An earthquake is a sudden tremor or movement of the earth's crust, which originates at or below the surface.

There are two main causes of earthquakes. They can be linked to explosive volcanic eruptions or can be triggered by tectonic activity, with the latter being the cause of most earthquakes.

Earthquakes in the UK are all tectonic quakes, triggered when the Earth’s crust is subjected to strain and eventually moves.

At the start of a quake there is a sudden movement within the Earth's crust and shock waves move out from that point.

The focus of the earthquake or tremor is often deep below the surface and difficult to map, so what is normally called the epicentre is taken as the point on the Earth directly above it.

There are two scales for defining the strength of an earthquake, the Mercalli Scale and the Richter Scale.

Earthquakes emit three types of waves which have their own characteristics.

Primary waves are identical in character to sound waves. The ground is forced to move forwards and backwards as it is compressed and decompressed.

Secondary waves travel more slowly and arrive after the P-waves. They move in all directions, at speeds which depend upon the density of the rocks through which they are moving. They cannot move through liquids. They can move walls and fences, leaving them 'S' shaped.

Surface waves are formed close to the epicentre and can only travel through the outer part of the Earth’s crust. They are responsible for the majority of the buildings damaged by earthquakes. The ground is made to move in a circular motion, causing it to rise and fall as visible waves move across the ground.

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