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Seismic waves are an expression of the energy released after an earthquake in the form of body waves and surface waves. When seismic energy is released, the first waves to propagate out are body waves that pass through the body of the planet. Body waves include primary waves (P waves) and secondary waves (S waves). Primary waves are the fastest seismic waves. They move through rock via compression, very much like sound waves move through the air. Particles of rock move forward and back during the passage of the P waves. Primary waves can travel through both fluids and solids. Secondary waves travel slower and follow primary waves, propagating as shear waves. Particles of rock move from side to side during the passage of S waves. Because of this, secondary waves cannot travel through fluids, including liquids, plasma, or gas.
When an earthquake occurs at a location in the earth, the body waves radiate outward, passing through the earth and into the rock of the mantle. A point on this spreading wavefront travels along a specific path which reaches a seismograph located at one of the thousands of seismic stations scattered over the earth. That specific travel path is a line called a seismic ray. Since the density (and seismic velocity) of the mantle increases with depth, a process called refraction causes earthquake rays to curve away from the vertical and bend back toward the surface, passing through bodies of rock along the way.
Surface waves are produced when P and S body waves strike the surface of the earth and travel along the Earth’s surface, radiating outward from the epicenter. Surface waves travel more slowly than body waves. They have complex horizontal and vertical ground movement that creates a rolling motion. Because they propagate at the surface and have complex motions, surface waves are responsible for most of the damage. Two types of surface waves are Love waves and Rayleigh waves. Love waves produce horizontal ground shaking and, ironically from their name, are the most destructive. Rayleigh waves produce an elliptical motion of points on the surface, with longitudinal dilation and compression, like ocean waves. However, with Raleigh waves rock particles move in a direction opposite to that of water particles in ocean waves.
Earth is like a bell, and an earthquake is a way to ring it. Like other waves, seismic waves bend and bounce when passing from one material to another, like moving from a dense rock to rock with even higher density. When a wave bends as it moves into a different substance, it is known as refraction, and when waves bounce back, it is known as reflection. Because S waves cannot move through a liquid, they are blocked by the liquid outer core, creating a shadow zone on the opposite side of the planet to the earthquake source.
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