Earthquakes in subduction zones rupture the plate boundary fault in discrete segments. One factor that may control this segmentation is topography on the downgoing plate, although it is controversial whether this is by weakening or strengthening of the fault. We use multichannel seismic and gravity data to map the top of the downgoing oceanic crust offshore central Sumatra, Indonesia. Our survey spans a complex segment boundary zone between the southern termination of the Mw = 8.7, A.D. 2005 Simeulue-Nias earthquake, and the northern termination of a major 1797 earthquake that was partly filled by an Mw = 7.7 event in 1935. We identify an isolated 3 km basement high at the northern edge of this zone, close to the 2005 slip termination. The high probably originated at the Wharton fossil ridge, and is almost aseismic in both local and global data sets, suggesting that while the region around it may be weakened by fracturing and fluids, the basement high locally strengthens the plate boundary, stopping rupture propagation.