Rayleigh- and Love-wave fundamental-mode group and phase velocities between World Wide Standard Seismograph Network Stations at Chiengmai, Thailand; New Delhi, India; Shiraz, Iran; and Helwan, United Arab Republic, have been obtained from data generated by an earthquake near the New Hebrides Islands. This epicenter and these seismic stations lie practically on one great circle path, the epicenter-to-station directions being confined to the narrow azimuthal window from 294.40° to 297.57°. Application of improved frequency-time analysis to carefully selected subsets of these data makes it possible to obtain results for periods extending to several minutes. The “impulse response” method is introduced for the estimation of interstation dispersion. This method is superior to the usual cross-correlogram method because spectral amplitudes are equalized to a greater degree.
These interstation Rayleigh- and Love-wave group- and phase-velocity results are consistent with shield-like upper mantle velocity models. A shear-wave upper mantle low-velocity zone is inferred for the region studied. The observed data support extension of the Indian shield beneath the thick sedimentary cover of the Indogangetic plains.