Earth's Interior and Seismic Waves

P-waves are longitudinal, or compressional, seismic waves. Particles in the material vibrate back and forth parallel to the direction of wave travel, creating alternating regions of compression and rarefaction. This particle motion allows P-waves to travel through solids, liquids and gases. Transverse displacement is characteristic of S-waves, while rotational motion and surface-wave motion do not describe the defining effect of P-waves.

S waves are transverse waves and cannot travel through liquids. Their absence in the outer core indicates that the outer core is liquid. P waves can travel through liquids, so their behaviour provides different evidence about the outer core. Exam tip: remember that S waves do not pass through liquids.

When P waves enter the outer core, their speed changes because the properties of the medium change sharply. This bends their path, and the bending is called refraction. Reflection would mean that the waves bounce back, whereas here their direction mainly changes while travelling through the new medium. Exam tip: P waves can travel through the liquid outer core, but their speed and direction change.