The south central Chilean margin regularly produces many of the world's largest earthquakes and tsunami, including the 2010 Mw 8.8 Maule and 1960 Mw 9.5 Valdivia events. In 2017, we acquired seismic reflection data along ~1,000 km of the margin using the R/V Langseth's 15 km long receiver array and 108.2 l (6,600 in3) seismic source to image structures associated with these ruptures. We focus on the Valdivia segment with the largest coseismic slip (~40 m). The outer 40 km of the forearc is an accretionary wedge constructed primarily of stacked sedimentary packages with irregular lengths and thicknesses and little along‐strike continuity. Forearc structures indicate that the accretionary wedge grows primarily through basal accretion of the downgoing trench fill. The décollement propagates along a weak boundary near the top of the trench fill but occasionally branches downward into the underthrust sediment along bedding horizons, peeling off slices that are underplated to the forearc. The shallow décollement level and the rarity of underplating events allow most of the trench sediment to subduct. As a result, only ~30% of the incoming sediment has been accreted since the Early Pliocene. This implies that, on average, ~1 km of sediment must subduct beyond the outer forearc, an inference that is supported by our seismic images. We propose that the thickness and great downdip and along‐strike extent of the underthrust layer, which separates the megathrust from the underlying roughness of the igneous ocean crust, ensures a smooth broad zone of strong coupling that generates the world's largest earthquakes and tsunami.