USGS scientific investigations focus on improving our understanding of the causes and effects of Pacific Northwest earthquakes, summarized below. In almost all cases a multi-disciplinary approach is required because near-surface evidence may be hidden by glacial deposits, extensive tree canopy, numerous water bodies. Also, the regions major faults do not appear to generate the frequent, small earthquakes recorded by our seismic networks and none have hosted a moderate or large earthquake in modern times. While good for our well-being, this low rate of recent moderate to large earthquakes leaves us with a lack of direct observations of damaging Pacific Northwest earthquakes.
For most faults of concern in the region, geophysical studies examine what large earthquakes and their impacts are plausible. Left mainly to geology are the problems of showing which of these earthquakes happen, how often they happen, and how large they get. We describe many of our investigations into the either of these two types of studies, noting that there is significant overlap between them.
Crustal faults in the Pacific Northwest are a serious, but poorly characterized hazard to the urban centers and major infrastructure of the U.S. Pacific Northwest (cities of Portland, Olympia, Tacoma, Seattle and Everett; major dams, bridges and other facilities throughout the state). Studies to identify and characterize faults east of the Cascades have only just begun within the last few years. We work with the USGS Geologic Hazards Team in Golden, CO, and independent researchers at Humboldt State University, Western Washington University, and Central Washington University.
Subduction Zone Earthquakes
Although the plates to the west of us are continuously converging toward the North American plate, the plate boundary between them appears to impede the sinking or ‘subduction’ of the denser, heavier western plates needed to accommodate this relative motion. The interface between the plates is stuck, or locked up, resulting in the build up of stresses along the interface. Eventually these stresses exceed the interface strength and all the motion that should have occurred while the plate was stuck, occurs within seconds as a major or great earthquake. These are known as Cascadia ‘megathrust’ earthquakes, and are much like those M>9 earthquakes that occurred near Sumatra in 2004 and Japan in 2011. The last Cascadia megathrust occurred on January 26, 1700.
We research the effects of great Cascadia earthquakes on land levels at Puget Sound and along the Pacific coast of Washington. Focused studies on Discovery Bay include analyses of microfossils, mapping and analysis of intertidal marsh stratigraphy, and radiocarbon dating of selected organic samples to estimate ages of strata and prehistoric events.
We continue to evaluate estimates of earthquake sizes and recurrence intervals of great plate-boundary earthquakes at the Cascadia subduction zone. Current studies evaluate the implications of turbidite deposits offshore and buried soils at estuaries of southwest Washington to improve age control on great earthquakes.
Deep (>30 km) earthquakes within the subducting Juan de Fuca, Gorda and Pacific plates also pose a significant seismic hazard to the region, as demonstrated by the nearly $2 billion in damages caused by the M=6.9 2001 Nisqually earthquake. Deep earthquakes have occurred beneath the Puget Lowland about every 30 years over the past 150 years, on average, including large events in 1949 and 1965. They are the most frequent source of strong ground shaking west of the Cascades, yet they are the least understood source of earthquakes, both in terms of their mechanisms and geographic distribution.
Scientists have only very recently learned that faults slip at highly varying rates. This discovery is important because the slippage relaxes the stresses generated as the tectonic plates continually move relative to one another, and because the fastest slipping faults emit damaging seismic waves in earthquakes. Slower slipping faults exhibit a variety of types of signals previously either not recognized or not observable without today’s instrumentation. These signals provide new clues and understanding about the processes involved in the loading and relaxation of stresses, which in turn will ultimately help us assess and reduce the hazard associated with the relaxation that occurs as damaging earthquakes.
Most of the highly populated areas of the Pacific Northwest sit atop large Cenozoic sedimentary basins that are expected to amplify some types of earthquake ground motions, in some cases by factors of 4 or more.
Most of the direct impact of earthquakes comes from the shaking, or ground motions, associated with the seismic waves they emit. The characteristics of these motions depend on the earthquake itself, how the waves travel through the Earth, and the local site response beneath the impacted structures. These motions can now be simulated with varying accuracy throughout the Pacific Northwest, with such simulations providing the basis of hazard maps and scenarios.
Geologic and Landslide Maps
The USGS and the Oregon Department of Geology and Mineral Industries are working to compile and publish the Portland Geologic Map and database, and a number of the landslide maps of the Portland area. Together with the GeomapNW project the USGS is publishing a series of geologic maps of the Seattle urban area. The USGS also participates in activities of the Oregon and Washington State Map Advisory Committee.