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, Mw 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 Mw 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.