Higher mode surface waves and their bearing on the structure of the earth's mantle

Abstract

A detailed numerical investigation of surface wave dispersion and particle motion associated with the higher Love and Rayleigh modes over realistic earth models has been carried out as a preliminary to the routine use of these waves in studies of the crust-mantle system. The suggestion that the so-called channel waves, such as the Lg, Li, and Sa phases, can be interpreted by higher mode group velocity dispersion curves is verified in detail. Furthermore, Sa should have a higher velocity across shield areas than across normal continental areas and a higher velocity across continents than across oceans. Higher mode Rayleigh wave data are presented for long oceanic paths to Pasadena. The observed data favor the CIT 11 model of Anderson and Toksöz (1963) over the 8099 model of Dorman et al. (1960) and indicate that under the Pacific Ocean the low-velocity zone extends to a depth perhaps as deep as 400 km followed by an abrupt increase in shear velocity.

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