An earthquake is a tremor of the earth's surface. On the earth lie huge earth plates, on which continents or parts of continents have space. These plates move. This creates energy and tensions. When the tensions become too great, the plates separate and the earth shakes.
Also, when a volcano erupts, the earth shakes. It also shakes when a meteorite, a chunk of rock from outer space, falls to the earth. Some earthquakes are also caused by humans: for example, during a large explosion. Earthquakes have also been caused by deep drilling in search of hot rock or water.
The Richter scale is used to measure the severity of an earthquake. Charles Richter from the USA helped invent it. The scale shows how much energy is released. A quake of magnitude 3 or worse can usually be felt. From magnitude 7, there is usually serious damage: houses collapse and people can be buried under them.