Near-ridge intraplate earthquakes in the Indian Ocean

Abstract

Moment release computation based on a detailed catalog of near-ridge earthquakes between 1912 and 1993 pertaining to the younger lithosphere in the Indian Ocean suggest that the overall seismicity level in the Central Indian Ocean is not significantly higher as compared to the seismicity pattern near the Southwest Indian Ridge (SWIR). The characteristic seismicity pattern along the Southeast Indian Ridge (SEIR) and Central Indian Ridge (CIR) in the Central Indian Ocean over a much wider zone represents the plate-wide stress distribution. Though the seismicity near Chagos Bank appears to be similar to that observed near the Atlantis II Fracture Zone along the Southwest Indian Ridge (SWIR), several lines of evidence, such as more frequent occurrence, geographically wide spread seismicity, consistent faulting pattern and plate motion inversion results, suggest that the Chagos Bank seismicity forms part of the plate-wide stress distribution in the Central Indian Ocean. The present study also demonstrates two examples of highly energetic sequences of events characterized by thrust faulting. While the thrust faulting events near the southern part of the Central Indian Ridge can be inferred due to thermoelastic stresses related to cooling of the lithosphere, the events near the Indomed Fracture Zone on the African plate occur due to slow relative motion along the boundary between Nubia and Somalia, which connects to the SWIR along a diffuse compressional segment. The available mechanisms confirm that the near-ridge seismicity is characterized by dominantly normal faulting in the Indian Ocean. The moment release computed for different ages suggests that a greater fraction of moment release takes place in the 15- to 35-My-old lithosphere.