The maximum size of thrust earthquakes at the world’s subduction zones appears to be limited by anelastic deformation of the overriding plate. Anelastic strain in weak forearcs and roughness of the plate interface produced by faults cutting the forearc may limit the size of thrust earthquakes by inhibiting the buildup of elastic strain energy or slip propagation or both. Recently discovered active strike-slip faults in the submarine forearc of the Cascadia subduction zone show that the upper plate there deforms rapidly in response to are-parallel shear. Thus, Cascadia, as a result of its weak, deforming upper plate, may be the type of subduction zone at wh ich great (moment magnitude approximate to 9) thrust earthquakes do not occur.