Another version of the upcoming megaquake when the Cascadia subduction zone snaps, written by Bruce Barcott in August 2011.
On the Oregon and Washington coasts, the S-waves turn the landscape into a rolling sea. Tourists struggle to stay on their feet. Older buildings shift off their foundations. In Seaside, the 1924 bridge that carries Broadway across the Necanicum River can’t handle this dance. It twists, buckles, and collapses.
People start checking their watches. Nobody can believe an earthquake could keep going this long. For that they can blame the unique features of the CSZ.
“Because there’s so much sediment on it, the CSZ is very smooth,” says Goldfinger. “Once it gets going, there are no irregularities on its surface to stop it. If there’s no reason for it to stop, it’ll just keep going until it dissipates all 300-odd years of elastic strain.”
Japan’s March 11 quake lasted more than five minutes. That’s longer than it takes a pot of coffee to brew. And that’s not good.
“Most modern buildings weren’t designed to withstand three to four minutes of shaking,” says Peter Yanev. One of the leading seismic-engineering consultants, Yanev has investigated more than a 100 quakes around the world. “Almost none of the buildings in Seattle were designed for a megaquake.”
Most unreinforced-masonry buildings in Portland and Seattle can survive a 45-second quake, like the magnitude-6.8 Nisqually quake that hit Seattle in 2001. But the longer they’re shaken, the weaker the structures become. “The difference between 40 seconds and four minutes is like the difference between a head-on collision at four miles an hour versus 40,” says Yanev.
That 45 second one was bad enough. I don’t look forward to a four minute one.
People in Seattle and Portland—those who have power and whose cellular networks are still functioning—watch live footage of the tsunami on their smartphones, shot by news helicopters. They wonder if it will hit the cities.
It probably won’t. To reach Portland, the tsunami would have to muscle its way up 75 miles of the Columbia River and hang a hard right at the Willamette River. Seattle is similarly protected by the topography of Puget Sound. The tsunami will likely slosh up the sides of the Strait of Juan de Fuca and expend its residual energy on the western shore of rural, sparsely populated Whidbey Island.
There could be a strange mini-tsunami effect in Puget Sound, however. Hydrologists call it a seiche. It’s like what happens when you kick a dog’s water bowl. The water sloshes back and forth in slowly diminishing waves. A handful of people who wander down to shore to watch the arrival of the tsunami will get sucked into the sound.
I can see it from here, thanks, assuming the whole place hasn’t pancaked.
It’s looking very pretty and tranquil right now.