When people think of earthquakes, they often picture the ground cracking open, roads and bridges buckling and breaking apart, buildings collapsing, and people being injured or killed. Such disasters frequently result from earthquakes But many earthquakes are so small that people can barely feel them. What causes earthquakes? Why are some earthquakes worse than others? These and many other questions have been answered by seismologists, the scientists who study shaking motions of the Earth.

Earthquakes are natural events on planet Earth. They occur as part of the geological processes that form the Earth's mountains, oceans, valleys, and plains. During the last hundred years or so, scientists have learned a great deal about these processes. They have learned about the causes of earthquakes, how to measure them, and where they occur. In learning about earthquakes, scientists have learned much about the Earth itself.

Causes of Earthquakes

An earthquake occurs because of geologic forces inside the Earth. These forces build up slowly. Eventually they become so strong that they cause rocks to break underground. When this happens, tremendous energy is released suddenly. The energy takes the form of motion that spreads out in all directions from the break, causing the ground to shake and move. This sudden release of energy and movement is what makes an earthquake so destructive.

There are two parts to the story of what causes earthquakes. The first concerns how forces in the Earth build up. The second explains why the ground breaks with a sudden motion rather than a slow shift.

When the Earth first formed, radioactive materials, such as uranium, potassium, and thorium, began to decay naturally in the planet's interior. This process of radioactive decay slowly heated the rocky material deep inside the planet. Heat attempts to rise. Thus this heated material has been moving slowly toward the Earth's surface. The movement is very slow—less than an inch or so each year. But over hundreds of millions of years, this slow movement adds up to distances of thousands of miles.

Deep inside the Earth temperatures are very high. The heated rocky material is flexible. It moves slowly and steadily. Nearer the Earth's surface, however, the rocky material becomes cooler and more brittle. It cannot move so easily. The slow, continual movement of material deep in the interior builds up. It exerts stronger and stronger forces on the brittle rocks near the surface, making them move as well. For an earthquake to occur, the rocks nearer the surface have to break suddenly—just as a dead stick breaks suddenly when it is bent too far.

People do not usually think of the ground as something that can break. But if you take a small piece of rock and squeeze it strongly enough in a metal vise, it will crack or crumble. An earthquake is like that cracking rock. But it takes place on a much larger scale. Often a break occurs in a place that has broken before, a fault. A fault is a break in the Earth's surface between two blocks of rock that have moved past each other.

When rock breaks along a fault, the pieces of rock slide past each other if the force is great enough. The motion may occur in a series of jerks, each one of which is an earthquake. In a large earthquake, the rock along the fault may move several feet in only a few seconds.

Over tens of millions of years, such geologic forces have caused massive changes in the Earth's landscape. Millions of earthquakes have moved large chunks of the planet's surface hundreds of miles. This process has changed the shape and location of the continents and oceans over the Earth's long history. It continues to do so today.

Seismic Waves

The sudden release of energy from an earthquake sends out several different shaking movements, or seismic waves. (The word "seismic" comes from a Greek word for "shaking.") Some of these seismic waves travel over the surface of the Earth. They are called surface waves. Others, called body waves, travel down through the Earth's deep interior before returning to the surface.

Surface Waves

If you throw a stone into a calm pond, it creates a brief splash. Then ripples spread outward for a much longer time. Something similar happens in the Earth when there is an earthquake. When rocks slip past each other along a fault, the initial movement is over very quickly. However, the energy that is released causes ripples that spread outward. These ripples are the surface waves. They gradually get weaker the farther they travel, until they eventually die out.

Seismic surface waves travel at speeds ranging from about 1 to 3 miles (1.6 to 4.8 kilometers) per second. The surface waves from small earthquakes usually go unnoticed except by sensitive instruments. But people who experience a large earthquake often describe a swaying or a rolling motion. This is the characteristic motion of seismic surface waves.

Body Waves

A seismic wave that travels through a material rather than over its surface is called a body wave. There are two basic types of seismic body waves: sound waves and shear waves. Everyone knows that sound travels through the air, but did you know that sound can also travel through water and even through solid rock? Sound waves are the fastest type of seismic wave. Shear waves travel slower than sound but a little faster than surface waves. Sound waves and shear waves are usually the first two types of seismic waves to arrive at any point after an earthquake. Thus they are called P-waves and S-waves, from the Latin for "first" (primus) and "second" (secundus).

P-Waves. The faster of the two types of body waves is the P-wave, or sound wave. When an earthquake occurs, sound waves travel through the interior of the Earth rather than over the surface. A sound wave is created in air, water, or rock as particles of material press rapidly together and then pull apart. The sound wave travels through the material as a series of squeezes and stretches. At the Earth's surface, sound waves travel through rock at about 3 miles (4.8 kilometers) per second. Deep inside the Earth, the speed can be almost three times faster. P-waves travel at different speeds depending on the type of material they pass through. Within the Earth they travel fastest in the region at the bottom of the Earth's mantle. In the fluid core they travel much slower.P-waves reach everywhere around the Earth within about 20 minutes.

S-Waves. The slower type of body wave, the S-wave, or shear wave, can be described in this way. Imagine that you have tied one end of a rope to a post and are holding the other end. When you shake your end of the rope from side to side, a shaking motion, like a wriggling snake, travels along the rope. S-waves are a similar type of shaking motion. With S-waves, material in the Earth moves sideways to the direction that the wave is traveling. S-waves cannot travel in liquid. They travel only in solid material. Therefore, when S-waves reach the fluid part of the Earth's outer core, they cannot continue. They stop, are reflected back, or convert to P-waves in the core. The shaking motions of S-waves are usually much larger than the motions of P-waves.

Probing the Earth's Interior. Scientists have known about the major layers of the Earth—the crust, mantle, and core —since the early 1900's. They have also discovered that the Earth must have both a solid inner core and a fluid outer core composed mainly of iron. This view of the Earth's interior is based on millions of measurements of the time it takes body waves to travel through the Earth. It is also based on knowledge of how substances of different densities, such as rock and molten iron, affect the speed and direction of body waves.

Measuring Seismic Motion

The instruments that measure seismic waves are called seismometers. The records they make are called seismograms. By studying seismograms, seismologists can determine where an earthquake occurred. By studying seismograms over many years, they have also determined in what regions of the world earthquakes are likely to occur. Over the Earth's long history, forces deep within the planet have moved huge sections of the Earth's crust from one place to another. These sections are called tectonic plates. The process by which they are moved is called plate tectonics.

Earthquakes, seismologists have discovered, generally happen along the boundaries of the tectonic plates. They may occur where plates are moving apart or moving past one another horizontally. Or they may occur where plates are moving toward one another in a process that forces one plate deep into the Earth.

There are many different types of seismometers. The principle behind all of them is very simple. They all measure the distance that the ground moves as a result of some seismic motion. The motion can be measured in two basic ways. One way is with a weight suspended from a spring. To measure vertical motion, seismologists note changes in the distance between the bottom of the weight and the base of the frame before the ground shakes and while the ground is shaking.

A second way to measure seismic motion is with a weight on a pendulum that swings sideways. Using this method, horizontal motion can be measured by noting changes in the distance between the side of the weight and the sides of the frame before and while the ground is shaking. In practice, seismologists often use a weight on a spring to measure vertical motions plus an east-west swinging pendulum and a north- south swinging pendulum to measure horizontal motions. They need all three systems because the ground can move in any direction during an earthquake.

Seismometers must be very sensitive because the seismic motions from distant earthquakes are often very small. A medium-sized earthquake in Alaska, for example, will produce a ground motion in New York of less than one millionth of an inch when the P-waves arrive. If sensitive seismometers are placed in a large city or near an ocean beach, traffic or ocean waves produce vibrations that will interfere with the detection of distant earthquakes. In areas that are seismically active, strong-motion seismometers are often used. These instruments can measure very intense seismic motions that would cause sensitive seismometers to go off the scale.

Locating an Earthquake

Thousands of seismometers are now operating at stations all around the world. Each is quietly waiting and "listening" for the seismic waves of a distant or a nearby earthquake. When an earthquake occurs somewhere on Earth, many of these seismometers will record the motions of the sound waves, shear waves, and surface waves that are produced. By measuring the size of these waves and the times at which they reach each seismic station, seismologists can determine where the earthquake occurred and how large it was.

Seismologists can locate the source of the waves—the place where the earthquake occurred—in several ways. One way is to interpret all the seismic motions recorded at each station and to backtrack along the path the motions traveled to find the common point from which all the seismic waves originated.

Another way to locate an earthquake is to study more than one type of seismic wave, taking into account the fact that different waves travel at different speeds. Both P-waves and S-waves, for example, start out at the same time like runners in a race. P-waves travel faster, however. The farther they travel, the greater the time difference between when they arrive at a seismometer and when the S-waves arrive. By measuring this time difference, seismologists can tell how far away the earthquake is from each station. They can then locate the source of the earthquake by studying the different times at which the waves reached different seismic stations.

The place on the Earth's surface where an earthquake occurs is called the epicenter. This is usually the place where the shaking is strongest. The place where the seismic waves actually originate is called the hypocenter. It lies below the epicenter. The hypocenter is the place where rock actually breaks along a buried fault. It is quite rare for an earthquake to occur on a fault that breaks through to the surface of the Earth. However, such surface faulting does sometimes occur—for example, in parts of California. Most earthquakes occur in the Earth's crust only a few miles below the surface. But about 10 percent of all earthquakes are deep, occurring more than 60 miles (about 100 kilometers) below the surface. Some hypocenters are as deep as 450 miles (about 700 kilometers).

The Size and Frequency of Earthquakes

Seismologists use three different ways to describe earthquake shaking and earthquake sizes. Intensity refers to the strength of shaking motions and the damage they can do. The intensity of shaking varies from place to place for the same earthquake. Seismologists use an intensity scale to describe the strength of seismic motions at a particular location. To describe how big the earthquake itself is, seismologists refer to the magnitude. They use a magnitude scale, which compares the sizes of different earthquakes. Or they use the seismic moment of the earthquake. Seismic moment describes an important combination of physical conditions at the earthquake source.

Earthquake Intensity

More than a hundred years ago, when people began studying earthquakes scientifically, they needed a practical way of describing the strength of an earthquake's shaking motions. They began by describing the pattern of damage to buildings and making maps to show different levels of damage at different places.

Seismologists call this approach the study of earthquake intensities. Today they usually work with twelve different damage zones, using what is called the modified Mercalli Intensity Scale. Each zone is assigned a Roman numeral. Each of the descriptions corresponds to a particular intensity of shaking.

After an earthquake, seismologists make intensity maps based on people's experiences and the types of damage that have occurred. By referring to old newspaper accounts and personal diaries, it is even possible to make intensity maps for past earthquakes.

Intensity and damage from an earthquake can be abnormally high in certain places because of the type of soil or surface. Areas with soft sedimentary layers of material are more susceptible to severe damage from shaking than surrounding areas of harder rock. Extensive damage is likely to occur in landfill areas. In these areas sandy material has been dumped into a lake or a bay to create a surface upon which buildings are constructed. Buildings in these areas should be specially strengthened.

The intensity scale describes the strength of seismic motions in different places. It does not tell whether the earthquake that caused the motions was large or small. Shaking at intensity III, for example, could occur near the epicenter of a small earthquake or at a great distance from a large earthquake. Earthquake Magnitude

In the 1930's, the American seismologist Charles Richter (1900-85) studied thousands of seismograms of earthquakes that had occurred in southern California. He realized that it would be useful to have a numerical scale for comparing the size of earthquakes that went beyond describing them as just large or small. Richter knew that he would have to take two things into account in devising such a scale. One was the distance from the epicenter. The other was the great difference in size of ground motion between small and large earthquakes.

The result of Richter's work was a method of assigning magnitude that we know today as the Richter scale. It is based upon a measurement of the size of the largest wave recorded on a certain type of seismometer that was commonly used in Richter's day. He took account of the distance from the epicenter by finding how to make a correction to the actual measurement. In this way he knew how big the seismic waves would be at a distance of exactly 100 kilometers (about 60 miles) from the epicenter. He also found a way to take account of the great difference in size of ground motion for different earthquakes. On his scale an increase of one unit (for example, from magnitude 7 to magnitude 8) means an increase in earthquake shaking by a factor of ten. Thus a magnitude of 8 is 100 times greater shaking than a magnitude of 6 and a million times greater than a magnitude of 2. He took 0 (zero) on his scale as the smallest earthquake he cared to work with at that time—100 million times smaller than an earthquake of magnitude 8. Very small earthquakes are rated up to about 2.5 on the Richter scale. Moderate earthquakes rated up to magnitude 5 can cause minor damage. Earthquakes of magnitude 6 and higher are major earthquakes. They can cause widespread damage and loss of life.

Today there are many different magnitude scales in addition to the Richter scale. All are based on ways of measuring the sizes of different seismic waves on different seismometers. The largest earthquakes on these scales range up to about magnitude 8 or 9. Using sensitive instruments, seismologists have detected earthquakes as small as magnitude –3 or –4. An earthquake with magnitude of –3 is 1,000 times smaller than magnitude 0, the smallest number on the original Richter scale.

Seismic Moment

Seismologists often prefer to describe the size of an earthquake in terms of the physical conditions at the earthquake source itself rather than in terms of the shaking it produces. To do this, they use seismic moment rather than magnitude. The seismic moment of an earthquake is determined from three factors. The first factor is the distance that rock slides along a fault surface after it breaks. This distance is called the fault slip. The second factor is the area of the fault surface that is actually broken by the earthquake. And the third factor is the measurement of how rigid the rocks are near the broken fault. All solid materials have a rigidity that can be measured. A strong rock such as granite is usually more rigid than a softer rock such as sandstone, which can be broken easily.

We determine the seismic moment of an earthquake by multiplying together the fault slip, the fault area, and the rigidity. The seismic moment describes the essential combination of physical quantities that really matters at the earthquake source and that determines how strong the seismic motions will be.

In the greatest earthquakes, the fault slip can be many feet and the fault area can be thousands of square miles. In the smallest measurable earthquake, on the other hand, the fault slip might be as small as a fraction of an inch and the fault area may be only a few square feet. If the rigidities are about the same, the largest seismic moment is a trillion times larger than the smallest.

Earthquake Frequency

Different-sized earthquakes occur with greater or lesser frequency depending on the magnitude. Seismologists have discovered that for each increase of one magnitude, there are generally about ten times fewer earthquakes. Worldwide, there are approximately 10,000 earthquakes of magnitude 4 or greater each year. There are only about ten earthquakes of magnitude 7 or greater each year.

Earthquake Prediction

Seismologists know that certain regions are more earthquake-prone than others. But they do not know for certain just when an earthquake will occur. Today much of the effort of earthquake prediction goes into studying the geology of the Earth and examining historical records of particular regions to determine exactly where and how often earthquakes occurred in the past. This information can then be used to make rough estimates of what to expect in the future in that same region. Such studies lead seismologists to think that there is between a 10 and 50 percent probability that a major earthquake will strike California within a person's lifetime.

Most earthquakes do little harm. But a few can cause great destruction and loss of life. Engineers study each major earthquake to learn how to build safer buildings, dams, and bridges so that destruction and loss of life can be reduced. Seismologists and other scientists continue their studies to learn more about what happens at the earthquake source and to discover more about the interior of the Earth.

Richards, Paul G., and Won-Young Kim. "Earthquakes." The New Book of Knowledge®. Grolier Online http://nbk-ada.grolier.com/cgi-bin/article?assetid=a2008600-h (accessed February 28, 2010).