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Knowledge of seismic velocities in the seismogenic part of subduction zones can reveal how material properties may influence large ruptures. Observations of aftershocks that followed the 2010 M w 8.8 Maule, Chile earthquake provide an exceptional dataset to examine the physical properties of a megathrust rupture zone. We manually analysed aftershocks from onshore seismic stations and ocean bottom seismometers to derive a 3-D velocity model of the rupture zone using local earthquake tomography. From the trench to the magmatic arc, our velocity model illuminates the main features within the subduction zone. We interpret an east-dipping high P-wave velocity anomaly (>6.9 km/s) as the subducting oceanic crust and a low P-wave velocity (<6.25 km/s) in the marine forearc as the accretionary complex. We find two large P-wave velocity anomalies (∼7.8 km/s) beneath the coastline. These velocities indicate an ultramafic composition, possibly related to extension and a mantle upwelling during the Triassic. We assess the role played by physical heterogeneity in governing megathrust behaviour. Greatest slip during the Maule earthquake occurred in areas of moderate P-wave velocity (6.5-7.5 km/s), where the interface is structurally more uniform. At shallow depths, high fluid pressure likely influenced the up-dip limit of seismic activity. The high velocity bodies lie above portions of the plate interface where there was reduced coseismic slip and minimal postseismic activity. The northern velocity anomaly may have acted as a structural discontinuity within the forearc, influencing the pronounced crustal seismicity in the Pichilemu region. Our work provides evidence for how the ancient geological structure of the forearc may influence the seismic behaviour of subduction megathrusts.