To understand controlling factors for seismogenesis and strain‐accumulation in subduction megathrusts we examine seismicity patterns across Nicoya, Costa Rica, to determine the overall and spatial variability in the earthquake frequency‐magnitude distribution along the interface. The mean reduction in earthquake activity with magnitude, b‐value, is higher (1.06) than global subduction zone averages (0.6–0.8), suggesting the interface here is weakly coupled. Strong spatial variations in b are anticorrelated (−0.53) with geodetic estimates of interface locking of E. Norabuena et al. (2004). High b prevails in two regions, including the subducted Fisher seamount chain. A broad zone of reduced b is observed at and offshore the central Nicoya coast; extending towards an imaged locked patch. These results suggest b‐value studies may be useful in identifying regions of increased interface locking in subduction zones, which may indicate regions capable of large slip in future large earthquakes.