Seismic anisotropy - a method for studying the fabric of deep continental lithosphere
Babuska, V.; Plomerova, J.
published: Jan 1, 1999
Open Access (paper can be downloaded for free)
Independent seismological results, obtained from shear-wave splitting, azimuthal variation of Pn velocities, P-residual spheres and dispersion of surface waves, have demonstrated to observed anisotropies originates in preferred orientation of olivine crystals in the mantle lithosphere. In comparison with oceanic regions, the large-scale fabric of the continental lithosphere is more complex. While the surface-wave data provide an integral property of a number of tectonic units, results on splitting of sub-vertically incident teleseismic shear waves show consistent orientations of the fast S waves within individual lithospheric blocks. In many provinces, the mean azimuth of the fast S polarization parallels the regional geological fabric at the surface (e.g. the strike of a mountain belt). The azimuth-incidence angle dependent terms of teleseismic P-wave residuals, cleared of the effects of source regions and deep mantle paths, show a systematic directional dependence of the relative residuals over distances of several hundred kilometres. Abrupt changes in orientations of the high- and low-velocity directions are related to important tectonic boundaries (deep-seated suture zones). The continental lithosphere related anisotropy scanned by P waves requires a tectonic block structure with dipping high- and low-velocity directions within the lithosphere. Present-day models and interpretations of individual observations of seismic anisotropy depend on the wavelength of different types of seismic waves, as well as on directions of the propagation (both as to azimuths and dips from the horizontal). A joint analysis of body waves - both anisotropic P-residual spheres and shear-wave splitting - allows detecting large-scale anisotropic structures approximated either by hexagonal or orthorhombic symmetries with inclined symmetry axes.