Subduction zone seismicity is highly variable. Great earthquakes occur at few subduction zones around the world, with significant variation in size and frequency of deep events. Interactions between overriding and subducting plates and slab pull strength for individual plates provide a framework for understanding these variations. Previous work suggests an inverse correlation between great earthquake moment release and the degree to which the subducted slab is connected to the surface plate. We find positive correlations between degree of plate-slab attachment and moment release from intermediate and deep earthquakes. This implies that shallow slab weakening that occurs at trenches where compressive stresses (and great earthquakes) dominate not only detaches slabs from plates, but is maintained as the slab descends, discouraging deep seismicity. Regions of low shallow moment release are consistent with extensional shallow stress regimes and undamaged slabs. Such slabs maintain mechanical strength during descent and deform seismogenically at depth.