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S-wave (secondary wave, shear wave, transverse wave) An elastic body wave in which particles oscillate about a fixed point but in a direction perpendicular to the direction of propagation of the wave energy. S-waves cannot travel through a fluid, since a fluid cannot support shear. In an isotropic and homogeneous medium, S-wave velocity (Vs) is given by: Vs = √(μ/ρ), where μ is the shear modulus and ρ the density of the material. S-waves travel at about half the speed of P-waves in a given medium, and can be polarized into SV- and SH-waves in which particle motion is restricted to the vertical and horizontal planes respectively. S-waves can also be generated from the non-normal incidence of P-waves on to an interface, when they are known as ‘converted waves’ and are mostly SV-waves. S-waves can also be converted into P-waves at an interface.