A study of seismographs has revealed that there are basically three types of seismic (earthquake) waves. Waves that travel through the Earth’s interior are collectively called body waves. Body waves can be split into two types:

• P (Primary) Waves: P, or primary waves are so-called because they travel the fastest of all seismic waves, and are the first to be recorded on a seismograph. A P wave causes rock particles in the Earth to move back and forth, or parallel to the overall direction of energy transfer. A P wave is identical to the kind of wave that carries sound energy. Waves that transmit energy in this manner are known under the general category of compressional waves.
• S (secondary) Waves: S waves are the second set of waves to be recorded on a seismograph; hence the name “secondary.” In an S wave, the rock particles travel up and down, or perpendicular to the overall direction of energy transfer. Radio waves and microwaves also transmit energy this way. Waves that transmit energy in this manner are known under the general category of transverse waves.

Surface Waves:The third kind of wave known as a surface wave. Surface waves travel only through the Earth’s outer layer, at or near the Earth’s surface, and are the slowest moving of all seismic waves. Their behaviour is quite complex and is fairly similar to the motion of water waves. It is the surface waves that are usually responsible for most of the damage and destruction caused by earthquakes.

It is fairly easy to distinguish the three kinds of waves because of their different speeds and arrival times at a seismograph. In granite, for example, P waves travel at approximately 6 km/s, S waves travel at 3.5 km/s, and surface waves travel at about 3 km/s.