Seattle Skyline Could Collapse in Big Quake

April 9, 2009 -- Someday soon, an earthquake of magnitude 9.0 or greater will hit the city of Seattle. It isn't a matter of 'if,' it's 'when.' And if a new study is right, the city's high-rise buildings are at greater risk of collapse than anyone ever knew.

The Pacific Northwest region of North America is home to perhaps the most dangerous fault on the planet, the Cascadia megathrust. Scientists believe the fault ruptures every 300 to 500 years, producing some of the most powerful quakes the world has ever seen.

The last time it broke, in the year 1700, it sent powerful tsunami crashing ashore as far away as Japan.

Engineers and city officials are aware of the threat that lurks in the Pacific Ocean ocean floor, less than 100 miles from downtown Seattle. The city building codes match those developed in California after the Northridge earthquake struck Los Angeles in 1994, and are regularly updated.

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But that may not be enough to prevent Seattle's skyline from devastation. In a first-ever computer simulation of ground motions during a giant earthquake, Jing Yang and Thomas Heaton of the California Institute of Technology in Pasadena have found that the city's high-rise buildings are at a disproportionately high risk of collapsing when the big one comes. They are scheduled to present their findings at the annual meeting of the Seismological Society of America today.

"There are around 900 high-rise buildings in the Cascadia region," Yang said, where 'high-rise' means anything over 10 stories. That includes buildings in Vancouver, British Columbia, and Portland, Ore., which also lie in the shadow of the megathrust.

Yang estimated that over half of those buildings were built before 1994, when steel beams were welded together using 'brittle' welds, which are weaker than the beams themselves. After building codes were updated, welders corrected the problem by switching to far stronger 'perfect' welds.