Relationship between the present-day stress field and plate boundary forces in the Pacific Northwest

The relationship between plate boundary forces and the observed stress field in the Pacific Northwest is established using numerical niodels of continental deformation. Because the orientation of the greatest horizontal principal stress throughout the Pacific Northwest differs considerably from the direction of convergence between the Juan de Fuca and North American plates. the relationship haween the stress field and forces acting along the subduction zone has been unclear. To address this relationship. a two-dimensional finite element muciei developed by Bird [I9891 is used that incorporates critical aspects of continental deformation such as a stratified rheology and interaction between thermal and mechiiniciil components of deformation, Boundary conditions arc specified in terms of either velocity or shear traction, depending on whether the computed shear stress at the plate boundary is less than or exceeds, respectively, a prcscribed limit, Shear-stress limits on the subduclion and transform plate boundaries are independently varied to determine the relative effect of forces along these boundaries on intraplate deformation. Results from this study indicate that the shear stress limit of both subduction and transform boitndaries is low, and that the intraplate stress field is attributed, in part, to the normal eomponenl of relative plate motion along the transform boundaries. Howeverthe models also indicate that although the subduction zone fault is weak. a minimum shear strength ( 2 10 MPa) for the fault is necessary to explain the observed stress field. The balance among forces along the tectonic boundaries of North America results in a surprising degree of variation in the present-day stress field,