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The subduction of topographic features has been linked both to earthquake generation and the segmentation of faults. Seismic imaging reveals a seamount subducted to 40-km depth below Sumatra that is associated with an aseismic zone, suggesting that at this location, the seamount reduces coupling of the slab and overriding plate. The subduction of large topographic features such as seamounts has been linked to plate locking 1 , 2 , 3 , 4 , 5 , 6 , 7 , earthquake generation 8 and segmentation 6 , as well as crustal erosion 9 , 10 , 11 at subduction zones. However, the role of subducted features in the generation of megathrust earthquakes has been difficult to discern because traditional imaging techniques are limited to the upper 12 km of the Earth’s crust 12 , whereas these ruptures initiate at depths of 20–40 km (ref.  13 ). Here we use a deeply penetrating imaging technique with a low-energy source to identify a seamount 3–4 km high and 40 km wide that has been subducted to a depth of 30–40 km below the Sumatra forearc mantle. We find that the seamount has remained intact despite more than 160 km of subduction, and that there is no seismic activity either above or below the seamount. We therefore conclude that the coupling between the seamount and overriding plate is weak and aseismic 14 . We suggest that the subduction of a topographic feature such as a seamount could lead to the segmentation of the subduction zone, which could in turn reduce the maximum size of megathrust earthquakes in these localities.