![]() Surface waves are produced when P and S body waves strike the earth’s surface and travel along the Earth’s surface, radiating outward from the epicenter. “ Seismic Waves” by the United States Geologic Survey is licensed under Public Domain. 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. That specific travel path is a line called a seismic ray. A point on this spreading wave-front travels along a specific path that reaches a seismograph located at one of the thousands of seismic stations scattered over the earth. 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. ![]() Because of this, secondary waves cannot travel through liquids, plasma, or gas. Particles of rock move from side to side during the passage of S waves. Secondary waves travel slower and follow primary waves, propagating as shear waves. Primary waves can travel through both fluids and solids. Particles of rock move forward and back during the passage of the P waves. They move through the rock via compression, very much like sound waves move through the air. Primary waves are the fastest seismic waves. Body waves include primary waves (P waves) and secondary waves (S waves). When seismic energy is released, the first waves to propagate out are body waves that pass through the planet’s body. Seismic waves are an expression of the energy released after an earthquake in the form of body waves and surface waves. (9 Crustal Deformation and Earthquakes – An Introduction to Geology, n.d.) “ Focus and Epicenter” by the Utah Geologic Survey is licensed under Public Domain. The focus is the point along the fault plane from which the seismic waves spread outward. The epicenter is also the location that most news reports give because it is the center of the area where people are affected. The epicenter is the location on the Earth’s surface vertically above the point of rupture (focus). ![]() More shaking is usually the result of more seismic energy released. The larger the displacement and the further it propagates, the more significant the seismic waves and ground shaking. The displacement produces shock waves, creates seismic waves. From the focus, the displacement propagates up, down, and laterally along the fault plane. The focus is always at some depth below the ground surface in the crust, and not at the surface. The focus, also called a hypocenter of an earthquake, is the point of initial breaking or rupturing where the displacement of rocks occurs.
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