GPS measurements of interseismic horizontal surface velocities reveal the degree of kinematic coupling of the plate boundary thrust along the Kamchatka subduction zone from about 51 degrees to 57 degrees N latitude. Inversions for the distribution of aseismic slip rate along the similar to 15 degrees NW dipping underthrust suggest a nonslipping plate interface in southern Kamchatka above similar to 50 km depth, along the segment that ruptured in the M-w = 9, 1952 earthquake. North of similar to 53 degrees N, the subduction interface experiences significant aseismic slip, consistent with the lower seismic moment release in M <= 8.5 earthquakes along this portion of the subduction zone. The GPS velocities are consistent with a boundary element forward model in which historic earthquake rupture zones are represented as locked asperities, surrounded by a zero shear stress subduction interface loaded by plate convergence. Models in which the complete rupture zones of historic earthquakes are considered locked greatly overpredict the degree of kinematic coupling. Reducing the area of the locked model asperities to the central 25% area of historic rupture zones fits the data well, suggesting that large earthquakes involve small fully locked core asperities surrounded by conditionally stable portions of the plate interface. Areas of low aseismic slip rate appear to be roughly correlated with areas of low isostatic gravity anomalies over offshore forearc basins, while less coupled portions of the Kamchatka subduction zone coincide with high-gravity anomalies offshore of two peninsulas, possibly related to the subduction of the Emperor-Meji seamount chain and the Kruzenstern fracture zone.