At periods greater than 1000 seconds, Earth’s seismic free oscillations have anomalouslylarge amplitude when referenced to the Harvard Centroid Moment Tensor faultmechanism, which is estimated from 300- to 500-second surface waves. By using morerealistic rupture models on a steeper fault derived fromseismic body and surface waves,we approximated free oscillation amplitudes with a seismic moment (6.5 x 1022NewtonImeters) that corresponds to a moment magnitude of 9.15. With a ruptureduration of 600 seconds, the fault-rupture models represent seismic observationsadequately but underpredict geodetic displacements that argue for slow fault motionbeneath the Nicobar and Andaman islands