USA: What if the megaquake happens when you’re in a Seattle high-rise? New study predicts stronger shaking
By Sandi Doughton
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Now, results are in from the M9 project, a four-year effort to better estimate shaking in Seattle from a magnitude 9 Cascadia quake — and the news is not reassuring. Researchers found the sedimentary basin under the city can amplify the type of ground motion that’s hardest on high-rises by a factor of two to five — much more than previous estimates.
In response, Seattle and Bellevue are boosting seismic standards for new buildings 240 feet or taller, or roughly 20 stories or more. But the prospect of stronger shaking also raises concerns about older high-rises, many constructed long before the region’s earthquake hazards were fully understood.
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Even before the new shaking estimates, older high-rises were known to have a greater risk of serious damage and collapse than buildings that meet modern seismic codes. Some are supported by steel frames with a type of welded joint now known to be fracture-prone, while others were built of poorly-reinforced concrete.
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But in Seattle, where the threat to tall buildings is higher than in California because of the power and nature of subduction zone earthquakes, there have been no efforts to identify potentially dangerous high-rises. The city also does not require retrofits for old brick buildings, a seismic menace California tackled decades ago.
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Among the more surprising findings is that shaking intensity in Seattle can vary by a factor of 10, depending on exactly how and where the 700-mile-long fault ruptures. The analysis also found that strong shaking will last for about two minutes, four times longer than most earthquakes, said USGS researcher Art Frankel, who conceived the project and led the ground motion estimates.
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