Slow slip events, or "silent" earthquakes, may relieve a significant amount of stress at many subduction plate boundaries, both downdip of the limit of seismogenesis, and within the seismogenic zone itself in cases where seismic energy release accounts for only a fraction of the plate tectonic displacement rate (Schwartz and Rokosky, 2007). Slow slip has been identified in several instances downdip of the landward limit of the seismogenic zone and is often accompanied by seismic tremor or low-frequency earthquake activity along and above the plate interface (referred to as "episodic tremor and slip", or ETS). Little is known, however, about the spatial distribution and history of slip between great earthquakes along the seismogenic thrust interface itself which lies mostly offshore. In this article we present formation pressure transients observed in two deep-sea boreholes near the toe of the subduction prism off Nicoya Peninsula, Costa Rica, which followed ETS events observed on shore by 1-2 weeks. The signatures of the transients are consistent with local slip on the shallow part of the thrust interface, with the underthrusting plate experiencing relaxation and the outer prism experiencing contraction. The delay between the tremor activity and the pressure transients observed c. 100 km seaward at the prism toe suggests either slow propagation across the seismogenic zone or delayed deformation at the outer part of the prism triggered by the slip beneath Nicoya. Such slip may serve generally to relieve stress at subduction zones, but also to increase stress in parts of the plate boundary where interseismic slip does not occur. Abstract Copyright (2011) Elsevier, B.V.