New inferences from higher mode data in western Europe and northern Eurasia

Abstract

Summary Stacking methods applied to wide arrays of long period stations provide a powerful tool to investigate the physical properties of the upper mantle by means of higher modes. In this paper, the UC-diagram technique of Cara (1976) is applied to a subset of records first used by Nolet (1975, 1976) to determine the dispersion of higher Rayleigh modes in western Europe. For the fundamental and first higher modes, the phase velocities found in the present study are very similar to those first obtained by Nolet (1976) but the second made exhibits slightly higher velocities in the period range 35–55 s. As a consequence, the low density channel proposed by Nolet (1977) under western Europe is no longer necessary to explain the higher mode data and further investigation will be necessary to obtain more details in the density distribution with depth. A new set of higher Love mode phase velocities has been also obtained in western Europe. As it has already been observed for the fundamental mode in several parts of the world, the phase velocities found here for the first and second higher modes are too large compared to those predicted by the models fitting the Rayleigh mode data. It is very difficult to interpret both the Love and Raylcigh wave data in terms of isotropic S velocity distribution and an anisotropy of the elastic parameters of the upper mantle is probable under western Europe. Another wide continental region, northern Eurasia, is investigated from higher Rayleigh mode phase velocities measured for the path between northern Japan and western European stations. A nearly constant 4.5–4.6 km s−1S velocity is required by these data in the uppermost 200 km of the upper mantle. Unlike the higher mode data obtained for the central and eastern United States by Cara (1978) or for western Europe in this paper, no wide low velocity channel is necessary to explain the higher mode data in this large arca.