In the last five years, there have been three great megathrust earthquakes in the Sumatra subduction zone, and a part of the zone is still locked. We have carried out deep seismic reflection surveys along the SW Sumatran margin and have imaged backthrusts along four profiles down to 15–20 km depth. We find that the seismic images of the backthrusts in the 2004 and 2007 great earthquake ruptured zones are brighter than those in the regions where the subduction zone is still locked. We suggest that this enhanced reflectivity is due to the increase of fluid contents along the reactivated backthrusts during or soon after the great earthquakes. If this interpretation is valid, then the backthrusts observed in the forearc basins could be used to monitor fluid movement from mantle wedge and earthquake precursors before and after an imminent great earthquake in the Sumatra locked zone.