Causes

An earthquake is the shaking and vibration of the crust due to movement of the Earth's plates (plate tectonics). Earthquakes can happen along any type of plate boundary.

Earthquakes are caused when the tension is released from inside the crust. This happens because plates do not move smoothly - sometimes they get stuck. When this happens a great deal of pressure builds up. When this pressure is eventually released, an earthquake tends to occur.

The point inside the Earth's crust where the pressure is released is called the focus. The point above the focus, on the Earth's surface is called the epicentre.

In an earthquake, energy is released in the form of waves. These are called seismic waves. The waves spread out from the focus. The strongest waves are found near the centre of the earthquake. This means that the most severe damage caused by an earthquake will happen close to the epicentre.

Diagram of an earthquake

The diagram above illustrates how the movement of two plates at the plate boundary creates pressure deep inside the Earth. This is the focus. This pressure travels up to the Earth’s crust and escapes at the epicentre of the earthquake. The pressure, released at the epicentre[an error occurred while processing this directive], travels outwards in all directions in rings (seismic waves). The waves are felt most strongly at the epicentre, becoming less strong as they travel further away.