N-S SHORTENING ACROSS THE OLYMPIC MOUNTAINS AND PUGET SOUND EVIDENCED BY GPS

The northern Washington segment of the Cascadia forearc can be divided into two very distinct regions based on the active deformation style and on seismicity in the crust. In the inner (eastern side) forearc, both intense crustal seismicity and active faults indicate clear N-S shortening of the Puget Lowland, between ~46.5 and ~48.5 °N. The outer (western side) forearc is marked by the presence of the Olympic Mountains accretionary complex, where the main direction of long-term shortening is debated. We use velocities derived from the WCDA and PANGA permanent GPS networks, as well as campaign results, to constrain the present-day and long-term deformation patterns of the northern Washington-southwestern British Columbia forearc. GPS strain rates and velocities show that this region is currently shortening at ~3 ± 1 mm/yr in the N-S direction and at ~10 ± 1 mm/yr in E-W. The former rate is in good agreement with inference of N-S crustal shortening based on shallow earthquake statistics in the Puget Lowland. The E-W shortening is often regarded as transient elastic loading from the locked Cascadia megathrust. We estimate the long-term deformation rates by subtracting from the GPS velocities the modeled interseismic loading of the margin by the locked megathrust. Assuming that the subduction-related interseismic deformation is purely elastic, the GPS long-term velocity vectors show N-S shortening at ~5 ± 1 mm/yr in the Puget Lowland and ~6.5 ± 1 mm/yr across the Olympic Mountains. Compared to the velocity of the Oregon forearc, this indicates that most of the northerly Oregon forearc motion is accommodated in this region. The E-W component of long-term deformation to the north and to the south of the Olympic Core rocks is insignificant. We discuss the uncertainties associated with the Cascadia subduction model and the impact of the elastic strain hypothesis on the estimate of long-term deformation in this region.