How are primary and secondary waves different?

Primary waves (P-waves) are compressional waves that travel fastest through the Earth, moving in the same direction as the wave propagation, and can travel through solids, liquids, and gases. In contrast, secondary waves (S-waves) are shear waves that move slower than P-waves and can only travel through solids, moving perpendicular to the direction of wave propagation. This fundamental difference in their motion and the types of materials they can traverse is what distinguishes them in the context of seismic activity.