3D Viscoelastic Models of Slip-Deficit Rate Along the Cascadia Subduction Zone

Abstract

Interseismic deformation in the Pacific Northwest is constrained by the horizontal crustal velocity field derived from the Global Positioning System (GPS) in addition to vertical rates derived from GPS, leveling, and tide gauge measurements. Such measurements were folded in to deformation models of fault slip rates as part of the 2023 National Seismic Hazard Model update. Here I build upon one of the contributing models, the viscoelastic earthquake-cycle model of Pollitz (2022, https://doi.org/10.1785/0220220137). This model permits inclusion of effects of time-dependent viscoelastic relaxation within earthquake cycles (i.e., “ghost transients”) and laterally variable elastic and/or ductile material properties. I leverage these capabilities to incorporate the Cascadia megathrust into Western U.S.-wide deformation models in which crustal fault slip rates are estimated simultaneously with slip deficit rates along the interplate boundary between the descending Juan de Fuca plate and North American plate. This effort includes construction of a margin-wide model of viscoelastic structure founded on the Slab 2.0 model and probes different models of the ductile properties of the surrounding oceanic asthenosphere, continental lower crust, and mantle asthenosphere. This results in new estimates of the distribution of slip deficit rate along the $\sim 1000$ km long margin, highlights the importance of correcting for glacial-isostatic adjustment effects, and permits assessment of sensitivity of results to assumed ductile properties.