These waves can travel for and cause damage at great distances. Once they start, these waves continue through the earth until their energy is used up. There are two basic types of seismic waves, and they travel at different speeds through the earth. The faster P waves move by alternately compressing and expanding the rock. 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 .