Description: The 27 February, 2010 Maule earthquake (Mw=8.8) ruptured ~600 km of the Nazca-South America plate boundary (33°S-38.5°S) and caused hundreds of fatalities and billions of dollars in structural losses. In order to better understand the coseismic rupture process of this devastating earthquake, we study the seismic velocity structure of the Nazca-South America subduction zone near the epicentral region. The results show a wedge shaped body ~40 km wide with typical sedimentary velocities (〈3.5 km/s) interpreted as an active accretionary prism composed of weak and fluid-rich sediments. Landward of the imaged accretionary prism, the velocity model shows an abrupt velocity-contrast suggesting a rheological and lithological change which is interpreted as a backstop. The backstop is coincident with the seaward limit of the aftershocks, defining the seismic front and up-dip limit of the megathrust earthquake. The latter is also observed along strike (33°S-38.5°S) where the seaward extension of more than 1000 aftershocks is located at roughly the same distance from the deformation front (~30 km). On the other hand, reflections from the top of the subducting oceanic crust constraint the location of the plate boundary and its dip angle to 10° and thus we estimate the hypocenter location at 22±1 km depth. Our seismic results provide crucial constraints for evaluating the capability of this area to target shallow tsunamigenic earthquakes and assessing earthquake and tsunami hazard.