Seattle Field Office

The spectacular scenery of the Pacific Northwest results directly from the active geological processes associated with being part of a subduction zone. The Pacific Northwest includes Washington, Oregon, northern California, and southwestern British Columbia, and geologically shares many similarities with the subduction zones of Japan and Chile.

large water fountain with mountain in the background
cartoon showing the Pacific Plate subducting under the North American Plate

Pacific Plate Subducting under the North American Plate (Public domain.)

Geologic Setting

Instrumental, written, and geological records from the region tell of earthquakes from three generalized sources. In Cascadia, strain arises from the interaction of three tectonic plates. The North America, Juan de Fuca, Gorda, Explorer, and (indirectly) the Pacific Plate all are moving with different trajectories. Earthquakes as large as magnitude 7 originate in the subducted Juan de Fuca plate, mainly beneath the Puget Sound region. Moderate and large earthquakes occur also on faults within the overriding North America plate, on both sides of the Cascades. Great earthquakes, as large as magnitude 9, happen on the boundary between these plates. Earthquakes from any of these sources may set off landslides, liquefaction, and tsunamis.

USGS earthquake studies of this region are intended to help reduce the losses earthquakes may cause.

Monitoring

The Pacific Northwest Seismic Network, operated by the Universities of Washington (UW) and Oregon (UO) in partnership with the USGS, monitors for earthquakes that threaten the PNW and in particular the urban corridor from Vancouver, British Columbia to Eugene, Oregon.

Paleoseismology

The Cascadia subduction thrust last produced a great earthquake in the year 1700. The most recent large earthquake on the Seattle Fault occurred close to the year 900. For these and other faults, the earthquake histories must be read from geological evidence, through ‘paleoseismic’ studies. USGS staff and its partners are addressing questions about great, megathrust earthquake recurrence, possible segment boundaries, and associated effects of ground shaking and fault rupture. They are examining paleo liquefaction, landslides, and land-level changes along the coastline associated with past great earthquakes. Further inland, scientists continue to characterize major active upper plate faults, including 13 faults in the Puget Lowlands of Washington that produced 27 prehistoric earthquakes in the last 15,000 years. On-going work east of the Cascade Mountians continues to advance knowledge of timing and rates of fault deformation events.

Earthquake Hazard Investigations

Pacific NW map with color contours

Seismic Hazard Levels in the Pacific Northwest. Red colors. indicate the highest level of hazard (Public domain.)

Significant gaps exist in our knowledge of basic fault characteristics and the modes in which faults accommodate plate motions in the Pacific Northwest region. These gaps limit the accuracy of needed seismic hazard and risk assessments, particularly for the region’s urban centers and major infrastructure (cities of Portland, Olympia, Tacoma, Seattle and Everett; major dams, bridges and other facilities throughout). The USGS and its partners employ analyses of remote sensing data such as LiDAR and InSAR, seismic reflection and refraction profiles, temporary and permanent seismic and geodetic network data, historic accounts of past earthquakes, and geological field mapping. Studies address faulting on time scales ranging from millennia (geological) to seconds (seismological).

Earthquake hazard assessments of eastern Washington are based on scant data, yet the region is home to a large amount of federal, state, and local/private infrastructure, and has experienced some of the largest historical earthquakes in the PNW in 1872 and 1936. Current work is evaluating regional tectonic models and deformation rates and leading to revised thinking on how the this region interacts with the rest of the Cascadia margin.

Urban Seismic Hazard Mapping and Ground-Motion Studies

USGS staff works with engineers and other users of USGS products to ensure that they are successfully applied to mitigate earthquake risk. Ground motion maps USGS provides are crucial for emergency management exercises, loss estimation, and improving community resilience. Example products include detailed seismic hazard maps of urban areas of the Pacific Northwest, simulated ground motions for expected large earthquakes, and various scenarios ground-motion maps requested by emergency management and other agencies.

Cascadia Recurrence

The USGS and its partners are evaluating recurrence intervals of Cascadia subduction zone earthquakes from multiple proxies for paleoearthquakes, including coastal subsidence and uplift, tsunami deposits, turbidites, and landslides. Studies are underway to provide contextual constraints on Cascadia-specific plate-interference properties from geodesy, source models and shaking from ground-motion modeling and seismic data, and potential linkages with crustal deformation. Findings will ultimately help refine and focus recurrence intervals on the CSZ that will in turn lead to an increase in the accuracy of earthquake hazard assessments in the Pacific Northwest.