Tectonic deformation in western Washington from continuous GPS measurements

The analysis of 3 years of continuous data from 7 permanent GPS stations along the western Washington section of the Cascadia Subduction Zone indicates that the direction of the observed horizontal velocities (with respect to station DRAO, nominally representing the stable North American continent) is roughly parallel to the relative plate convergence direction of the Juan de Fuca and North America plates and that their magnitude decreases away from the trench. Most of this deformation can be attributed to interseismic strain accumulation due to the locking of the thrust interface. When the dislocation model predictions are subtracted from the observed velocities, there is evidence for an additional N-S oriented contraction at a rate of similar to 4 mm/yr over a distance of 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 is most likely related to the occurrence of shallow earthquakes in western Washington that are characterized by predominantly N-S oriented maximum principal stress.