Chilean-type convergent margins have many large (M > 7.6) earthquakes, whereas Marianas-type ones do not. This dichotomy is enigmatic if the plate interface is viewed as a thin frictional decollement, whereas it becomes understandable if it is viewed as a relatively thick, sediment-filled shear zone, which thins or thickens arcward depending on subduction speed and sediment supply. Chilean-type margins have thick trench fills, and their shear zones generally thin arcward from inlets as much as several thousand metres high, the most pronounced thinning being located near backstops. Tall (up to several kilometres) seamounts are subducted essentially intact to relatively great depths and confining pressures before jamming into the roof of the channel and becoming seismogenic asperities. Their near-basal ruptures can generate large thrust-type earthquakes, mainly concentrated in seismic fronts near backstops. Marianas-type margins, in contrast, have thin trench fills, and their shear zones generally thicken arcward from inlets that can be as little as 300 m high. Seamounts are truncated near the inlet at low confining pressures and generate only small earthquakes. After passing the inlet, they do not touch the roof and therefore cannot generate large earthquakes. A similar mechanism may explain seismic gaps at sediment-poor regions of subduction zones.