An earthquake is the sudden, quick shaking of the ground. It happens when rock under the ground breaks and moves. The shaking spreads out from the middle in every direction. It is like ripples from a rock thrown into a pond.

The outside layer of the Earth is broken into big pieces called plates. They push and slide against each other very slowly. Sometimes they get stuck.

A fault is a crack in the rocks. One side of it can move past the other.

Most earthquakes happen where the edges of Earth's plates meet.

A big earthquake under the sea can cause a tsunami. A tsunami is a series of ocean waves. Out in the deep ocean, the waves are not very tall. They grow taller as they get near land.

An earthquake can come at any time of year. It comes with no warning. No one can say exactly when one will happen.

Families and classes practice. They do drills of drop, cover, and hold on.

When the ground shakes, drop, cover, and hold on. If a strong table or desk is close, crawl under it to cover. Hold on until the shaking stops. Standing in a doorway is not the safe move.

If you are inside, stay inside. Do not run outside while the ground is shaking. Stay away from windows. Stay away from heavy things that could fall, like bookcases. If you are outside, move to an open place away from buildings, trees, and wires.

Smaller earthquakes called aftershocks can come after a big one. So be ready to drop, cover, and hold on again.

Words to know

• plates: big earth pieces
• aftershocks: later small quakes
• tsunami: giant ocean waves
• drills: safety practice