Elastic Rebound Theory
An earthquake is just the shaking of the ground, and many things can cause earthquakes. Much effort has been devoted to detecting underground nuclear tests by identifying the earthquake waves produced. Mining cave-ins, conventional explosions at quarries and mines or other places, and other events can cause earthquakes. The deepest earthquakes, which are very rare but often among the biggest ones, may have a phase-change or "implosion" origin, which we'll discuss later.
However, most earthquakes are produced by elastic rebounds. We’ve already seen that rocks are moving around on the planet and that the pull-apart action has allowed Death Valley to drop down. We will see that other motions occur as well, with one group of rocks moving past another. Where rocks are warm and soft, they flow. When cold and hard, they cannot flow.
Consider, for example, two large pieces of rock, such as southwestern California and the rest of the state. The southwestern part of the state, from Los Angeles to San Francisco, and the adjacent ocean floor are moving northwest relative to the rest of the state. The break separating the different parts is called the San Andreas Fault. (Both sides are moving westward, but the southwest side has an additional bit of northwesterly movement relative to the northeast side. Faults may go east-west, north-south, or any other direction, may be vertical or angled, and the rocks may move vertically or horizontally or in-between across the fault.) The forces that move the rocks are huge and applied over large areas so that far from the fault the motion is smooth. But at the fault, rough patches can get stuck against each other and become locked for a while. The rocks then bend. This bending is elastic—it can spring back. Eventually, the stress on the rough spots becomes too great, the fault “let's go”, and the bent rocks “spring back”. The springing back is very rapid, in the same way as for a spring or a rubber band. Displacements of several feet (more than a meter) or more are possible in much less than a second. A building sitting on the rocks near the fault can be subjected to very large accelerations and may fall apart.
Such an earthquake will shake rocks beyond a fault. This is achieved through seismic waves—one piece of rock pushes another next to it, which pushes one next to it, and so on. Two major types of seismic waves are P or push, and S or shear. The P-wave is an ordinary sound wave. It represents a push-pull in the direction the wave is moving. A P-wave moving to the north will shake a mineral grain north-south-north-south. An S-wave moves slower than the corresponding P-wave. When an S-wave moves to the north, the mineral grains are shaken east-west-east-west or up-down-up-down (or some combination). A shear wave is similar to the wave you generate by shaking a rope. A piece of the rope moves up and down or back and forth, but the wave moves along the rope. The “wave” at football games is the same way—you stand up and sit down, but the wave moves along the bleacher bench. A P-wave may start a building shaking in one way, and then the S-wave hits the building and starts shaking it a different way, making the building more likely to break and fall down. (Earthquakes also make surface waves, which move more slowly than shear waves and go along the surface of the Earth like wind-driven waves on the ocean, rather than going through the Earth the way P- and S-waves do. The surface waves can also contribute to breaking buildings.)
S-waves don’t travel through liquids at all. (Wiggle one piece of liquid to the side, and the moving piece slides freely past the next piece rather than wiggling it.) Recall that earlier we claimed that the outer core of the Earth is liquid. You may have asked, “How does anyone know that?” The answer is that, after a really big earthquake, P-waves can be detected all over the Earth. But S-waves are missing across the Earth from the quake, in places reachable only by passing through the core, as shown by the figure. So, we know that the outer core is liquid because it transmits P-waves but not S-waves. And, the outer core is nearly spherical because no matter where an earthquake occurs on the planet, there is a zone on the other side of the Earth in which S-waves are absent. (Learning that the inner core is solid is tougher; one of the pieces of evidence is based on wave conversions. The P-wave that passes through the outer core loses some energy in making an S-wave when the P-wave hits the inner core; this S-wave passes through the solid inner core, makes a new P-wave when it hits the liquid outer core again, and that P-wave travels on to the surface. The delay associated with the slower motion of the S-waves allows this to be figured out. But don’t worry about wave conversions, or the other evidence for a solid inner core, in an introductory course such as this one.)