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TITLE: Seismic Waves

[Photo: Direction of P wave]
Seismic waves are shock waves that radiate outward from an earthquake. There are three main types:

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.

  [Photo: Direction of S wave]

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

  [Photo: Direction of surface wave]

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.

  [Photo: Seismic Waves]

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Smithsonian National Museum of Natural History Department of Mineral Sciences website Credits