P-wave

In seismology, a class of seismic wave that passes through the Earth in the form of longitudinal pressure waves at speeds of 6-7 kps/3.7-4.4 mps in the crust and up to 13 kps/8 mps in deeper layers, the speed depending on the density of the rock. P-waves from an earthquake travel faster than S-waves and are the first to arrive at monitoring stations (hence primary waves). They can travel through both solid rock and the liquid outer core of the Earth.

P-waves are longitudinal waves; that is, they travel in the same direction as the rock is moving back and forth. P-waves cause one part of the rock to compress together and another section to be pushed apart. This process is known as compression and rarefaction, similar to the propagation of sound waves.