Subduction megathrust earthquakes show complex rupture behaviour and large lateral variations of slip. However, the factors controlling seismic slip are still under debate. Here, we present 2-D velocity depth tomographic models across four trench-perpendicular wide angle seismic profiles complemented with high resolution bathymetric data in the area of maximum coseismic slip of the M-w 8.8 Maule 2010 megathrust earthquake (central Chile, 34 degrees-36 degrees S). Results show an abrupt lateral velocity gradient in the trench-perpendicular direction (from 5.0 to 6.0 km/s) interpreted as the contact between the accretionary prism and continental framework rock whose superficial expression spatially correlates with the slope-shelf break. The accretionary prism is composed of two bodies: (1) an outer accretionary wedge (5-10 km wide) characterized by low seismic velocities of 1.8-3.0 km/s interpreted as an outer frontal prism of poorly compacted and hydrated sediment, and (2) the middle wedge (similar to 50 km wide) with velocities of 3.0-5.0 km/s interpreted as a middle prism composed by compacted and lithified sediment. In addition, the maximum average coseismic slip of the 2010 megathrust event is fairly coincident with the region where the accretionary prism and continental slope are widest (50-60 km wide), and the continental slope angle is low (<5 degrees). We observe a similar relation along the rupture area of the largest instrumentally recorded Valdivia 1960 M-w 9.5 megathrust earthquake. For the case of the Maule event, published differential multibeam bathymetric data confirms that coseismic slip must have propagated up to 6 km landwards of the deformation front and hence practically the entire base of the middle prism. Sediment dewatering and compaction processes might explain the competent rheology of the middle prism allowing shallow earthquake rupture. In contrast, the outer frontal prism made of poorly consolidated sediment has impeded the rupture up to the deformation front as high resolution seismic reflection and multibeam bathymetric data have not showed evidence for new deformation in the trench region. (C) 2017 Elsevier B.V. All rights reserved.