Upper mantle structure in the Southeast Indian Ocean: A surface wave investigation

Abstract

Surface waves recorded at Dumont D'Urville (DRV) in Terre Adelie have been analyzed to determine the upper mantle structure between Antarctica and the Southeast Indian Ridge. Rayleigh wave group velocities have been determined in the period range 15–60 s by using the single station method applied to earthquakes located on the Southeast Indian Ridge. An 8% decrease of group velocity is observed when passing from paths coming from the north of DRV to those coming from the east. Variations in both the lithospheric thickness and the upper mantle shear velocity are needed to explain this group velocity decrease by means of isotropic models. The corresponding change in the upper mantle structure does not appear to be correlated with the age of the seafloor. It suggests the existence of a low velocity region, several hundred kilometers in extent, in which transform faulting is active. A strong azimuthal anisotropy of Rayleigh wave velocity in the investigated area could also, but only partially, account for the observed velocity variations.