The most prolific source of moderate and large earthquakes along the mid-ocean ridge system is located near the Atlantis II Transform along the Southwest Indian Ridge. A remarkable swarm of 48 earthquakes was recorded from 1925 to 1933, which included 29 events with M≥6.0 and 4 events with M=7.0. Examination of the global earthquake catalog suggests this is the largest known earthquake swarm. Relocated events cluster around 33.8 ¿S, 58.0 ¿E, approximately 100 km east of the southern intersection of the Atlantis II Transform and the spreading ridge, and epicentral uncertainties derived from both standard least squares and resampling techniques preclude locations along the plate boundary. Multiple event relocation shows that seven 1977--1983 events (mb 5.1--5.6) are distributed across a 40-km-diameter region at the location of the 1925--1933 swarm events. Mechanisms determined for three of the recent events using waveform inversion indicate normal faulting with variable strike-slip components at depths of 12--17 km; neither the nodal planes nor the tensional axes show consistency between the three mechanisms. Grid search fits to body waveforms from the larger 1925--1933 swarm earthquakes suggest normal faulting mechanisms similar to the recent events, with depths of 11--20 km and seismic moments ranging up to 1.8¿1027 dyn cm.

The earthquake depths and tensional mechanisms are consistent with release of thermoelastic stress, which may be enhanced at this located by thermal bending moments associated with the transform fault. The total moment release of the 1925--1933 swarm (~7¿1027 dyn cm) is larger than the cumulative moment of all known oceanic intraplate seismicity since 1963 (~3¿1027 dyn cm). This suggests that intraplate stress release is dominated by infrequent but highly energetic episodes of seismicity and that estimates of intraplate moment release taken from observations over a limited time period underestimate intraplate seismicity levels. ¿ American Geophysical Union 1990