Splitting of an incident S-wave into two (or more) waves with different polarizations, also called birefringence, S-wave splitting, mode splitting, double refraction (Crampin, 1981). In a polar anisotropic (transversely isotropic) medium, for each travel direction only two orthogonal polarizations of plane shear waves are allowed (although they are not necessarily orthogonal to the propagation direction). An S-wave of arbitrary polarization entering such a region in a direction other than along the symmetry axis splits into two S-waves (one of which is quasi-shear) that propagate at different velocities S1 for the faster, S2 for the slower (often written S1, S2). For horizontal thin-layer anisotropy, the two waves are the SH- and qSV-waves, the qSV not necessarily being polarized perpendicular to the propagation direction. For parallel fracturing the S1 mode that is polarized parallel to the fractures travels faster than the S2 mode that is polarized perpendicular to the fracture direction (see Figure B-4), See also polar anisotropy, Thomsen anisotropic parameters, and anisotropy (seismic).