GPS velocities from the Alaska Peninsula are modeled to determine the extent of locking on the Alaska-Aleutian subduction interface. The observations, which span from the Semidi Islands to Sanak Island, encompass the 1938, M w 8.3, rupture zone and the transition into the Shumagin gap. Model parameters are optimized using a simulated annealing method. Coupling variation along strike of the plate interface show a nearly fully locked (90%) subduction zone at the Semidi Islands, decreasing to about 30% locked at the Shumagin Islands, and freely slipping to the west of the Shumagins. Independent rupture of the Shumagin segment could produce repeated M w 7.6 earthquakes, unless a significant fraction of the slip on the interface occurs as afterslip following large earthquakes. Southwest directed velocities at most of the sites may be attributed to clockwise rotation of a Bering block.