Subduction plate age as a control on elastic upper-plate thickness in Cascadia: Insights from interseismic GNSS observations and implications

Understanding the viscoelastic structure of subduction zones is essential for assessing seismic hazards and understanding subduction-zone dynamics. However, the influence of lateral variations in elastic upper-plate thickness (Hc) remains poorly constrained and is often overlooked. In this study, we use two-dimensional forward viscoelastic earthquake-cycle models to fit both horizontal and vertical GNSS observations. We identify a clear trade-off between locking depth (D) and Hc in both components. To resolve this ambiguity, we incorporate constraints from thermal models and tremor distributions along the Cascadia subduction zone. As a novel result extending beyond previous kinematic models, our results reveal a systematic northward increase in Hc from ~20 km to ~30 km. This trend correlates with increasing oceanic plate age and likely reflects variations in the subaccretion and wedge cooling processes along the trench-parallel direction. In contrast, D remains relatively uniform at ~10 km, consistent with previous findings. These results demonstrate the robustness of our approach for simultaneously constraining Hc and D, and suggest it may be applied to other subduction zones. Lateral variations in Hc significantly affect crust deformation and should not be ignored in earthquake-cycle models. Accounting for these heterogeneities improves estimates of Hc and D, and enhances our understanding of megathrust locking, seismic hazard potential, and the physical conditions controlling episodic tremor and slip events.