The P wave is the seismic wave that travels fastest through Earth and arrives first at a seismograph.
P means primary because these waves are recorded before the other major body wave, the S wave. P waves are compressional: particles move back and forth in the same direction as the wave travels, somewhat like sound waves through a spring.
Unlike S waves, P waves can pass through solids, liquids, and gases. This property allows them to travel through both Earth’s liquid outer core and solid inner core, although their speeds and paths change at internal boundaries.
Arrival times help seismologists locate earthquakes. The larger the gap between P-wave and S-wave arrivals, the farther the recording station is from the earthquake, though at least three stations are normally needed to locate it precisely.