October 23, 2013 – SEATTLE – You already know Seattle is due for a devastating earthquake, but a new study shows one more thing to worry about: Landslides. Seattle is full of slope-side real estate with gorgeous views of Lake Washington and Puget Sound. It’s also primed for possible disaster, thanks to these very same hilly areas that could hurtle into the water during the next big earthquake. Published Tuesday in the Bulletin of the Seismological Society of America, the study found that damage from earthquake-triggered landslides will be worse and more widespread in Seattle than previously thought. The study focused on the Seattle Fault, a 30-mile fracture that runs east-west through Seattle, under CenturyLink Field and over to Issaquah. Capable of inflicting mass damage, it’s due for another rupture, but no one knows when. “A major quake along the Seattle Fault is among the worst case scenarios for the area since the fault runs just south of downtown,” said Kate Alltstadt, a University of Washington doctoral student and co-author of the study.
Then there’s Seattle’s rain and craggy topography, a recipe for landslides. Allstadt and the research team wondered: How would a magnitude 7.0 quake along the Seattle Fault affect the city’s crumbly slopes? Answer: Catastrophe. The study found that thousands of landslides would ravage Seattle’s coastal bluffs and southern neighborhoods. More than a 1,000 buildings would be in hazard zones of collapsing hills, the study found. But that when the soil was dry. A far worse threat loomed when hills were soggy. More than 8,000 buildings would be in potential danger if a major earthquake hit after a rainy spell. “A lot of people assume that all landslides occur in the same areas, but those triggered by rainfall or human behavior have a different triggering mechanism than landslides caused by earthquakes, so we need dedicated studies,” said Allstadt, who’s also a seismologist with the Pacific Northwest Seismic Network. The Seattle Fault last ruptured in 900 A.D., at an estimated magnitude 7.4. It wreaked geologic havoc, sending chunks of forest into Lake Washington. Seattle was not yet a city then, and the next rupture has the deadly potential to bring down thousands of homes. When will this happen? One estimate puts the quake recurrence at every 750 years. Another says every 200 to 12,000 years. One thing is known: Seattle is due; the fault last ruptured more than 1,100 years ago. –Seattle PI