Influence of Subduction Interface Geometry on Surface Displacements and Slip Processes in Cascadia

The subduction interface geometry is particularly important for estimating interplate coupling and surface geodetic motion, which has significant implications for seismic hazard mapping. Several published Cascadia subduction interface geometries derived from different seismic data sets vary significantly from one another. However, results from deformation models that use the different interface geometries are rarely systematically compared. Here, we assess the impact of subduction interface geometry on surface motion predictions, slip inversion results, and interface coupling estimates from four published Cascadia subduction interface geometries. We isolate the effect of the interface geometry on the predicted surface motion by applying uniform unit slip or Gaussian slip patterns to each interface geometry and calculate the predicted displacements at locations of GNSS stations. The forward model-predicted horizontal displacements can differ by >20% and show azimuthal differences up to 10°; such differences correlate spatially to geometric differences amongst the interface realizations. Inversions of surface displacements estimated using a Gaussian distribution of slip, mimicking an earthquake, recover the applied slip distribution with differing spatial patterns and residuals of up to 38% of the maximum applied slip. Block models that use the four interface realizations produce coupling estimates on the interface with regions of significant coupling (>50%) that differ noticeably in down-dip extent and lateral continuity. The results we present suggest that models utilizing interface geometry as an input, such as earthquake and tsunami models, should consider comparing models with differing interface geometries to critically evaluate model uncertainty stemming from this fundamental input.