SEATTLE – A new analysis shows more than 100,000 people are at risk from a tsunami on the Northwest coast – but the outlook isn’t uniformly grim.
In many communities, residents should be able to make it to high ground in time simply by walking at a brisk pace.
Tsunami surges are expected to slam into some parts of the coast within 15 to 30 minutes of an earthquake on the Cascadia Subduction Zone, the offshore fault where two tectonic plates collide.
Published Monday in the Proceedings of the National Academy of Sciences, the analysis takes the most comprehensive look yet at the threat along the 700-mile-long coast of Washington, Oregon and Northern California – and finds surprising variability.
“People always say: Give me a list of the worst towns and we’ll focus on them,” said U.S. Geological Survey geographer Nathan Wood, lead author of the study. “We wanted to point out that all of these communities have issues, but they’re different issues.”
By identifying specific vulnerabilities, the researchers hope to help residents and local leaders better target their education and preparedness efforts.
For example, the 20,000 people who live in Aberdeen and Hoquiam in Washington make up the largest at-risk population on the coast. The quake is expected to leave most roads across the region impassable, but the analysis shows that 90 percent of the Aberdeen-Hoquiam population could make it to high ground by walking at a normal pace.
Crank the speed up a notch and the potential survival rate exceeds 99 percent.
So the biggest payoff in those communities would come from ensuring everyone knows the best evacuation routes – and that most citizens are fit, Wood said.
Across the Northwest, the analysis estimates 21,562 people live in areas where they won’t have time to evacuate on foot walking at 2.2 miles per hour. Speeding up to 3.4 mph would lower that number to 16,000.
“Just by getting people to move faster, you can save thousands of lives,” Wood said.
But the study also confirms that most residents of Ocean Shores in Grays Harbor County, Washington, and in Seaside, Oregon, would not be able to evacuate in time even if they ran like the wind.
In communities like that, the main focus should be on design and construction of tsunami evacuation structures, Wood said.
Ocosta School District near Westport, Washington, is building the region’s first such structure: a gymnasium designed to survive the quake and tsunami and allow hundreds of people to shelter on its roof.
Chuck Wallace, deputy director for emergency management in Grays Harbor County, said he was glad the analysis focused attention on smaller coastal communities at high risk, like Grayland. During clam-digging season and the summer, hundreds of people could be trapped on the beaches with no easy way out, he said.
But even in places such as Aberdeen, where there’s high ground nearby, some people would still have a hard time making it to safety, Wallace cautioned.
“It’s much tougher when you’re dealing with school kids or people you have to put in wheelchairs,” he said.
The analysis examined 73 coastal communities or county areas and grouped them into three categories, based on the population at risk, the number of businesses and critical facilities in the tsunami zone, and the avenues for evacuation.
The first category comprises small settlements with high ground near at hand. The second includes places such as Aberdeen, with larger numbers of people but good evacuation routes. The third includes towns such as Ocean Shores, with relatively high populations and few ways out.
Fifty-six percent of the residents at risk live in Washington, which is also home to half the businesses at risk.
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