The 1942 Southwest Indian Ocean earthquake is the largest ever recorded on an oceanic transform. Its mechanism has essentially the expected pure strike-slip geometry. Using long-period surface wave techniques, the seismic moment is found to be 1.3¿1028 dyn-cm; directivity suggests southwestward rupture propagation over approximately 130 km. This shock is then much smaller than previously reported on the basis of its high unified Richter magnitude (M=8.3). It does not show the slow energy release occasionally found in large transform fault events. Scaling laws and thermal modeling suggest that seismic slip extended at least to the 600 ¿C isotherm. ¿ American Geophysical Union 1987