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.

The Blackfoot Reservoir volcanic field (BRVF), Idaho, USA, is a bimodal volcanic field that has hosted explosive silicic eruptions during at least two episodes, as recently as 58 ka. Using newly collected terrestrial and marine gravity data, two large negative anomalies (−16 mGal) are modeled as shallow (<1 km) laccoliths beneath a NE-trending alignment of BRVF rhyolite domes and tuff rings. Given the trade-off between density contrast and model volume, best-fit gravity inversion models yield a total intrusion volume of 50−120 km3; a density contrast of −600 kg m-3 results in model intrusion volume of 63 km3. A distinctive network of 340°−360° trending faults lies directly above and on the margins of the mapped gravity anomalies. Most of these faults have 5−10 m throw; one has throw up to ∼50 m. We suggest that the emplacement of shallow laccoliths produced this fault zone and also created a ENE-trending fault set, indicating widespread ground deformation during intrusion emplacement. The intrusions and silicic domes are located 3−5 km E of a regional, 20 mGal step in gravity. We interpret this step in gravity as a change in the thickness of the Upper Precambrian to lowermost Cambrian quartzites in the Meade thrust sheet, part of the Idaho-Wyoming Thrust Belt. Silicic volcanism in the BRVF is a classic example of volcanotectonic interaction, influenced by regional structure and creating widespread deformation. Exogeneous and endogenous domes are numerous in the region. We suggest volcanic hazard assessments should account for potentially large-volume silicic eruptions in the future.