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Keep reading to find out:
How are earthquakes formed?
What happens when an earthquake starts?
What happens to the waves earthquakes form?
How do you measure seismic waves?
How do you locate an earthquake?
How is its size measured?
How frequently do earthquakes occur?
Can you predict an earthquake?

How are earthquakes formed?
When Earth was 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 up the rocky material deep inside the planet. Since heat attempts to rise, 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, where temperatures are very high, the heated rocky material is flexible, so it moves slowly and steadily. Nearer the earth's surface, however, the rocky material becomes cooler and more brittle, and it cannot move so easily. The slow, continual movement of material deep in the interior builds up and 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 like a stick that breaks suddenly when it is bent too far.

You don't 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 on a much larger scale. Often a break occurs in a place that has broken before, on what scientists call a fault 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 will move by sliding 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, as 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 Earth's long history, and it continues to do so today.

Go inside the earth to learn the anatomy of an earthquake with this interactive map and diagram.

There are thousands of earthquakes every year — most of which are so minor we don't even feel them. In fact, there are approximately 8,000 earthquakes EVERY DAY, but they hardly register in intensity.

What happens when an earthquake starts?
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 and 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 short splash and 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 — called surface waves — that spread outward. These waves gradually get weaker the farther they travel, until they eventually die out. These waves travel at speeds ranging from about 1 to 3 miles 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, which 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 a type of shaking called 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 wave. Shear waves travel slower than sound but a little faster than surface waves. Because sound waves and shear waves are usually the first two types of seismic waves to arrive at any point after an earthquake, they are called P-waves and S-waves, from the Latin for "first" (primus) and "second" (secundus).

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, however, the speed can be almost three times faster. P-waves also 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. Within the fluid core they travel much more slowly. P-waves reach everywhere around the Earth after about 20 minutes.

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 is actually moving sideways to the direction that the wave is traveling. S-waves cannot travel in liquid; they can travel only in solid material. Therefore, when S-waves reach the fluid part of the earth's outer core they stop or are reflected back, or they convert to P-waves in the core. The shaking motions of S-waves are usually much larger than the motions of P-waves.

In 1663 the European settlers experienced their first earthquake in America.

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