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What Are Waves of Energy That Travel Through the Earth?

Waves of energy that travel through the earth are called seismic waves. These energy waves are produced by earthquakes or large explosions and detected by seismometers and recorded by seismographs. There are two categories of seismic waves: body waves and surface waves. Body waves, called P and S waves, can propagate through the earth's interior, while surface waves, called Love and Rayleigh waves, can move only along the surface.

P Waves
- P waves, also known as pressure waves or primary waves, are body waves. P waves are longitudinal waves that propagate via compression and are the fastest seismic waves. Particles are displaced in a direction parallel to the movement of the energy, called the direction of wave propagation. This motion is similar to the way a slinky moves. Pressure waves can propagate through both solid material, such as rock, and fluids, such as water and magma. Primary waves are sound waves, although their frequency is too low for human ears to hear. It is not uncommon for animals, such as dogs, to hear this low-frequency sound prior to the arrival of subsequent S waves and surface waves. The parts of a P wave are called compressions and rarefactions.
S Waves
- S waves, also known as shear waves or secondary waves, are also body waves. S waves are transverse waves that travel at approximately 60 percent to 70 percent of the speed of P waves, always arriving after the primary wave. In a secondary wave, particles are displaced in a direction perpendicular to the direction of wave propagation, either vertically or horizontally. This motion is similar to vibrating one end of a taut rope. Unlike P waves, shear waves can travel only through solid rock and not through fluids. The parts of a secondary wave are called crests and troughs.
Love Waves
- Love waves are surface waves that are named after A.E.H. Love, a British mathematician, who formulated the model for this type of wave in 1911. Surface waves have a lower frequency than body waves. Their frequency decreases with depth, thereby limiting them to a shallow path along the surface of the earth. Although they arrive after body waves, surface waves are almost exclusively responsible for earthquake damage. Love waves are the fastest of the surface waves, displacing particles in a transverse, horizontal direction. This produces the side-to-side shaking that is felt during an earthquake.
Rayleigh Waves
- Rayleigh waves are surface waves that are named after Lord Rayleigh, who mathematically predicted the existence of this type of wave in 1885. Rayleigh waves are slower than Love waves but can be much larger. Particles in a Rayleigh wave are displaced vertically, as well as horizontally, in a direction parallel to the motion of the wave. This type of wave is similar to a wave on the ocean, with an elliptical, rolling motion. Most of the shaking felt during an earthquake is the result of Rayleigh waves. The combined side-to-side, up-and-down and back-and-forth motion of surface waves are extremely destructive to surface structures.

Nature