Crustal Earthquakes

Shallow earthquakes in the Pacific Northwest, with depths no greater than about 35 km, are caused by the rupture of faults within the North American Plate. Crustal earthquakes occur throughout the region, but they occur most frequently where the crust is deforming the fastest. Earthquakes along crustal faults are driven by the rotation and shearing of the North American Plate due to the forces imposed by the subducting Juan de Fuca Plate, the Pacific Plate grinding north along the North American Plate's western edge in California, and extension in the Basin and Range Province further inland.

The magnitude of an earthquake is related to how big a fault it ruptures, and the biggest crustal faults in our region could produce earthquakes with magnitudes as large as M7.5. It is also common for crustal earthquakes to produce a number of aftershocks, including ones large enough to be potentially damaging. The Puget Sound region has the highest risk for large crustal earthquakes in the Pacific Northwest, though evidence has been found for large earthquakes in eastern Washington near Wenatchee, Yakima, and Richland, and in Oregon near Klamath Falls and Scotts Mills.

Crustal earthquakes are potentially the most damaging to the urban areas of the Puget Sound because several major population centers are located near or on top of known crustal faults. This means the seismic wave amplitudes will not be attenuated, or diminished, by long propagation paths between the earthquake source and major cities, as is the case for Cascadia Subduction Zone and deep intraplate earthquakes. Also, surface rupture offsets () can inflict further damage beyond the shaking.

Seattle in particular is threatened by the Seattle Fault, which runs right through the city. It traverses beneath Lumen Field, west across Bainbridge Island to Hood Canal, and east out past Issaquah roughly following I-90. This fault produced a large earthquake that left a geologic record of surface offsets in the year 900 AD. It's only one of a series of active large crustal faults in the Puget Lowland, pictured in the image below. Unlike deep or subduction zone earthquakes, it is difficult to know how often earthquakes happen along crustal faults. Recurrence intervals for crustal earthquakes can range from a few hundred to thousands of years.

Locating faults can be rather difficult, especially in the Pacific Northwest. Most of their length can be buried beneath vegetation, infrastructure, and water. Smaller earthquakes and modern tools like LiDAR are useful in refining the location of suspected faults. You can view a full map of known faults in the region by going to our Recent Events Map and clicking the icon for Map Layers under the zoom controls in the upper right of the map, and selecting the "Faults" overlay.