Seismic zones beneath consuming plate margins have a flattened upper section, which has been attributed to the growth of the accreted material beneath the innter trench slope. Comparison of well-studied seismic zones show that the width of this flattened section is proportional to the trench-volcanic chain separation in nearly all arcs. The bend, or zone of maximum curvature between upper and lower sections, lies in front of the volcanic chain and behind the trench slope break. Combined geological and seismological data indicate that the bend location remains nearly fixed during subduction and that accretion at the trench causes progressive loading and depression of the descending lithosphere. Concurrent migration of the positive-negative free air gravity anomaly couple and the approach to isostatic conditions over the earlier trench positions are further observational support for this conclusion. Mechanical modeling of the load applied by accretion to an elastic lithosphere not only shows the plausibility of this concept but also indicates that in wider accretionary prisms the geometry of the outer trench slope and outer rise is controlled by loading rather than by deeper level interplate stresses. This latter point is also supported by the lower dips of outer trench slopes and low amplitudes of outer rises observed seaward of wide accretionary prisms. The growth of the flattened seismic zone can persist through several subduction episodes, the indication being that the slab remains unbroken and stressed after subduction ceases and that much of the mass imbalance is retained for long periods of time.