P waves (primary or pressure waves) are the fastest and can pass through solids and liquids. They move by temporarily compressing rocks in the direction the waves are traveling.

S waves (secondary or shear waves) are slower than P waves and travel only through solid rock. They temporarily wiggle the rocks perpendicular to the direction in which the waves are traveling. S waves cannot pass through Earth’s outer core because it is liquid.

Surface waves are the slowest and travel along Earth's surface. Their rolling motions cause the most structural damage.

How Do Seismic Waves Behave?

They speed up as they descend through Earth because rocks become denser with depth. The denser a rock, the faster a seismic wave travels through it.

When seismic waves reach a boundary between rock layers of markedly different densities, some of the energy is reflected back toward the surface and some refracted (or bent) to still deeper levels.