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

Abstract

Dispersion of Love and Rayleigh waves hemm 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 Ephe-izc"y, are consistent. with oceanic Love wave dispersion data for periods from 10 and 800 seconds to wihLi 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 calcul ions for a spherical earth are consistent with oceanic Love and Rayleigh wave data, apparent ci repancics which result from computations for flat layers are resolved without recourse to difference in SH and SV velocities in the suboceanic mantle. Recent calculations and observaions of Love and Rayleigh wave dispersion by Brune and Dorman for the mantle beneath the Canadian shield v:wed (1) that a low-velocity zone is required and (2) that no difference bet SH and SV ,*ciocuje• is neeoed. Thus their study and this paper show that these two conclusions are applicable to large areas of the world. The wave guide of thc 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 cont"e-,ts confirms previous results which indicate that channel velocities are smaller beneath the oceans.Introduction. The physical properties of the as 10 seconds, because of the penetration of the crust and mnanifc beneath oceanic areas can be wave disturbance into the low-velocity channel. investigated with the aid of Love and RayleighIn fact, sphericity can resolve the Apparent diswaves. In this study theoretical phase and crepancies discussed by Ewing and Landisman group velocities for a model whinih contains a [19611 which result from computations of Love low-velocity layer are compared with experiand Rayleigh wave dispersion for flat layers. mental data for a broad range of pcriodr. The When sphericity is taken into account the're is effect of sphericity on Love and Rayhovh waves fo lorger a necessity for a difference in SH and has been taken into account in the theoretical SV velocities in the suboceanic mantle. calculations.Recently Brune and Dorman [1962], using In this paper considerable emphasis has high-precision Rayleigh and Love wave data, been placed on the Love wave solution for found that no difference between SH and SV a spherical ea.rth as compared with the result velocities is nee~nd for the mantle beneath the ...

Continental structure and drift

“…Sutton, Ewing & Major (i960) find results similar to those obtained by Dorman for surface waves travelling across the North Pacific. Comparable results have been obtained with Love wave dispersion curves by Sykes, Landisman & Saito (1962) and Kovach & Anderson (1962). Takeuchi et al (1962) find that the Lehmann model fits su propagation across continents.…”

Thickness Of The Earth's Crust Beneath The Pacific-Antarctic Ridge

“…Love waves are not affected at all by the water, and are affected by lowvelocity bottom sediment only at periods below about 12 sec (Sykes et at:, 1962). They thus give unique values of crustal thickness over both contInental and oceanic paths.…”

Study of Love and Rayleigh waves from earthquakes with fault plane solutions or with known faulting. Part 1. A phase difference method based on a new model of earthquake source

“…The numbers are given ill units of second per 100 kin. The values for the oceanic path are based on the theoretical model of Sykes et al (1962), and those for the continental path are based on Press's case 6EG (Press, 1960). The values in brackets in Column 12 are based on Alexander's model (Alexander, 1963).…”