What's an Earthquake?
Earthquake in Peru
Earthquakes happen when the moving tectonic plates that make up the surface of the Earth move apart or bump into each other, or slide under each other. This movement tears apart the surface of the Earth, or crunches it up. Most often, this just means a little shaking for a few seconds, and nothing very serious happens.
Several times a year, though, somewhere in the world there is enough movement to really shake the earth a lot, and the earthquake is serious enough to knock down buildings. When the buildings fall on people, many people can be killed in a few minutes. The strongest earthquakes can break trees in half.
The Richter scale (or ML scale) rates earthquakes on an exponential scale, so that if an earthquake is rated 1, you can hardly feel it, but an earthquake rated 2 is ten times as strong as an earthquake rated 1, and an earthquake rated 3 is ten times as strong as an earthquake rated 2. Only a few people feel a level 1 earthquake. In a level 2 earthquake, a few people who are resting may feel it, especially if they're near the top of a tall building. Nearly everyone will feel a level 5 earthquake, and some dishes and windows will break. At level 6, heavy furniture moves around, and many people will feel frightened, but there's not really much damage. In a level 8 earthquake, many buildings will fall down.
A 6.8 earthquake in 2001 in Geiyo, Japan
Because most of the Earth is covered by oceans, earthquakes often happen in the ocean. Usually an earthquake in the ocean just shakes the water and people don't notice. But sometimes the water pulls all together into a huge wave called a tsunami (tsoo-NAMM-ee).
Bibliography and further reading about earthquakes: