Seismic Waves - P Waves

P Waves are a type of seismic wave called a body wave - they are called this because they travel through the body of the Earth. See surface waves.

A P wave travels the 'fastest' of all of the waves we monitor. It therefore arrives first at an earthquake detector. For this reason, these waves are called primary waves (hence the letter "P")

P waves are 'compressional' or longitudinal waves; that is, the medium vibrates parallel to the direction that the wave energy is travelling.