Seismic anisotropy -- the state of the art: II

Abstract

The theory, causes, observations, and possible applications of seismic anisotropy in the Earth have developed considerably since the previous state of the art paper was published in 1977. The behaviour of waves in layered anisotropic media is now much better understood and the evidence for seismic anisotropy indicates that anisotropy is likely to be present throughout much of the crust and upper mantle. The top few hundred kilometres of the mantle appears to be anisotropic with the orientations aligned by the present or palaeo stress-field. The upper part of the crust is frequently anisotropic, probably due to cracks differentially aligned by the non-lithostatic stresses. The possibility of being able to monitor crack geometry by seismic techniques opens a wide range of applications in currently important activities.

Measurements of mantle wave velocities and inversion for lateral heterogeneities and anisotropy: 3. Inversion

“…For a general review of that evidence, the reader is referred to two special issues of the Geophysical Journal of the Royal Astronomical Society (Bamford and Crampin, 1977;Crampin et al, 1984). On a regional scale, Pn studies have revealed that in the oceanic lithosphere, P waves traveling in the oceanic plate spreading direction are consistently faster (by about 5%) than those traveling perpendicular to the spreading direction (e.g., Hess, 1964;Raitt et al, 1969).…”

Preliminary Shear-Wave Splitting Results in the Broader Aigion Area (Greece) During 2013

“…When shearwaves enter an anisotropic medium, they split into two distinct components, presenting discernible seismic velocities. The phenomenon has been attributed to several causes, such as the lattice preferred orientation of minerals in sedimentary rocks (Valcke et al, 2006) or in accordance with the orientation of sedimentary grains (Crampin et al, 1984). In settings dominated by tectonic processes, it can be interpreted by the Extensive Dilatancy Anisotropy (EDA) model, according to which fluid-saturated microcracks, oriented parallel to the maximum horizontal compressive stress component, exist in the rockmass and control the anisotropy direction (Crampin, 1978).…”

Wave field simulation for heterogeneous transversely isotropic porous media with the JKD dynamic permeability

“…The first theory of wave propagation in a fluid-saturated porous medium was put forward by Biot [8,9]. Porous media appearing in geophysical application are anisotropic on account of the presence of micro-cracks, fluidfilled cracks [15] and thin layering [1]. Anisotropy has a very strong effect on wave propagation.…”