Seismic anisotropy for understanding the dynamics of crust and upper mantle

Abstract

Seismic anisotropy refers to the directional dependence of seismic velocity which is present in the Earth's crust, mantle, and inner core, as revealed by numerous observations of shear-wave splitting of radially polarized (S, Ps, SKS and SKKS) waves and gives important constraints on the Earth's internal dynamics. Whenever a seismic wave enters an anisotropic medium, it splits into two horizontally polarized (V SH and V SV) seismic waves. Anisotropy is measured by the angle (Φ) between the geographic north and the fast polarization direction (FPD) and the time difference (δt) between the fast and the slow polarization directions. Splitting measurements are especially applicable to the upper mantle, and to a lesser extent to the crust. Seismic anisotropy in the upper mantle are often attributed to a strain-induced lattice-preferred orientation (LPO) of olivine that occurs during mantle flow. Olivine is elastically anisotropic and the general alignment of the ‘fast’ a-axis of olivine grains results in detectable seismic anisotropy. Tectonic and flow deformation results in anisotropic fabric through crystallographic alignment of anisotropic minerals. The radially polarized SKS and SKKS core phases could be used to obtain extremely simple in situ measurements of azimuthal anisotropy. In case of the crust, the tectonic and flow deformation results in anisotropic fabric through crystallographic alignment of anisotropic minerals. Crustal anisotropy can be measured by the Moho converted Ps phases and direct S waves. The radially polarized body waves are useful to determine azimuthal anisotropy but for the radial anisotropy surface waves (Rayleigh waves and Love waves) are more useful. This chapter provides an overview of the shear-wave splitting, basic principle of anisotropy, the causative factors for shear-wave splitting, types of shear-wave splitting, waveforms used to determine the anisotropy of different layers of the Earth. At the end, there is a case study of seismic anisotropy for crust and upper mantle beneath the northwest Himalaya and eastern part of the Ladakh-Karakoram zone and a comparison of the crustal and upper mantle anisotropy.