate,” Corina Allen, chief hazards geologist at the Washington Geological Survey, said Wednesday while presenting the study’s results.
“So, understanding that and having vertical evacuation structures’ relocation out of the tsunami zone, and tsunami mitigation, is critical for life safety and much of coastal Washington, where time is very limited for this tsunami event.”
Allen, featured guest at the Clallam County Economic Development Council’s weekly newsmakers’ Coffee Chat program, discussed tsunami inundation maps produced by the National Oceanic and Atmospheric Administration, the University of Washington and the state Department of Natural Resources, which oversees the Geological Survey.
The maps, released in January, show the impact of a 9.0-magnitude offshore Cascadia Subduction Zone earthquake. Such a quake is estimated by researchers to hit much of the West Coast up to every 600 years. The Pacific Northwest is in year 322 of that cycle. The last one hit Jan. 26, 1700.
Allen said there is one vertical evacuation structure in Washington state. It is perched on top of the Ocosta School District gym outside of Westport. It was funded by a voter-approved school bond.
The 35,000-square-foot rooftop haven — the first in the U.S. — holds 1,000 people, cost $13 million and sits 55 feet above sea level, according to Seattle-based Degenkolb Engineers, which designed the project.
It includes classrooms and a kitchen, reinforced concrete stair towers and drilled piles (www.degenkolb.com/projects).
The Shoalwater Bay Tribe on Willapa Bay is building a vertical evacuation tower through the Federal Emergency Management Agency (FEMA).
A FEMA program, Building Resilient Infrastructure in Communities, allows vertical evacuation structures as an expenditure for communities with an active hazard mitigation plan. They are applied for through counties.
“Vertical evacuation structures are really critical for coastal communities that don’t have time to evacuate and they have a large area of inundation,” she said.
According to a recent study by the state Emergency Management Division and the University of Washington, about 50 to 85 vertical evacuation structures are needed by coastal communities.
“We have 1½, and we need 70-plus more, so there’s a long way to go,” Allen said.
The Cascadia earthquake will be centered in a fault zone where the oceanic Juan de Fuca Plate slides under the North American Plate from off Vancouver Island in British Columbia to north of Mendocino, Calif.
The temblors occur every 300 to 600 years, Allen said.
“We are definitely within the window of when the next large earthquake could happen,” she said.
“Or it could be another 300 years from now. It’s hard to say. Geology works in mysterious ways.”
When it does hit, estimated timelines for the tsunami to hit in Clallam and Jefferson counties are 80 minutes to Dungeness, 85 minutes to Miller Peninsula, 90 minutes to Blyn and 95 minutes to Discovery Bay.
Waves could exceed 20 feet within the Strait of Juan de Fuca.
According to the study, “New Tsunami Hazard Maps of the Olympic Peninsula ” (www.washingtonstategeology.wordpress.com), the first wave would reach Port Angeles with a wave height of 20 feet.
Significant flooding would occur on the Lower Elwha Klallam reservation and on Ediz Hook.
The Sequim area could see inundation of about 10 feet in Dungeness. Discovery Bay has the largest potential for flooding at a predicted 33 feet, likely blocking or destroying areas of U.S. Highway 101.
Four different tsunami deposits have been found there, carbon dated to indicate the impact of past earthquakes.
“Tsunami wave inundation would likely continue over 8 hours and remain hazardous to maritime operations for more than 24 hours,” the study said, estimating coastline and Strait of Juan de Fuca currents would exceed 9 knots.
The shoreline likely would be covered in debris, mud, logs, trees and pieces of buildings, Allen said.
The March 11, 2011, earthquake and tsunami in Japan provides a good example of a post-tsunami future, she said.
“If you look at Japan or other places after the tsunami happened, you can just see if there’s, depending on what exists there today, if that’s what will be there following the tsunami, but it won’t be standing,” Allen said.
“Likely it will be damaged, broken, collapsed.”
Allen’s hourlong presentation is at www.clallam.org.
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Senior Staff Writer Paul Gottlieb can be reached at 360-452-2345, ext. 55650, or at pgottlieb@peninsuladailynews.com.
