Earthquakes are one of the most dramatic and deadly natural hazards.
Violent shaking and vibrations of the Earth's crust.
They strike without warning, and can cause catastrophic damage.
But what causes them?
It's all down to the movement of the Earth's tectonic plates.
Earthquakes can occur along any type of plate boundary.
The plates don't move smoothly, but can get stuck, so pressure builds up...
They can then slip suddenly, causing a violent release of pressure –
as an earthquake.
The plate boundary where the earthquake occurs is called the fault line.
Inside the Earth's crust, the point where the pressure is released is called the focus.
And directly above the focus, on the Earth's surface, is the epicentre – where the most severe damage occurs.
Earthquake energy is released in seismic waves, which spread out from the focus like ripples on a pond.
These vibrations can be measured using a seismometer, and the magnitude recorded using the Richter scale.
A reading of one or two on the Richter scale is very common, but a quake measuring seven or eight is devastating.
The more severe earthquakes can destroy whole cities, and change landscapes forever.
They can also trigger secondary effects, like fires, landslides, and tsunamis.
And the initial quake is always followed by a series of smaller aftershocks, as the rock along the fault line adjusts.
If we could predict when and where earthquakes will happen, it could save millions of lives – but there's still no reliable way of doing this.
They remain a shocking and unpredictable force of nature.