JOAN CARTAN-HANSEN, HOST: The earth averages about 55 earthquakes every day.

But what is an earthquake?

Find out.

[MUSIC] An earthquake happens when the ground shakes.

And they usually start deep underground.

The earth is made up of three main layers.

The center, or core, which is very hot.

Around the core lies the mantle, a thick layer of mostly rock and some liquid.

And floating on top of the mantle is the earth's thin outer layer, called the crust.

The crust is made up of separate slabs called tectonic plates.

The solid rock of the crust is under constant pressure from the movement of the plates.

As a result, giant cracks or fault lines are created.

When plates try to slide past each other along a fault line, sometimes they get stuck, the pressure builds up until the plates suddenly breaks apart causing a powerful ground movement we call an earthquake.

STUDENT: You can see why an earthquake can cause a lot of damage.

Put two wood blocks together and cover them with a layer of sand.

Put a few buildings and animals on top, and then move one of the blocks.

And that's what an earthquake looks like.

CARTAN-HANSEN: The spot on the earth's surface reached by the first wave is called the epicenter.

From the epicenter, seismic waves or shockwaves spread outward.

Scientists measure earthquakes with a special tool called a seismograph.

The seismograph's pin records shockwaves by drawing lines on a piece of paper.

When a wave is closer or stronger, the line is longer.

When the wave is farther away, or weaker, the line is shorter.

Another thing scientists look for is the intensity and magnitude of an earthquake.

The intensity is the damage that people can see and feel.

The magnitude is how strong the surface of the earth shakes.

STUDENT: Scientists use a special scale called a Richter scale to measure the intensity and the magnitude of an earthquake.

The higher the number the more severe the earthquake.

Say this marble represents a magnitude one earthquake and a golf ball would be a three, a baseball would be a four and a soccer ball would be a five.

The largest earthquake in the world so far happened in Chile.

That quake measured 9.5 on the Richter scale, so if it were a ball, it would be about this size.

CARTAN-HANSEN: One of the most famous earthquakes in U.S. history was the 1906 earthquake in San Francisco.

That earthquake leveled 500 city blocks, killed 3,000 people, and left 225,000 people homeless.

One of the largest earthquakes in Idaho history was the 1983 quake near the town of Challis.

Two children died when a storefront collapsed on top of them.

And in that quake, Mount Borah, Idaho's highest peak rose about a foot and then fell back into place.

Earthquakes just don't happen on land.

They can happen in the ocean, too.

And can be just as destructive.

The shifting of the ocean floor can cause a giant wave called a tsunami.

Because a tsunami contains so much energy, it can race across the ocean for thousands of miles at speeds of more than 500 miles per hour, and when it reaches land, the height of the wave can be over 100 feet tall.

Scientists have no way of predicting earthquakes.

So it's a good idea to have a disaster kit at home.

You should have a fire extinguisher, a first aid kit and enough water so everyone in your family can have enough to drink for three days.

You should have an emergency plan, so everyone knows where to meet after an earthquake.

And make sure you or your parents have a person out of town you can contact, so they can let everybody else know you are ok. During an earthquake, if you are inside, get under a strong table and stay away from windows.

And if you are outside, stay away from buildings, trees, and power lines.

Scientists are studying earthquakes in hopes of someday learning how to predict when they will happen.

That could save lots of lives.

But for now, we don't know when the next earthquake will strike.

The best thing you can do is find out what an earthquake is and then be prepared.

If you want to learn more about earthquakes, check out the science trek website.

You'll find it at science trek dot org.

[MUSIC] ANNOUNCER: Presentation of Science Trek on Idaho Public Television is made possible through the generous support of the Laura Moore Cunningham Foundation, committed to fulfilling the Moore and Bettis family legacy of building the great state of Idaho.

By the Idaho National Laboratory, mentoring talent and finding solutions for energy and security challenges, by the Friends of Idaho Public Television and the Corporation for Public Broadcasting.