Mantle shear wave velocities determined from oceanic Love and Rayleigh wave dispersion

Abstract

Dispersion of Love and Rayleigh waves has been computed for a model of the suboceanic crust and mantle. An important feature of this model is a low-velocity layer in the upper mantle. These calculations, which include the effect of sphericity, are consistent with oceanic Love wave dispersion data for periods from 10 and 800 seconds to within a few hundredths of a kilometer per second. The effect of sphericity on oceanic Love waves is of great importance for periods as short as 10 seconds because of the penetration of these waves into the low-velocity channel. Computations for the same model, which consists of homogeneous isotropic layers, are also in accord with the oceanic Rayleigh wave data for periods from 30 to 150 seconds to within a few hundredths of a kilometer per second. The results can be further improved by minor modifications of the model. Since the calculations for a spherical earth are consistent with oceanic Love and Rayleigh wave data, apparent discrepancies which result from computations for flat layers are resolved without recourse to a difference in SH and SV velocities in the suboceanic mantle. Recent calculations and observations of Love and Rayleigh wave dispersion by Brune and Dorman for the mantle beneath the Canadian shield showed (1) that a low-velocity zone is required and (2) that no difference between SH and SV velocities is needed. Thus their study and this paper show that these two conclusions are applicable to large areas of the world. The wave guide of the G wave for periods from 60 to 300 seconds is the free surface and the steep gradient in shear velocity in the upper half of the mantle. At shorter periods the fundamental oceanic Love mode exhibits large particle amplitudes in the low-velocity channel. A comparison of the upper mantle beneath oceans and continents confirms previous results which indicate that channel velocities are smaller beneath the oceans.