An usually large number of off-ridge earthquakes have occurred within a broad region astride the Southeast Indian Ridge. The source mechanisms of none of these earthquakes, in lithosphere from 4 to 27 m.y. old, have been determined with a formal inversion technique based on matching observed and synthetic P and SII waves. All events occur in the oceanic mantle, from 8 to 22 km below the seafloor. The seismic moment and duration of faulting obtained from the inversion constrain the stress drop to lie between a few tens of bars and few hundred bars for most of the events. All of the off-ridge earthquakes are characterized by normal-faulting mechanisms, with the T axes nearly parallel to the nearby ridge axis, in sharp contrst to typical mid-ocean rfidge or intraplate earthquakes. A variety of possible tectonic processes which might produce normal faulting in young oceanic lithosphere are considered as possible causes, including bending stresses, thermal stresses of various origins, different subsidence at a fracture zone, off-ridge volcanism, latitudinal plate motions on a nonspherical earth basal shear stress from small-scale convection, and plate-wide perturbations of the stress field due to the continental collision between India and Asia. No single mechanism is adequate to explain all the observed off-ridge normal-faulting earthquakes. An apparent asymmetry in the observed seismicity with respect to the ridge suggests that the earthquakes on the Antarctic and Indian plates are caused by different processes. The Antrctic plate events can be satisfactorily attributed to the release of thermal and bending stresses associated with a residual depth anomaly between the Kerguelen Plateau and the Southeast Indian Ridge, a region which has been suggested as the site of a horizontal ''pipe'' for flow in the asthenosphere from the Kerguelen hot spot to the vicinity of Amsterdam and St. Paul islands on the Southeast Indian Ridge. The off-ridge earthquakes in the Indian plate are speculatively attributed to part of the plate-wide response of the lithosphere to the ongoing continental collision.