In subduction zones, onshore geodesy provides the main data used to map seismic locking on the plate interface. Based on observations at the Cascadia subduction, we propose a new offshore control by establishing a rationale for the co-location of the locking depth and the shelf break, not the coastline as previously proposed. The erosive shelf of a subduction margin results from the combination of continuous uplift and active wave erosion. The long-term uplift is driven by 1) the non-recoverable fraction of interseismic deformation that mimics the pattern of elastic deformation and 2) continental uplift (e.g. isostasy). We combine a wave erosion model with an elastic deformation model to show how the hinge line that marks the transition from interseismic subsidence to uplift pins the location of the shelf break. The width of the shelf is then set by the amount of coastal retreat under wave erosion. A global compilation of subduction zones with well-resolved locking depths confirms our model with shelf breaks lying much closer to the locking depth than coastlines (mean distances landward of the locking depth of 4.7 and 43.1 km respectively). The morphology of a subduction margin integrates thus hundreds of seismic cycles and it can inform seismic coupling stability through time.