If you pick up an ordinary stick and play with it, you’ll find that you can bend it only so far before it suddenly breaks. Most substances, including rocks, will also undergo some degree of plastic deformation, bending, before they display brittle deformation: rupture, or breaking. Deep in the Earth’s crust, especially near the edges of the continents, rocks are constantly being subjected to forces that cause them to bend or stretch. When a bend gets too sharp or a rock stretches too thin, CRACK! the rock breaks and an earthquake occurs.
When you break the stick you hear a sound, which is nothing more than energy that travels through the air to your ears as sound waves. When a rock breaks, its sudden rupture also creates energy: a lot of energy. Since the rock is buried, that energy travels throughout the planet in the form of seismic waves. Strong waves created by large breaks cause the ground above and near the point of breakage to shake and tremble in an earthquake. The “center” of the earthquake, the point immediately above the break on the surface, is called the epicenter.
Seismic waves are broadly separated into two groups, body waves and surface waves. Humans are most concerned with the surface waves, which radiate out from the epicenter much like the circular ripples that spread outward when an object is dropped into still water. Very near the epicenter, surface waves can cause the ground to ripple and shake, toppling buildings and shattering roads. That’s because the ground ripples like waves in water, creating circles of alternating high and low areas at the surface; circles that move away from the center of the disturbance. Just like ripples in water, seismic surface waves become smaller as the distance from the epicenter increases. That explains why a large earthquake in California might be felt in Nevada, but the same quake can only be sensed by delicate instruments in Iowa.
As dangerous and fascinating as earthquakes may be, the scientists who study earthquakes and the interior structure of the earth,are more interested in the other type of seismic waves; the body waves. Seismologists, as they are known, carefully study the body waves that spread throughout the earth when a quake occurs. The energy contained in body waves propagates, or spreads, in several forms, but the two most important types are called P waves and S waves. They get their names from P for “primary,” because this wave is so fast that it arrives at recording stations first; and S for “secondary,” because it arrives second.
The two body wave types are quite different, but both can be described with the help of a single child’s toy; the Slinky. Hold the ends of a Slinky at arms length apart, then have a friend pull the center down a few inches and release it. The wire coils will move up and down, at right angles to the length of the coil. This motion is characteristic of a “shear” wave, and it is the kind of motion particles follow when disturbed by an S wave. Now, take the Slinky, bunch a few coils together at one end and release them. If you watch carefully, you will see the coils bunch together all along the Slinky to the other hand (and perhaps back). This is the motion of a “compressional” wave, and it is the motion particles follow when a P wave passes. The sound waves that you heard when you broke your stick were compressional waves, too.
Compressional waves (P waves) travel through the Earth at 4 to 7 kilometers per second, while shear waves (S waves) move only about 2 to 5 kilometers per second. Surface waves are slower still, but they cause most of the property damage in an earthquake.
Seismologists use a worldwide network of devices called seismographs to record the passage of the P and S waves generated by an earthquake. By carefully comparing the difference in arrival of the body waves, a seismologist can determine how far away the quake occurred. From three or more such measurements, a team of seismologists can pinpoint the location with considerable accuracy.
The difference between S waves and P waves extends to more than velocity. First, because S waves cause the particles they encounter to move perpendicular to the wave’s direction of travel, S waves cannot travel through liquids. This fact helped early seismologists discover that there is a liquid layer within the Earth’s interior, because it cast a “shadow” where no S waves arrived, even though P waves came straight through. Careful measurements allowed the scientists to estimate the depth to the top of this liquid layer.
Another property of waves is that they are affected by refraction. When a body wave crosses the boundary between layers of different densities, the wave bends slightly. By combining thousands of measurements from around the world after each earthquake, seismologists have been able to generate an accurate model of the Earth’s layers, from the center to the surface. This layering includes a solid inner core, a liquid outer core, and layers of mantle and crust. All this information has come from the study of seismic waves!