An earthquake is a suddden jolt followed by strong shaking that can last a few seconds or up to a couple of minutes. It's caused when tension is released from inside the crust and pressure builds along plate boundaries. When this pressure is eventually released, an earthquake can occur.
The most severe damage caused by an earthquake will happen close to the epicentre, the point where the pressure is released on the Earth’s surface.
Earthquakes can happen at any time of year, day or night. Each year, there are about a million earthquakes around the world. However, only about 100 of these will cause serious damage.