Slowness surface for hexagonal (transversely isotropic) crystal
Slowness surfaces (1/v as a function of propagation direction) reveal anisotropy: quasi-P (qP) is fastest, quasi-S1 (qS1) and quasi-S2 (qS2) are shear modes. In hexagonal symmetry (5 independent c_ij), the surfaces are rotationally symmetric about the c-axis. Anisotropy causes shear wave splitting, exploited in seismic imaging.