The particles move back and forth in the same direction the wave is travelling. P waves can travel through solids, liquids, or gases. When P waves travel in air, they are called sound waves. In most rock types, P waves travel between 1.7 and 1.8 times as fast as the second kind of seismic waves, S waves. The particles vibrate at right angles to the direction in which the wave is travelling.
S waves can travel only in solids.
Seismic waves are detected and recorded by an instrument called a seismograph . From many such records obtained throughout the world, we can determine how fast seismic waves travel at various depths, from the earth's surface to its core. Seismic waves generally travel faster in denser rock. When an earthquake takes place, many people want to know where it occurred and how it was. Although the area in which an earthquake is felt can be very large, the place of rock rupture is very localized.