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 Mw 7.7 earthquake. To the south, the 2012 Mw 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.

Links between hydrology and sliding of the Greenland Ice Sheet (GrIS) are poorly understood. Here, we monitored meltwater’s propagation through the entire glacial hydrologic system for catchments at different elevations by quantifying the lag cascade as daily meltwater pulses traveled through the supraglacial, englacial, and subglacial drainage systems. We found that meltwater’s residence time within supraglacial catchments-depending upon area, snow cover, and degree of channelization-controls the timing of peak moulin head, resulting in the two hour later peak observed at higher-elevations. Unlike at lower elevations where peak moulin head and sliding coincided, at higher elevations peak sliding lagged moulin head by ~2.8 hours. This delay was likely caused by the area’s lower moulin density, which required diurnal pressure oscillations to migrate further away from subglacial conduits to elicit the observed velocity response. These observations highlight the supraglacial drainage system’s control on coupling GrIS hydrology and sliding.