Seismotectonics of the Central Indian Ridge, Western Indian Ocean

A detailed analysis of earthquakes along the slow-spreading Central Indian Ridge (CIR) is carried out by comparing the seismicity pattern with different morpho-tectonic segments of the ridge. The seismicity pattern is correlatable well with the bathymetric trend of major fracture zones and longer spreading ridge segments in the central part of the ridge between 7°S and 19°S. The frequencymagnitude relation of earthquakes along four major fracture zones and two spreading segments suggest that the spreading segments, in general, are characterised by higher b-values than the fracture zones. The difference in b-values can be attributed to changes in deformation pattern and faulting mechanism between the two regions.

Source mechanisms for 54 major earthquakes occurring along the ridge mostly show either normal faulting with one of the nodal planes parallel to the local spreading segment, or strike-slip motion on steeply dipping nodal planes parallel to the transform. However, a careful analysis of their locations with the morphotectonic details of the ridge reveals few anomalous mechanisms located near the ridge - transform intersections or along geometrically complex transform faults. The reverse faulting mechanisms at the ridge-transform intersections near Sealark F.Z. and Argo F.Z have been inferred to be due to thermal stresses associated with the differential cooling of the lithosphere, The normal faulting earthquakes along the 12°12'S F.Z. might be due to an extensional offset of the fault. Another normal faulting event located along the Marie-Celeste EZ, outside the Principal Transform Displacement Zone(PTDZ) indicates the geometrical complexity of the fault.