Regional and Local Patterns of Upper-Plate Deformation in Cascadia: The Importance of the Down-Dip Extent of Locking Relative to Upper-Plate Strength Contrasts

The details of subduction zone locking place constraints on the characteristics of megathrust events. Due to the lack of significant present-day seismicity along the Cascadia subduction interface, geodetic data are used to assess subduction locking along the margin. We isolate the subduction signal from other tectonic signals within the Cascadia GPS field, to assess the details of plate-interface locking. Apparent coupling determined by a simple homogenous elastic half-space inversion cannot everywhere reproduce the subduction component of the GPS field. Consequently, we explore the relationships among upper-plate strength, locking depth and the resulting surface velocity signal using 2D finite element models. When the upper plate is composed of a weak material, trenchward of a strong backstop, we find that the down-dip limit of locking relative to the location of the weak-to-strong transition controls how upper-plate deformation is spatially distributed. If locking extends into the stronger material, as observed in central Cascadia, the surface velocity field propagates farther inland than expected from a simple homogeneous elastic model. In contrast, in southern Cascadia, because locking terminates within the weak accretionary margin, upper-plate shortening is localized within the weaker material, particularly in the region between the end of locking and the strong Klamath terrane. This behavior provides a possible mechanism for producing the high (geodetic and permanent) uplift rates, plate-motion-parallel shortening, and crustal exhumation observed in many active and fossil subduction zones.