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Description: In subduction zones, onshore geodesy provides the main data used to map seismic locking on the plate interface. We propose a new offshore control by establishing the co-location of the shelf break and the locking depth based on the Cascadia subduction. The erosive shelf of a subduction margin results from continuous uplift and active wave erosion. The long-term uplift is driven by 1) the non-recoverable fraction of interseismic 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. 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. Subduction margin morphology integrates hundreds of seismic cycles and informs seismic coupling stability through time.