Based on the model of oblique convergence, we predicted the slip rate on the megathrust plate boundary in the western Sunda arc and compared it with the slip distribution of the 2004 M w 9.0 Sumatra-Andaman earthquake. The slip directions on the megathrust plate boundary, which are necessary for predicting the slip rate, were determined using the fault plane solutions of medium-sized earthquakes occurring for the last 30 years in the 2004 rupture zone. At long wavelengths the predicted slip rate correlates well with the latitudinal variation of the coseismic slip, suggesting that kinematics described by the model of oblique convergence operated over a long interseismic period and that the seismic coupling coefficient has been generally uniform along the strike of the rupture zone. Using the data for the same period of time, we estimated the total seismic moment released by the earthquakes on the Sumatra fault and its submarine continuation located on the backarc side of the Nicobar and Andaman islands. The ratio of the observed seismic moment to the seismic moment predicted from the model of oblique convergence is small. The residual may either be stored for generation of future earthquakes or was taken up by the fault creep at shallow depths.