Between February 1991 and May 1992, an earthquake swarm including 33 teleseismically recorded events occurred about 300 km west of the axis of the southern East Pacific Rise. Joint hypocentral relocations show that the best located events were in a 10-km-wide band between and parallel to two seamount chains. New, simulated-annealing inversion procedures were developed to find the moment tensors of three of the largest events that then served as master events for focal mechanism determination of the smaller events. Path/station corrections were applied to the spectra of surface waves from the smaller events, and the corrected spectra were linearly inverted for the moment tensors. The new procedures made it possible to derive focal mechanisms for 32 of the 33 earthquakes, ranging in size from Mw 4.2 to 5.9. All of the events have double-couple components that are nearly pure normal faulting with a wide range of strikes of the nodal planes. Most have a non-double-couple component that may have been caused by simultaneous slip on nearly randomly oriented fault planes. The individual mechanisms and net horizontal extension that is nearly equal in all directions are consistent with the release of thermal stresses in the cooling oceanic lithosphere. ¿ American Geophysical Union 1996