An inverse dislocation model of surface deformation in western Washington

Active horizontal and vertical deformation in western Washington is modeled using an inverse dislocation model. The modeling technique uses published horizontal crustal velocities, derived from campaign and continuous GPS data, and uplift rates, calculated from repeated historical leveling surveys. The plate interface is represented by both screw and edge dislocations divided into segments. The rate of slip deficit accumulation on each segment is estimated by inverting the surface deformation data using a linear least squares procedure. The model predicts the data well, with an overall RMS residual of 0.85 mm/year. The horizontal residual vectors show no discernable pattern and no significant uniform regional strain and rigid rotations can be identified. The estimated slip deficit accumulation rates suggest that the plate interface is locked in a broad region beneath the coastline and possibly partially locked further downdip. The maximum slip deficit rate was estimated at 32.1±1.9 mm/year or approximately 91% of the plate convergence rate. The partially locked zone appears to be accumulating slip deficit at 8.2±1.9 mm/year.