Crust and upper-mantle velocity structure beneath the northern and central Indian Ocean from the phase and group velocity of Rayleigh and Love waves

Abstract

Fundamental-mode Rayleigh and Love waves generated by nine earthquakes, which occurred in the central Indian Ocean (with epicentres extending to 40° S) and recorded at seven WWSSN stations of central Asia, have been used to determine the phase and group velocity along various paths across the northern and central Indian Ocean. The dispersion characteristics of Rayleigh and Love waves show a thick crust of 23 km thickness. This anomalous crustal thickening, having quasi continental-oceanic structure, may be explained by assuming the gradual transformation of top mantle material into material having either crustal like velocity or slightly lower than Moho velocity (P-wave velocity 7.72 km s −1, S-wave velocity 4.45 km s −1). The velocity structure down to a depth of 200 km has been obtained across the northern and central Indian Ocean. A low-velocity zone of 90 km with P-wave velocity of 7.85 km s −1 and S-wave velocity of 4.37 km s −1 is estimated to be centred at a depth of 78 km from the water surface. This low shear velocity in the mantle may be caused by the partial melting and elevated temperature there. The observed dispersion data for Rayleigh and Love waves also exhibit a strong anisotropy along different paths in the crust and upper mantle. The observed dispersion data for Rayleigh and Love waves that cross the Ninety-east Ridge are characterized by lower phase and group velocities, as compared to waves having paths in a more western direction than this ridge axis. However, the phase velocities for Rayleigh and Love waves are characterized by lower values across the Ninety-east Ridge, as compared to the normal oceanic structure.