Seismic anisotropy in the upper mantle

Abstract

Abstract Seismic wave propagation in anisotropic media is fundamentally different from isotropic propagation, displaying variation of characteristics with direction, and, perhaps more diagnostic, coupling between motion in the sagittal plane and horizontal transverse motion. Such coupling has been observed for higher mode surface waves indicating anisotropy in the upper mantle beneath much of Eurasia. Synthetic seismograms show that the passage of P or SV body waves through an anisotropic upper mantle will write SH signatures on the wavetrains generated at both isotropic/anisotropic and anisotropic/isotropic interfaces. These signatures have not yet been positively identified on seismograms, but once observed, may be interpretated in terms of depth, thickness, orientation, and degree of anisotropy. The widespread anisotropy already demonstrated implies that the interpretation of every seismic wave which penetrates the upper mantle must be modified for anisotropy if the observations are made with sufficient resolution.