Where Can Seismic Anisotropy Be Detected in the Earth’s Mantle? In Boundary Layers...

Abstract

During the last 30 years, considerable evidence of seismic anisotropy has accumulated demonstrating that it is present at all scales, but not in all depth ranges. We detail which conditions are necessary to detect large-scale seismic anisotropy. Firstly, minerals must display a strong anisotropy at the microscopic scale, and/or the medium must be finely layered. Secondly, the relative orientations of symmetry axes in the different crystals must not counteract in destroying the intrinsic anisotropy of each mineral, and there must be efficient mechanisms of orientation of minerals and aggregates. Finally, the strain field must be coherent at large scale in order to preserve long wavelength anisotropy. Part of shallow anisotropy can be related to the past strain field (frozen-in anisotropy), however the deep anisotropy is due to the present strain field. All these conditions are fulfilled only in boundary layers of convective mantle.We review in this paper, the seismic data sets which provide insight into the location at depth of large-scale anisotropy from the D¦-layer up to the lithosphere. In addition to the well-documented seismic anisotropy in the lithosphere and asthenosphere, there is new evidence of seismic anisotropy in the upper (400-660 km) and lower (660 -900 km) transition zones and in the D¦-layer. Nonetheless the bulk of the lower mantle seems close to isotropy. If we assume the hypothesis that seismic anisotropy is associated with boundary layers in convective systems, these observations strongly suggest that the transition zone is a boundary layer which makes the pasage of matter between the upper and the lower mantle difficult. However, this general statement does not rule out flow circulation between the upper and lower mantles. Finally, the geophysical, mineral physics and geological applications are briefly reviewed. An intercomparison between surface wave anisotropy and body-wave anisotropy data sets is presented. We discuss the scientific potential of seismic anisotropy and how it makes it possible to gain more insight into continental root, deformation and geodynamics processes.

Deep Earth Structure - Upper Mantle Structure: Global Isotropic and Anisotropic Elastic Tomography

“…The different observations related to anisotropy, at different scales, are reviewed in Babuska and Cara (1991), Montagner (1998), Park and Levin (2002) and in Chapter 2.16.…”

“…Between plate boundaries, oceans display very large areas with a large positive radial anisotropy such as in the Pacific Ocean (Ekströ m and Dziewonski, 1998), characteristic of an overall horizontal flow field. This very large anisotropy in the asthenosphere might be the indication of a strong deformation field at the base of the lithosphere (Gung et al, 2003), corresponding to the upper boundary layer of the convecting mantle (Anderson and Regan, 1983;Montagner, 1998). The nature of the boundary between lithosphere and asthenosphere (usually named LAB) has been questioned, and the large shear-wave velocity reduction with the rapid increase in radial anisotropy at the LAB requires a partially molten asthenosphere consisting of horizontal melt-rich layers embedded in meltless mantle (Kawakatsu et al, 2009).…”

The Spectral-Element Method, Beowulf Computing, and Global Seismology

“…This wealth of data has led to the construction of improved models of velocity heterogeneity, anisotropy, and attenuation in Earth. These models have provided important constraints on Earth's composition and physical processes (1)(2)(3). Nevertheless, improvements in seismic models will require the development, implementation, and application of methods that accurately incorporate the effects of mantle and crustal velocity and density heterogeneity on seismic wave propagation.…”

Crustal anisotropy of Taihang Mountain Range using azimuthal variation of receiver functions

“…Since Crampin and Booth (1985) discovered the phenomenon of shear wave splitting in 1980s, the technique of shear-wave splitting has become a main and effective method in studying the crust and mantle anisotropy. Anatomizing anisotropy information contained in the seismic waves can provide useful information about the Earth's interior structure, deformation (Tommasi et al, 1999), dynamic processing (Montagner, 1998) and mantle convection (Karato, 1998). Therefore, seismic anisotropy plays an increasingly important role during earth science research, such as theoretical seismology, exploring seismology, geodynamics and earthquake catastrophology.…”