Subduction zone tsunamis require significant co-seismic slip in the shallow, offshore plate interface near the trench, possibly related to the degree of prior interseismic coupling. In Nicaragua, a large tsunami was associated with the1992 M_w 7.7 earthquake. To the south, the 2012 M_w 7.6 earthquake in the Nicoya peninsula of Costa Rica did not generate a tsunami. The disparate behavior between these two adjacent segments of the Central American megathrust remains unexplained. A stress-constrained model of slip deficit applied to the interseismic surface velocity field in Nicoya suggests a slip deficit rate in the updip portion of the megathrust between 0.8-8.5 cm/yr, suggesting that large tsunamis are possible here. Limited GPS data in Nicaragua can be reconciled by an offshore locked zone that matches the shallow rupture defined by the model of the 1992 tsunami. Sea-floor geodesy would allow much better near-trench constraints on slip deficit in both regions.