Three-dimensional viscoelastic interseismic deformation model for the Cascadia subduction zone

Contemporary deformation of the Cascadia forearc consists of an elastic int erseismic strain build-up as part of the subduction earthquake deformation "cycle" and a secular deformation primarily in the form of are-parallel tra nslation and clockwise rotation of forearc blocks. A th ree-dimensional (3- D) elastic dislocation model, constrained by vertical deformation data, was developed previously to study the interseismic deformation. In this study, we develop a 3-D viscoelastic finite element model for the Cascadia subduc tion zone to study the temporal and spatial variations of interseismic defo rmation, and we compare the model results primarily with horizontal geodeti c deformation observations. The model has an elastic lithosphere/slab and a viscoelastic mantle which has a viscosity of 10(19) Pa s as constrained by recent postglacial rebound analyses. For comparison, we adopt a seismogeni c zone geometry that was used in the previous elastic dislocation model, an d we test the effects of different estimates of relative plate motion on th e model predictions. Interseismic deformation is simulated by assigning a b ackslip rate to the locked zone of the subduction fault, preceded by an ear thquake rupture of the same zone. Based on preliminary model results, we dr aw the following conclusions: (1) The deformation rate decreases through th e interseismic period. A seaward motion is predicted for inland sites early in the interseismic period, an effect of postseismic creep of the mantle. (2) Model strain rates 300 years after the earthquake are consistent with t he observed values, regardless of the plate motion models used. The horizon tal velocities in northern Cascadia decrease landward at a slower rate than predicted by the elastic dislocation model, providing a better fit to obse rvations. (3) Oblique subduction causes strain partitioning. As a result, t he direction of local maximum contraction is much less oblique than plate c onvergence. The northerly direction of the GPS velocities in southern Casca dia represent a northward translation of the forearc. The secular deformati on of the forearc may be partially accommodated through earthquake deformat ion cycles, but it may be better modeled as a process independent of the ea rthquake cycle.