Anisotropy in deeper parts of the Earth

Abstract

The oceanic lithosphere exhibits a clear seismic anisotropy. In continents, there are many observations which strongly suggest that seismic anisotropy is present in large enough volumes of the upper crust and subcrustal lithosphere to make direct seismic investigations possible. In deeper parts of the Earth, evidence of seismic anisotropy is more difficult to obtain. At sublithospheric depths, hypothesis testing thus plays an important role in interpretations of seismic anomalies. Interpretations of the seismic anomalies which are suspected to be due to anisotropy in the asthenosphere are commonly conducted in the framework of preferred orientations of upper mantle minerals. Due to its large abundance, to its strong elastic anisotropy and its ability to be preferentially oriented in a shearing deformation zone, olivine plays a major role in modeling the asthenospheric anisotropy in connection with the present flow pattern of the upper mantle convection. Effects of other mineralogic constituents, such as orthopyroxene and clinopyroxene in the upper mantle, and higher pressure phases and/or constituents with different chemistry at greater depths have to be taken into account (Anderson, 1989). Finally, in the deepest part of the mantle, it is not impossible that region D” at the core-mantle boundary is also anisotropic. Several recent studies concerning both normal mode data and inner core PKIKP body waves have led different authors to suggest that the inner core might also be anisotropic. Seismic anisotropy of the liquid outer core is very unlikely but, even there, it has been suggested that a well organized pattern of convection in the liquid core with a rain of solid particles in cold regions could induce heterogeneities in the seismic velocity pattern which might show up as an apparent seismic anisotropy! Seismic anisotropy has thus been invoked at all depths as a possible explanation of observed seismic anomalies.