| | M. Meghan Miller1, Daniel J. Johnson1, Charles M. Rubin1, Herb Dragert2, Kelin Wang2, Anthony Qamar3, and Chris Goldfinger4 |
High-precision GPS geodesy in the Pacific Northwest provides the first synoptic view of the along-strike variation in Cascadia margin kinematics. These results constrain interfering deformation fields in a region where typical earthquake recurrence intervals are one or more orders of magnitude longer than the decades-long history of seismic monitoring and where geologic studies are sparse. Interseismic strain accumulation contributes greatly to GPS station velocities along the coast. After correction for a simple elastic dislocation model, important residual motions remain, especially south of the international border. The magnitude of northward forearc motion increases southward from western Washington (3-7 mm/yr) to northern and central Oregon (~9 mm/yr), consistent with oblique convergence and geologic constraints on permanent deformation. The margin-parallel strain gradient, concentrated in western Washington across the populated Puget Lowlands, compares in magnitude to shortening across the Los Angeles Basin. Thus crustal faulting also contributes to seismic hazard. Farther south in southern Oregon, northwestward velocities reflect the influence of Pacific-North America motion and impingement of the Sierra Nevada block on the Pacific Northwest. In contrast to previous notions, some deformation related to the Eastern California shear zone crosses northernmost California in the vicinity of the Klamath Mountains and feeds out to the Gorda plate margin.
1Department of Geological Sciences, Central Washington University, Ellensburg, Washington
2Geological Survey of Canada, Pacific Geoscience Center, Sidney, British Columbia, Canada
3Geophysics Program, University of Washington, Seattle, Washington
4College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon
 | Tectonics, Vol. 20, No. 2, Pages 161-176 April 2001. (3.3 MB) |
 | Selected tables of PANGA and other GPS station velocities. |
 | Selected maps showing GPS-determined velocities. |