In July 2005, about 6 months after the 2004 great Sumatra-Andaman earthquake, 20 ocean bottom seismometers were deployed in the northern Sumatra area. More than 1000 events were identified during the 12 day recording period. After relocation, the seismicity shows different patterns on each side of the Lower Splay Fault. East of this feature and beneath the Aceh basin and fore arc, the deep earthquakes outline the subduction megathrust fault. West of it, the aftershocks distribution is strongly influenced by the active N-S oceanic fracture zones of the subducted plate. Two N-S trending clusters of 10- to 50-km-deep earthquakes observed below the lower wedge are interpreted as reactivated oceanic fracture zones. The postseismic activity suggests a significant influence of the N-S active fracture zones of the oceanic plate on the toe of the wedge explaining the dextral wrenching of wedge thrusts in the morphology. The megathrust fault does not extend to the trench but outcrops east of these active oceanic fractures and seems to merge upward along the Lower Splay Fault where shallow earthquakes are observed.