Abstract
Response of the upper mantle structure near the East Pacific Rise to Rayleigh and Love waves for periods less than 200 s is investigated through the group velocity partial derivatives with respect to changes of shear and compressional wave velocities and density. A noteworthy characteristic of the partial derivatives with respect to shear wave velocity is that the maximum amplitude of the group velocity partial derivatives of Rayleigh waves of the first two modes is about twice that of the phase velocity partial derivatives. Using the group velocity partial derivatives of Rayleigh and Love waves, the structures of the asthenosphere are determined and discussed with special respect to accuracy and resolution. The detailed structures exhibit significant discrepancies between the profiles of vertically and horizontally polarized shear wave velocities at depths of 60-90 km in the asthenosphere, with a minimum vertically polarized shear wave velocity of 4.0 km/s at those depths. The profile of horizontally polarized shear wave velocity does not possess a clear boundary between the lithosphere and the low velocity zone.
