Focal mechanisms of earthquakes in the Indian Ocean and adjacent regions

Focal mechanism solutions are presented for 22 earthquakes that occurred in the Indian Ocean, on the Macquarie ridge, in the East African rift valleys, in the South Atlantic, and on the Azores-Gibraltar ridge during the period 1963 to 1966. The first motions P and PKP and the polarization (or first motions) of S waves were used for this investigation. Eleven of these earthquakes occurred on or close to that portion of the mid-oceanic ridge that extends from the Gulf of Aden to south of Australia. The mechanism solutions of these 11 earthquakes are characterized by large components of either strike-slip faulting or normal faulting. The inferred directions of sea-floor spreading are nearly northeast on the portion of the ridge (the Carlsberg ridge) extending from the Owen fracture zone to 20°S, 68°E, whereas they are almost north-south on the portion of the ridge extending from the latter point to south of Australia. These directions of spreading are in close agreement with the results obtained from studies of magnetic anomalies. Earthquakes on the Macquarie ridge are characterized by either thrust faulting, strike-slip faulting, or a combination of the two. Strike-slip solutions were obtained for an earthquake on the Ninetyeast ridge and for one other earthquake in the South Atlantic near Bouvet Island. The mechanism solution of one earthquake on the western end of the Azores-Gibraltar ridge is characterized by a combination of normal faulting and strike-slip faulting whereas another near the coast of Portugal shows thrust faulting. Thirteen earthquakes of this study indicate a large component of strike-slip motion. At places where fracture zones are known to exist, the nodal plane striking nearly parallel to the fracture zone was chosen as the fault plane. The sense of strike-slip motion is the same as that predicted for transform faults; it is opposite to the sense of motion that is expected for a simple offset of the ridge. The results of this study furnish additional support for the hypotheses of transform faulting, sea-floor spreading, and continental drift.