Following a large (M_s=7.0), interplate earthquake on 15 July 1981 in the Vanuatu (New Hebrides) Islands, the area defined by the aftershocks dramatically increased over a two week period of time. The aftershocks were located using the Cornell/ORSTOM seismograph network that has been monitoring earthquakes with magnitudes larger than mb=2.9 in the region since mid-1978. In the first few hours following the mainshock, the area defined by the aftershocks was consistent with the rupture zone of a typical interplate earthquake with the observed seismic moment of 5.8 x 1026 dyne-cm. However, during the 16 days following the mainshock, the area of the aftershock zone increased to approximately 10 times the area of expected coseismic rupture. The final aftershock zone included earthquakes that were located within the overriding and descending plates as well as along the interplate fault zone. The migration of aftershocks away from the rupture zone of the 1981 mainshock indicates that, at least in some cases, the aftershock zone can overestimate the extent of coseismic rupture.