An earthquake can come at any time of year with no warning, and no one can say exactly when one will happen. An earthquake is the sudden, quick shaking of the ground. It happens when rock under the ground breaks and moves. To see why the rock breaks, we have to look at the big pieces that make up the surface of our planet.

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. They stick at their edges because of friction, so they can get stuck instead of sliding along smoothly. 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. The slip happens along a fault, which is a crack in the rocks of the Earth where one side can move past the other. In California, the Pacific Plate and the North American Plate slide past each other at about two inches a year.

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. The Ring of Fire is a huge ring of earthquakes and volcanoes around the Pacific Ocean.

Once an earthquake starts, the shaking spreads out from the middle in every direction, like ripples from a rock thrown into a pond. The spot on the ground straight above where an earthquake starts is called the epicenter.

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. Magnitude is one number for the size of the earthquake itself, and it does not change from place to place.

Can scientists say when the next big one will come? Not exactly. An earthquake can come at any time of year with no warning. They can tell how much strain a fault is building and where past quakes happened, and from that they can figure the chance of future ones.

People can get ready for something that cannot be put on a calendar. Engineers make buildings safer by designing new ones to handle shaking and by strengthening old ones. A phone alert from an early-warning system can come seconds before the shaking, in time to drop, cover, and hold on. Families and classes practice drop, cover, and hold on in earthquake drills.

When the ground shakes, the safe moves are drop, cover, and hold on. That keeps you from being knocked down and lowers the chance of being hit by falling objects. Standing in a doorway is not the safe move. Smaller earthquakes called aftershocks can come after a big one, so be ready to drop, cover, and hold on again.

A tsunami is a series of ocean waves, and a big earthquake under the sea can cause one. Out in deep ocean, tsunamis are not very tall, but they grow taller as they near land. So if you are near the ocean and feel strong shaking, go to higher ground once the shaking stops, because a tsunami may follow.

Words to know

• strain: stretch that builds
• fault: crack in rock
• magnitude: quake size
• epicenter: spot above start
• seismograph: shaking recorder