<P> Secondary waves (S - waves) are shear waves that are transverse in nature . Following an earthquake event, S - waves arrive at seismograph stations after the faster - moving P - waves and displace the ground perpendicular to the direction of propagation . Depending on the propagational direction, the wave can take on different surface characteristics; for example, in the case of horizontally polarized S waves, the ground moves alternately to one side and then the other . S - waves can travel only through solids, as fluids (liquids and gases) do not support shear stresses . S - waves are slower than P - waves, and speeds are typically around 60% of that of P - waves in any given material . </P> <P> Seismic surface waves travel along the Earth's surface . They can be classified as a form of mechanical surface waves . They are called surface waves, as they diminish as they get further from the surface . They travel more slowly than seismic body waves (P and S). In large earthquakes, surface waves can have an amplitude of several centimeters . </P> <P> Rayleigh waves, also called ground roll, are surface waves that travel as ripples with motions that are similar to those of waves on the surface of water (note, however, that the associated particle motion at shallow depths is retrograde, and that the restoring force in Rayleigh and in other seismic waves is elastic, not gravitational as for water waves). The existence of these waves was predicted by John William Strutt, Lord Rayleigh, in 1885 . They are slower than body waves, roughly 90% of the velocity of S waves for typical homogeneous elastic media . In the layered medium (like the crust and upper mantle) the velocity of the Rayleigh waves depends on their frequency and wavelength . See also Lamb waves . </P> <P> Love waves are horizontally polarized shear waves (SH waves), existing only in the presence of a semi-infinite medium overlain by an upper layer of finite thickness . They are named after A.E.H. Love, a British mathematician who created a mathematical model of the waves in 1911 . They usually travel slightly faster than Rayleigh waves, about 90% of the S wave velocity, and have the largest amplitude . </P>

Seismic waves which travel only through the outer layer of the earth are called