An earthquake swarm on the Chagos—Laccadive Ridge and its tectonic implications

Summary. An unusual, isolated swarm of earthquakes occurred on the Chagos‐Laccadive Ridge in the Central Indian Ocean between 1965 and 1970. Sixteen earthquakes were “located at approximately 6.0° S, 71.3°E on the steep west facing scarp of the Chagos Bank. This swarm forms the only major seismic activity in the period 1963–76 on the otherwise aseismic ridge. The mechanisms of the three largest earthquakes were studied using body‐ and surface‐wave data. All are extremely similar shallow normal fault events on an east—west fault plane. Such faulting is difficult to reconcile with that expected from either the trend of the Chagos‐Laccadive Ridge or the spreading direction at the nearby Central Indian Ridge. The swarm may have occurred at depth on a cross fracture remaining from the breakup of the Chagos Bank and the Mascarene Plateau and the formation of the present Central Indian Ridge.