Get Ready to Rumble

New seismic studies show sway time for Seattle skyscrapers.

By Karen Quinn May 22, 2009 Published in the June 2009 issue of Seattle Met

THIS HAS US ALL shook up: Two new studies—both drawing on geologic evidence of the massive magnitude 9 earthquake that shook the Northwest at 9pm on January 26 in the year 1700—present an ever grimmer view of our seismic future.

The first study is a computer simulation from San Diego State University. On a purple map, a blue fissure erupts just off Vancouver Island, runs down the Washington coast like a spilled bottle of Windex, and pours into Puget Sound; blue represents where the ground will move at one-and-a-half feet per second. Kim Olsen, professor of geological sciences at SDSU, suggests that the loose sediment in the Puget Sound area will undergo up to five full minutes of ground shaking.

A California Institute of Technology study is even scarier, especially if your office is on the 43rd floor. During a mag 9 shake-up, steel high-rises—like the ones that make up Seattle’s impressive skyline—will likely collapse. They’ll continue to sway for another “45 seconds after ground shaking stops,” says Thomas Heaton, Caltech professor of geophysics and civil engineering. That, combined with the four to five minutes of swaying the structures would undergo while the earthquake is actually occurring, he says, is enough to topple the buildings.

Our buildings will sway for another 45 seconds after ground shaking stops.

Why the focus on Seattle? “The city presents an interesting case for us because we’ve never seen a magnitude 9 earthquake in a modern city before,” says Heaton. And given the time that has elapsed since the 1700 megaquake and the instability of the Cascadia fault zone, “it’s safe to assume that we’ll get a chance to see it happen.”

Heaton and his team estimate that there are more than 900 skyscrapers in the Pacific Northwest cities of Portland, Seattle, and Vancouver, BC, and half of them were constructed prior to the earthquake building codes established in 1997.

Hearing the fear in our voice, Heaton turned the questions on us: “What kind of structure are you in right now?” A seven-story brick building. “Do you know when it was built?” Nope. His advice: find out. “That could be useful information for you in the future.” PS: It’s vintage 1910, with a 2003 retrofit.

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