Let's find out why the ground shakes. An earthquake is the sudden, quick shaking of the ground. It happens when rock under the ground breaks and moves. The outside layer of the Earth is broken into big pieces called plates. What makes the rock break? The answer starts with those plates, and with how they move.

The plates push and slide against each other very slowly. They can get stuck. They stick at their edges because of friction. Strain builds up in the rock like a rubber band pulled too tight. Then the stress gets bigger than the friction. The rock slips and sends out energy as waves. The waves shake the ground.

Let's look at the place where the rock breaks. A fault is a crack in the rocks of the Earth. One side of the crack can move past the other. The spot on the ground straight above where an earthquake starts is called the epicenter. That is the spot that sits right over the start of the quake.

Now let's see where earthquakes happen. Most earthquakes happen where the edges of Earth's plates meet, and earthquakes and volcanoes mostly happen along those edges. Plate movement also builds mountains. In California, the Pacific Plate and the North American Plate slide past each other at about two inches a year.

A tsunami is a series of ocean waves. A big earthquake under the sea can cause one. So if you are near the ocean and feel strong shaking, go to higher ground once the shaking stops. Do that because a tsunami may follow the shaking. Higher ground is the place to be when a tsunami may come.

When the ground shakes, the safe moves are drop, cover, and hold on. If a strong table or desk is close, crawl under it to cover. Hold on until the shaking stops. This keeps you from being knocked down. It lowers the chance of being hit by falling objects. Standing in a doorway is not the safe move.

Inside, stay inside, and do not run outside while the ground is shaking. Outside, stay outside and move to an open place away from buildings, trees, and wires. Stay away from windows, and stay away from heavy furniture that could fall too, like bookcases. Stay put until the ground stops shaking.

An earthquake can come at any time of year with no warning, and no one can say exactly when one will happen. So families get ready ahead of time with an emergency kit, and they make a plan for where to meet if they get separated. Being ready starts before anything shakes.

Words to know

• epicenter: spot above start
• friction: rubbing that sticks
• plates: big crust pieces
• tsunami: giant ocean wave

The short version

An earthquake is the sudden, quick shaking of the ground. It happens when rock under the ground breaks and moves. Earth's outside layer is broken into plates that push and slide very slowly, and strain builds where they get stuck until the rock slips.

Quick facts

• Most earthquakes happen where the edges of Earth's plates meet.
• A fault is a crack in the rocks where one side can move past the other.
• The spot on the ground straight above where an earthquake starts is called the epicenter.
• When the ground shakes, the safe moves are drop, cover, and hold on.