SEISMIC WAVE

vibration generated by an earthquake, explosion, or similar phenomenon and propagated within the Earth or along its surface. Earthquakes generate four principal types of waves; two, known as body waves, travel within the Earth, whereas the other two, called surface waves, travel along its surface. Interpretation of seismograms (recorded traces of the amplitude and frequency of seismic waves) yields information about the Earth and its subsurface structure; artificially generated seismic waves are used in oil and gas prospecting. The P wave Of the body waves, the primary, or P, wave has the higher speed and would thus reach a seismic recording station faster than the secondary, or S, wave. P waves, also called compressional or longitudinal waves, give rock particles a back-and-forth motion along the path of propagation, thus stretching or compressing the rock as the wave passes any one point; these waves are like sound waves in air (see also longitudinal wave). P waves travel at speeds from about 6 kilometres (3.7 miles) per second in surface rock to about 10.4 km/sec near the Earth's core 2,900 km below the surface; as the waves enter the core, the velocity drops to about 8 km/sec, increasing to about 11 km/sec near the centre of the Earth. The speed increase with depth results from increased hydrostatic pressure as well as from changes in rock composition and phase; in general, the increase causes P waves to travel in curved paths, concave upward. The S wave S waves, also called transverse or shear waves, cause rock particles to move back and forth perpendicular to the direction of propagation; as the wave passes, the rock is distorted first in one direction and then in another. The speed of S waves increases from about 3.4 km/sec at the surface to 7.2 km/sec near the boundary of the liquid core, within which they cannot be propagated; indeed, their absence is a compelling argument for the liquid nature of the outer core. Like P waves, S waves travel in curved paths, concave upward; they vibrate at a slower rate but with greater amplitude. The Love wave Of the two surface waves, Love wavesnamed after A.E. Love, who first predicted their existencetravel faster. They are propagated in a surface layer that overlies a solid rock layer with different elastic properties. Displacement of a rock particle by the wave is entirely perpendicular to the direction of propagation and has no vertical or longitudinal components. The energy of Love waves, like that of other surface waves, spreads from the source in two directions rather than in three, and so these waves produce a strong record at seismic stations even when originating from distant earthquakes. The Rayleigh wave The other principal surface waves are called Rayleigh waves after Lord Rayleigh, who first theorized their existence. Rayleigh waves travel on the free surface of an elastic solid. Their motion is a combination of longitudinal and vertical vibration that gives an elliptical motion to the rock particles. Of all seismic waves, Rayleigh waves and other surface waves have the strongest effect on seismographs.