seismic wave

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Energy wave generated by an earthquake or an artificial explosion. There are two types of seismic waves: body waves that travel through the Earth's interior; and surface waves that travel through the surface layers of the crust and can be felt as the shaking of the ground, as in an earthquake.

Seismic waves show similar properties of reflection and refraction as light and sound waves. Seismic waves change direction and speed as they travel through different densities of the Earth's rocks.

Body waves
There are two types of body waves: P-waves and S-waves, so-named because they are the primary and secondary waves detected by a seismograph. P-waves, or compressional waves, are longitudinal waves (wave motion in the direction the wave is travelling), whose compressions and rarefactions resemble those of a sound wave. S-waves are transverse waves or shear waves, involving a back-and-forth shearing motion at right angles to the direction the wave is travelling (see wave).

Because liquids have no resistance to shear and cannot sustain a shear wave, S-waves cannot travel through liquid material. The Earth's outer core is believed to be liquid because S-waves disappear at the mantle-core boundary, while P-waves do not.

Surface waves
Surface waves travel in the surface and subsurface layers of the crust. Rayleigh waves travel along the free surface (the uppermost layer) of a solid material. The motion of particles is elliptical, like a water wave, creating the rolling motion often felt during an earthquake. Love waves are transverse waves trapped in a subsurface layer due to different densities in the rock layers above and below. They have a horizontal side-to-side shaking motion transverse (at right angles) to the direction the wave is travelling.

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