Earthquake hazards in the Pacific Northwest come from a variety of fault sources related to the convergence of the North American and Juan de Fuca tectonic plates.
Across much of the Pacific Northwest, active structures are visible in the landscape. The Yakima Folds form elongate east-west mountains. Back-arc extension in Oregon is expressed as sharp north-trending normal fault escarpments. Offshore, the trench and spreading ridges are dominant features.
Red lines here are known Quaternary-active faults and folds (USGS). Black circles are cities >10,000 pop.
The North American tectonic plate rotates clockwise as it overrides the subducting Juan de Fuca plate (McCaffrey et al., 2013). Gradients between populations of these GPS data are locations where active intraplate deformation may occur as faults or folds.
Neotectonic mapping of fault scarps and landslides highlights the complex nature of surface deformation related to recent earthquakes (Bennett et al., 2016 - AGU poster).
In Fall 2015, we excavated trenches across a scarp in the Burbank Creek area of Yakima River Canyon. This scarp is likely related to back-thrusting along the Umtanum Ridge anticline and thrust fault, a component of the Yakima Folds. USGS Volunteer, Tabor Reedy, pictured.
In Summer 2016, we excavated trenches across a scarp in the Wynoochee River Valley, related to the Canyon River fault. Kate Scharer (USGS) and Harvey Kelsey (Humboldt State Univ.) are seen examining the complex fault zone exposure.
Ongoing research along the Seattle fault zone will further examine fault scarps and folds on Bainbridge Island and Kitsap peninsula that continue offshore into the Puget Sound. Map from USGS-NAGT intern research project by Mattie Reid (Reid et al., 2016).