A thrust-type subduction-zone earthquake of Mw 7.6 ruptures an area of ∼6000 km2, has a seismic slip of ∼1 m, and is nucleated by the rupture of an asperity ∼25 km across. A model for thrust-type subduction-zone seismicity is proposed in which basaltic seamounts jammed against the base of the overriding plate act as strong asperities that rupture by stick-slip faulting. A Mw 7.6 event would correspond to the near-basal rupture of a ∼2-km-tall sea-mount. The base of the seamount is surrounded by a low shear- strength layer composed ofsubducting sediment that also deforms between seismic events by distributed strain (viscous flow). Planar faults form in this layer as the seismic rupture propagates out of the seamount at speeds of kilometres per second. The faults in the shear zone are disrupted after the event by aseismic, slow viscous flow of the subducting sediment layer. Consequently, the extent of fault rupture varies for different earthquakes nucleated at the same seamount asperity because new fault surfaces form in the surrounding subducting sediment layer during each fast seismic rupture.