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.
In Yang and Heaton's simulation the crucial difference was laid bare; a giant earthquake shook buildings for up to five minutes, and those with brittle welds were five times more likely to collapse.
The key is that giant quakes generate a lot of low-frequency shaking, often as slow as one strong, wobbling shake per second. This motion particularly affects taller buildings, which begin to resonate with the shaking, leaning further and further as their weight lolls back and forth. If the building tilts much beyond five degrees from vertical, its steel frame will bend, and eventually it will collapse.
"I definitely see this as being a useful study," Barb Graff, director of the Office of Emergency Management for the City of Seattle. "It's not quite at the same level of tangible experience that makes you change building code, but if it gets tested further in the community and becomes more concrete down the road, it could change code."
However, Graff stressed that engineers at the city's Department of Planning and Development are acutely aware of the seismic hazards posed by the Cascadia and nearby Seattle faults, and comb the latest research for information relevant to upgrading earthquake protection in buildings.
"There are still many unknowns and uncertainties in our simulations," Jing Yang of the California Institute of Technology said. "But we want to express our concern about the possibility of building collapse. Residents and insurance companies should know these buildings are not strong enough to prevent collapse."
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