The Role of Elastic Heterogeneity for Inferring Seismic Cycle Deformation at the Nicoya Peninsula

Subduction of the Cocos beneath the Caribbean Plate generates a spectrum of plate boundary deformation, including megathrust earthquakes and slow-slip events. These processes are well constrained on the Nicoya Peninsula, Costa Rica, where dense seismic and geodetic instrumentation has been deployed. Here, we estimate plate boundary deformation using an adjoint-based, mixed finite-element method in which elastic heterogeneity is allowed. Our framework can be used to invert for interseismic coupling, coseismic slip, and aseismic creep. We show that a two-fold rigidity contrast between the subducting slab and the weaker overlying plate reduces inferred fault slip by 10-20% compared to a homogeneous model, consistent with earlier studies for different regions. This suggests that omitting material heterogeneity may lead to overestimation of fault locking and slow-slip events. We also explore more realistic rigidity structures derived from seismic tomography, and assess whether heterogeneity affects inferred slip patterns and not just amplitudes, as well as resolution limits for locking in the presence of heterogeneity.