The P wave is the type of seismic wave that arrives first at a recording station during an earthquake.
“P” stands for primary because these waves are the first major body waves detected by seismographs. They are compressional waves: particles move back and forth in the same direction the wave travels. This motion alternately squeezes and expands the material.
P waves can travel through solids, liquids, and gases. That property distinguishes them from S waves, which cannot pass through liquids. P waves generally travel faster than S waves, though their speed varies with the material’s density and elasticity.
Seismologists use the arrival-time gap between P and S waves to estimate the distance from a station to an earthquake. Data from at least three stations can then help locate the epicenter. Surface waves, including Love and Rayleigh waves, usually arrive later and can produce especially damaging ground motion near the surface.