The Pacific Northwest is an active subduction zone. Because of this tectonic setting, there are three distinct earthquake source zones in earthquake hazard assessments of the Seattle area. Offshore, the broad sloping interface between the Juan de Fuca and the North America plates produces earthquakes as large as magnitude 9; on the average these events occur every 400-600 years. The second source zone is within the subducting Juan de Fuca plate as it bends, at depths of 40-60 km, beneath the Puget lowland. Five earthquakes in this zone this century have had magnitudes greater than 6, including one magnitude 7.1 event in 1949. The third zone, the crust of the North America plate, is the least well known. Paleoseismic evidence shows that an event of approximate magnitude 7 occurred on the Seattle fault about 1000 years ago. Potentially very damaging to the heavily urbanized areas of Puget Sound, the rate of occurrence and area over which large magnitude crustal events are to be expected is the subject of considerable research.
Additional publication details
No longer so clueless in seattle: Current assessment of earthquake hazards