GPS-determination of along-strike variation in Cascadia margin kinematics: Implications for relative plate motion, subduction zone coupling, and permanent deformation

Copyrighted by American Geophysical Union.High-precision GPS geodesy in the Pacific\ud Northwest provides the first synoptic view of the along-strike\ud variation in Cascadia margin kinematics. These results constrain\ud interfering deformation fields in a region where typical\ud earthquake recurrence intervals are one or more orders of magnitude\ud longer than the decades-long history of seismic monitoring\ud and where geologic studies are sparse. Interseismic\ud strain accumulation contributes greatly to GPS station velocities\ud along the coast. After correction for a simple elastic dislocation\ud model, important residual motions remain, especially\ud south of the international border. The magnitude of northward\ud forearc motion increases southward from western Washington\ud (3-7 mm/yr) to northern and central Oregon (~9 mm/yr), consistent\ud with oblique convergence and geologic constraints on\ud permanent deformation. The margin-parallel strain gradient,\ud concentrated in western Washington across the populated\ud Puget Lowlands, compares in magnitude to shortening across\ud the Los Angeles Basin. Thus crustal faulting also contributes\ud to seismic hazard. Farther south in southern Oregon, northwestward\ud velocities reflect the influence of Pacific-North\ud America motion and impingement of the Sierra Nevada block\ud on the Pacific Northwest. In contrast to previous notions,\ud some deformation related to the Eastern California shear zone\ud crosses northernmost California in the vicinity of the Klamath\ud Mountains and feeds out to the Gorda plate margin