A seismic wave is a wave that travels through the Earth, often as the result of an earthquake or explosion. Seismic waves are studied by seismologists, and measured by a seismograph.
Body waves travel through the interior of the Earth. They follow curved paths because of the varying density and composition of the Earth's interior. This effect is similar to the refraction of light waves. Body waves transmit the preliminary tremors of an earthquake but have little destructive effect. Body waves are divided into two types: primary (P) and secondary (S) waves.
- P waves are longitudinal or compressional waves, which means that the ground is alternately compressed and dilated in the direction of propagation. These waves generally travel twice as fast as S waves and can travel through any type of material. Typical speeds are 330m/s in air, 1450m/s in water and about 5000m/s in granite.
- S waves are transverse or shear waves, which means that the ground is displaced perpendicularly to the direction of propagation, alternately to one side and then the other. S waves can travel only through solids. Their speed is about 58% of that of P waves in a given material.
Surface waves are analogous to water waves and travel over the Earth's surface. They travel more slowly than body waves. Because of their low frequency, they are more likely than body waves to stimulate resonance in buildings, and are therefore the most destructive type of seismic wave. There are two types of surface waves: Rayleigh waves and Love waves.
Rayleigh waves, also called ground roll, are surface waves that travel as ripples similar to those on the surface of water. The existence of these waves was predicted by John William Strutt, Lord Rayleigh, in 1885. They are slower than body waves.
Love waves are surface waves that cause horizontal shearing of the ground. They are named after A.E.H. Love, a British mathematician who created a mathematical model of the waves in 1911. They are usually slightly faster than Rayleigh waves.
A quick and dirty way to determine the distance from a location to the orgin of a seismic wave is to take the difference of arrival time from the P wave to the S wave in seconds and multiply by 8 kilometers per second.
Other modes of wave propagation exist than those described in this article, but they are of comparatively minor importance.
Last updated: 02-10-2005 16:34:12
Last updated: 05-03-2005 17:50:55