As a seismologist with extensive experience in the study of seismic waves, I can explain the differences between P and S waves. P waves, also known as primary waves, are the fastest type of seismic wave and the first to be detected by seismographs during an earthquake. They are compressional waves that move in a longitudinal manner, meaning they cause particles in the Earth's crust to vibrate back and forth in the same direction as the wave is moving. Because of their nature, P waves can travel through all three states of matter: solids, liquids, and gases. S waves, on the other hand, are called secondary waves because they arrive after P waves at seismograph stations. S waves are slower and are transverse waves, which means they cause particles to vibrate perpendicular to the direction of the wave's travel. Unlike P waves, S waves cannot travel through liquids or gases; they can only move through solids. In summary, the key differences between P and S waves are their speed, the direction of particle movement, and the types of material they can travel through. read more >>

This is why S-waves arrive as secondary waves at the Earth's surface. There is another important difference between P-waves and S-waves. Although both can pass through solid rock, only P-waves can also pass through gases and liquids.read more >>