P waves can travel through Earth’s solid inner core but not its liquid outer core. P, or primary, waves are compressional body waves that move material back and forth in the direction of travel.
Unlike S waves, P waves can pass through solids, liquids, and gases. Their ability to cross liquids helps seismologists detect the outer core, which is liquid, and their behaviour in the inner core provides evidence that it is solid. P waves slow down and refract when they cross boundaries between Earth’s layers.
The outer core creates a P-wave shadow zone rather than blocking every P wave. Waves are strongly bent as they enter and leave the core, leaving some angular distances with no direct P-wave arrivals. Careful analysis of these patterns helped establish Earth’s layered interior.
P waves usually arrive before S waves at a recording station because they travel faster. Their first arrival can help locate an earthquake, but full earthquake analysis also uses later wave phases and readings from multiple stations.