A study of the spatial and temporal distributions of the seismicity in the central New Hebrides island arc shows that the Efate region (including the interplate boundary and the upper plate) can be considered as a single block under stress. Repeated activation of very specific zones within this block as indicated by seismic clusterss is temporally correlated with larger than average energy release rates. These relative high rates are associated with main shocks with magnitude (Ms, mb)>5.0 in the upper part of the interplate boundary and indicate that slip at the plate boundary is not steady. No correlation is found with main shocks located in the lower part of the interplate boundary, where slip occurs more freely. We propose that these observations are consistent with a buckling process with a wavelength of about 170 km within the overriding plate. Buckling or bending of the upper plate may be due to the impingement of a seamount on the subduction system, elevating the stress level of the Efate region, which behaves like a single mechanical entity. ¿ American Geophysical Union 1992