Upper mantle structure from teleseismic P wave arrivals in Washington and northern Oregon
Teleseismic P wave travel time residuals are used to detect lateral velocity heterogeneities in the upper mantle beneath Washington and northern Oregon. The results of an inversion for three-dimensional velocity variations resolves an east dipping high-velocity zone that we interpret as the subducting Juan de Fuca plate. The plate is characterized by 3–8% higher velocities than those in the surrounding upper mantle. Inversion of the travel time data and ray trace modeling indicate that the plate extends to a depth of 200–300 km. The plate dips at a moderate angle of 45° to the east-northeast beneath the central Washington Cascade Range north of Mount Rainier, with 5% faster velocities than the surrounding upper mantle. Beneath the North Cascade Range of Washington, the plate strikes to the northwest and has 6–8% faster velocities than the upper mantle to the west. South of 47°N, beneath the Cascade Range in southern Washington and northern Oregon, the plate dips steeply to the east and has 3–4% faster velocities than the surrounding upper mantle. Based on changes in the geometry and velocity structure of the subducted Juan de Fuca plate east of about 123°W, we propose that the subducted slab is segmented into three sections beneath Washington and northern Oregon.