Tectonic deformation in western Washington state from global positioning system measurements

Western Washington state is adjacent to the Cascadia Subduction Zone (CSZ), which stretches from northern California to southern British Columbia. More than three years of continuous and "campaign" style GPS measurements in western Washington indicate that the direction of the observed horizontal velocities is roughly parallel to the direction of relative plate convergence of the Juan de Fuca (JDF) and North America (NA) plates and that the relative plate convergence and velocities decrease away from the deformation front. The average maximum principal strain rate for the entire study area calculated from the GPS velocities indicates compression of 0.05 +/- 0.01 m strain/yr in the direction N75°E.;Most of the observed velocities can be attributed to locking of the CSZ thrust interface causing the accumulation elastic strain as the next great subduction earthquake approaches. Three-dimensional elastic dislocation modeling results suggest that the seismogenic zone along the Washington section of the CSZ is ∼10% wider than the earlier published values of 90 km; this can theoretically cause higher levels of ground shaking in the densely populated Puget basin of western Washington.;Although the subduction related signal dominates the observed GPS velocity field, there is also evidence for an additional north-south oriented compression. To isolate this signal from the subduction related signal the dislocation model predictions must be subtracted from the observed velocities, yielding a "residual" velocity field that suggests the presence of N-S oriented compression at a rate of ∼4 mm/yr over a N-S distance of approximately 250 km. This signal presumably represents a more long-term deformation pattern than the periodic accumulation and release of elastic strain connected with subduction earthquakes and most likely is related to the occurrence of shallow earthquakes in western Washington, which are characterized by predominantly north-south oriented maximum principal stress directions.