13.2: The Epicenter, Focus, and Waves
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An earthquake is like a telegram from the Earth. It sends a message about the conditions beneath the Earth’s surface. The shaking or trembling experienced during an earthquake is the result of a rapid release of energy within the Earth, usually as a result of movement along geologic faults. Think back to the strike-slip fault from the Crustal Deformation chapter. Rocks on either side of the fault are sliding past each other. As they move in opposite directions, the rocks become deformed, as they will bend slightly and build up pressure. Eventually, they will reach a breaking point. Once the strength of the rock has been exceeded, the rocks will snap back to their normal shape, releasing all that stored energy as an earthquake. The more energy that has been stored, the larger the earthquake is. Remember the stress-strain diagram from Crustal Deformation. When rocks are under too great of a stress, they undergo brittle failure (the earthquake). The strength of the rock has been exceeded at this point.
Earthquakes originate at a point called the focus (plural foci). From this point, energy travels outward in different types of waves. The place on the Earth’s surface directly above the focus is called the epicenter (Figure 13.2). Earthquake foci may be shallow (less than 70 km from Earth’s surface) to deep (greater than 300 km deep), though shallow to intermediate depths are much more common. Earthquake frequency and depth are related to plate boundaries. The vast majority (95%) of earthquakes occur along a plate boundary, with shallow focus earthquakes tending to occur at divergent and transform plate boundaries, and shallow to intermediate to deep-focus earthquakes occurring at convergent boundaries (along the subducting plate). The earthquakes associated with convergent boundaries occur along Wadati-Benioff zones, or simply Benioff zones, areas of dipping seismicity along the subducting plate (Figure 13.3).
As an earthquake occurs, two different types of waves are produced: body waves, so termed because they travel through the body of the Earth, and surface waves that travel along the Earth’s surface (Figure 13.4). There are two types of body waves. P-waves, or primary waves, are compressional waves that move back and forth, similar to the action of an accordion. As the wave passes, the atoms in the material it is traveling through are being compressed and stretched. Movement is compressional parallel to the direction of wave propagation, which makes P-waves the fastest of the seismic waves. These waves can travel through solids, liquids, and gases because all materials can be compressed to some degree. S-waves, or secondary waves, are shear waves that move material in a direction perpendicular to the direction of travel. S-waves can only travel through solids and are slower than P-waves. A similar motion to S-wave motion can be created by two people holding a rope, with one snapping the rope quickly. Alternately, you can also think of this wave movement similar to the wave created by fans in a stadium who stand up and sit down. Body waves are responsible for the jerking and shaking motions felt during an earthquake.
Surface waves are slower than body waves and tend to produce more rolling sensations to those experiencing an earthquake, similar to being in a boat on the sea. Because surface waves are located at the ground’s surface where humans (and their structures) are located, and because they move so slowly, which bunches them up and increases their amplitude, they are the most damaging of seismic waves. Love waves are the faster surface waves, and they move material back and forth in a horizontal plane that is perpendicular to the direction of wave travel (see Figure 13.4). Buildings do not handle this type of movement well, and Love waves may be responsible for considerable damage to structures. Rayleigh waves make the Earth’s surface move in an elliptical motion, similar to the movement in a sea wave. This results in ground movement that is up and down and side-to-side.