An earthquake can come at any time of year with no warning. No one can say exactly when one will happen. That is why being ready matters so much. An earthquake is the sudden, quick shaking of the ground, and it happens when rock under the ground breaks and moves.

Why does rock break and move? The outside layer of the Earth is broken into big pieces called plates. The plates of Earth's crust push and slide against each other very slowly, and they can get stuck. They stick at their edges because of friction. When the stress gets bigger than the friction, the rock slips and sends out energy as waves that shake the ground.

Strain builds up in rock like a rubber band pulled too tight, until the rock suddenly breaks and slips. A fault is a crack in the rocks of the Earth where one side can move past the other. When rock slips along a fault, the ground shakes.

Where do earthquakes happen? Most earthquakes happen where the edges of Earth's plates meet. Earthquakes and volcanoes mostly happen along plate boundaries, and plate movement also builds mountains. So the edges of the plates are the places to watch. Think of the plates as a big puzzle of rock, and the edges as the places where the pieces push on each other.

How do people find out about an earthquake? A machine called a seismograph records the shaking of the ground. Seismographs and the stations around them show where and how big an earthquake was. The spot on the ground straight above where an earthquake starts is called the epicenter. The shaking spreads out from the epicenter in every direction, like ripples from a rock thrown into a pond.

When the ground shakes, the safe moves are drop, cover, and hold on. Drop to the floor. 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 and lowers the chance of being hit by falling objects. You may have heard that a doorway is the safest spot. It is not. Drop, cover, and hold on instead.

Inside, stay inside and do not run outside while the ground is shaking. Stay away from windows and from heavy furniture that could fall, like bookcases. The kitchen is a poor place to be because things can fall on you. Outside, move to an open place away from buildings, trees, and wires.

After a big earthquake, smaller ones called aftershocks can come. Aftershocks are smaller earthquakes that come after a big one, so be ready to drop, cover, and hold on again. Knowing this ahead of time helps you stay safe when the ground shakes again. Hold on until the shaking stops, each time.

A big earthquake under the sea can cause a tsunami, a series of ocean waves. In deep ocean they are not very tall, and they grow taller as they near land. If you are near the ocean and feel strong shaking, go to higher ground once the shaking stops, because a tsunami may follow. Even then, when the ground shakes, the safe moves are drop, cover, and hold on.

People can get ready before an earthquake comes. Some places can warn people. A phone alert from an early-warning system can come seconds before the shaking, in time to drop, cover, and hold on. Engineers make buildings safer by designing new ones to handle shaking and by strengthening old ones. Families can get ready too, with an emergency kit and a plan for where to meet if they get separated.

Words to know

• plates: big crust pieces
• friction: rubbing that sticks
• seismograph: shaking recorder
• aftershocks: later small quakes
• engineers: building designers