Seismic anisotropy

Seismic anisotropy is the dependence of seismic wave speed on direction, commonly produced in the mantle when flowing rock aligns its olivine crystals.

Olivine lattice-preferred orientation: A preferred alignment of olivine crystal axes within a rock. Mantle flow aligns olivine crystals, producing the directional elastic properties measured as anisotropy.
SKS splitting: Shear-wave splitting measured in core-transiting SKS phases recorded at seismic stations. SKS measurements are widely used to map mantle anisotropy beneath stations.
Olivine: A magnesium-iron silicate mineral that is abundant in Earth's upper mantle. Its crystal structure deforms and aligns during mantle flow, creating much mantle anisotropy.
Seismic heterogeneity: Spatial variation in seismic properties from one location to another. Heterogeneity varies properties across space, whereas anisotropy varies them with direction.
Seismic wave polarization: The orientation of particle motion in a seismic wave relative to its direction of travel. Anisotropic structure can rotate polarization and split shear waves into differently polarized arrivals.
Shear-wave splitting tomography: A method that combines shear-wave splitting observations to image anisotropic structures inside Earth. It turns directional travel-time and polarization data into spatial models of mantle fabric.
Dislocation creep: A mode of crystal deformation in which dislocations move through a mineral lattice. It can rotate olivine crystals into preferred orientations under mantle stress.
Seismic isotropy: The condition in which seismic properties are identical in every direction at a point. It is the directional uniformity that seismic anisotropy departs from.
Shear-wave splitting: The separation of a shear wave into two differently polarized waves traveling at different speeds. It is a common observable signature of seismic anisotropy in the mantle.
Mantle flow: The slow circulation of Earth's solid mantle driven by internal heat and forces. Olivine alignment can preserve a record of the direction and pattern of mantle deformation.

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