Since 1900, six earthquakes of magnitude 5.0 or greater have been recorded as having caused damage in Snohomish and Island counties, or were centered within the counties’ borders.
Where the faults lie
Two significant faults run through Snohomish and Island counties. The South Whidbey Island Fault Zone consists of several intermittent faults that run from the Strait of Juan de Fuca southeast across Whidbey Island and south Snohomish County to Woodinville. The Devils Mountain Fault runs from the Strait of Juan de Fuca east near Deception Pass and across the Skagit-Snohomish county line southeast to Darrington.
Geologic evidence shows that large quakes have happened in the South Whidbey and Devils Mountain fault zones in the distant past. One happened in 1950 in the South Whidbey fault zone but data is inconclusive about the fault of origin, said Bill Steele, coordinator of the University of Washington Seismology Lab.
Quakes on these faults carry a potential magnitude of 7.5, which could cause severe damage because of their proximity to the surface, according to a natural hazard report done for Snohomish County in 2010. A quake in the Cascadia Subduction Zone 50 miles off the Pacific coast could inflict a quake of up to 9.5 in magnitude, causing catastrophic damage and capable of starting a tsunami, according to the report.
Major modern-era Puget Sound quakes
View Major modern-era Puget Sound earthquakes in a larger map
Fault locations are approximate.
SOURCES: University of Washington Seismology Lab, U.S. Geological Survey, state Division of Geology and Earth Resources