Characteristics
Rayleigh waves are a type of surface wave that travel near the surface of solids. Rayleigh waves include both longitudinal and transverse motions that decrease exponentially in amplitude as distance from the surface increases. There is a phase difference between these component motions.
The existence of Rayleigh waves was predicted in 1885 by Lord Rayleigh, after whom they were named. In isotropic solids these waves cause the surface particles to move in ellipses in planes normal to the surface and parallel to the direction of propagation – the major axis of the ellipse is vertical. At the surface and at shallow depths this motion is retrograde, that is the in-plane motion of a particle is counterclockwise when the wave travels from left to right. At greater depths the particle motion becomes prograde. In addition, the motion amplitude decays and the eccentricity changes as the depth into the material increases. The depth of significant displacement in the solid is approximately equal to the acoustic wavelength. Rayleigh waves are distinct from other types of acoustic waves such as Love waves or Lamb waves, both being types of guided wave in a layer, or longitudinal and shear waves, that travel in the bulk.
Rayleigh waves have a speed slightly less than shear waves by a factor dependent on the elastic constants of the material. The typical speed is of the order of 2–5 km/s. Since Rayleigh waves are confined near the surface, their in-plane amplitude when generated by a point source decays only as, where is the radial distance. Surface waves therefore decay more slowly with distance than do bulk waves, which spread out in three dimensions from a point source.
In seismology, Rayleigh waves (called "ground roll") are the most important type of surface wave, and can be produced, for example, by ocean waves, by explosions or by a sledgehammer impact.
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