We examine the amplitude and distribution of slip on vertical frictionless faults in the zone of concentrated shear strain that is characteristic of transform plate boundaries. We study both a 2D and a 3D approximation to this strain field. Mean displacements on ruptures within the zone of concentrated shear strain are proportional to the shear strain at failure when they are short, and are limited by plate displacements since the last major earthquake when they are long. The transition between these two behaviors occurs when the length of the dislocation approaches twice the thickness of the seismogenic crust, approximately the breadth of the zone of concentrated shear strain observed geodetically at transform plate boundaries. This result explains the observed non-linear scaling relation between seismic moment and rupture length. A geometrical consequence of the 3D model, in which the strain-field tapers downward, is that moderate earthquakes with rupture lengths similar to the thickness of the crust tend to slip more at depth than near the surface. Seismic moments estimated from surface slip in moderate earthquakes (M≤7) will thus be underestimated. Shallow creep, if its along-strike dimension is extensive, can reduce a surface slip deficit that would otherwise develop on faults on which M<7 events are typical. In the absence of surface creep or other forms of off-fault deformation great earthquakes may be necessary features of transform boundaries with downward-tapering strain-fields. ¿ American Geophysical Union 1994