There are three types of waves that are created when stress is released as energy in earthquakes: P, S, and surface waves. The P wave, or primary wave, is the fastest of the three waves and the first detected by seismographs. They are able to move through both liquid and solid rock. P waves, like sound waves, are compressional waves, which means that they compress and expand matter as they move through it. S waves, or secondary waves, are the waves directly following the P waves. As they move, S waves shear, or cut the rock they travel through sideways at right angles to the direction of motion. S waves cannot travel through liquid because, while liquid can be compressed, it can't shear. S waves are the more dangerous type of waves because they are larger than P waves and produce vertical and horizontal motion in the ground surface. Both P and S waves are called body-waves because they move within the Earth's interior. Their speeds vary depending on the density and the elastic properties of the material they pass through, and they are amplified as they reach the surface. The third type of wave, and the slowest, is the surface wave. These waves move close to or on the outside surface of the ground. There are two types of surface waves: Love waves, that move like S waves but only horizontally, and Rayleigh waves, that move both horizontally and vertically in a vertical plane pointed in the direction of travel.
Last modified on 8/13/98 by Maggi Glasscoe (firstname.lastname@example.org)