(bī, ri frin’ j∂ns) Splitting of an incident S-wave into two waves with different polarizations, also called shear-wave splitting, S-wave splitting, mode splitting, and double refraction (Crampin, 1981). In a transversely isotropic medium, for each travel direction only two orthogonal polarizations of plane shear-waves are allowed (although they are not necessarily transverse 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. For horizontal thin-layer anisotropy, the two waves are the SH- and qSV-waves, the qSV not necessarily being polarized transverse 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 with a component perpendicular to the fracture direction (see Figure B-4). See also transverse isotropy, Thomsen parameters, and anisotropy (seismic).