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.