Active deformation near the Nicoya Peninsula, northwestern Costa Rica, between 1996 and 2010: Interseismic megathrust coupling

Abstract

We use campaign and continuous GPS measurements at 49 sites between 1996 and 2010 to describe the long‐term active deformation in and near the Nicoya Peninsula, northwestern Costa Rica. The observed deformation reveals partial partitioning of the Cocos‐Caribbean oblique convergence into trench‐parallel forearc sliver motion and less oblique thrusting on the subduction interface. The northern Costa Rican forearc translates northwestward as a whole ridge block at 11 ± 1 mm/yr relative to the stable Caribbean. The transition from the forearc to the stable Caribbean occurs in a narrow deforming zone of ∼16 km wide. Subduction thrust earthquakes take 2/3 of the trench‐parallel component of the plate convergence; however, surface deformation caused by interseismic megathrust coupling is primarily trench‐normal. Two fully coupled patches, one located offshore Nicoya centered at ∼15 km depth and the other located inland centered at ∼24 km depth, are identified in Nicoya with the potential to generate an Mw 7.8 1950‐type earthquake. Another fully coupled patch SE of Nicoya coincides with the rupture region of the 1990 Nicoya Gulf earthquake. Interface microearthquakes, non‐volcanic tremor, low‐frequency earthquakes, and transient slow‐slip events generally occur in the intermediately to weakly coupled regions.