Page last updated at 13:10 GMT, Thursday, 28 January 2010

How to measure earthquakes

There are hundreds of earthquakes around the world each year but only a few cause serious damage.

earthquake graphic

The figures above seek to measure an earthquake in terms of the energy it releases.

The scale used to measure earthquakes is unusual. For example, the difference in strength between an earthquake of magnitude five and a magnitude six earthquake is much more dramatic than a rise of just one unit would suggest.

In fact a magnitude six earthquake possesses 32 times more energy than a magnitude five quake, as seismologists use a logarithmic scale to record these natural disasters.

That means that a gap of two steps, from five to seven, represent an earthquake nearly 1,000 times stronger.

Those quakes likely to cause the most destruction measure 7.0 and above.

The 2004 earthquake which triggered the Asian tsunami was the third biggest quake since 1900. It measured 9.3.

There are an estimated 20 major quakes in the world every year according to the US Geological Survey.

The Haiti quake measured 7.0 and because the epicentre was so close to the ill prepared capital, Port-au-Prince, the damage was severe, and over 200,000 people died as a result.

The death toll in Haiti is in stark contrast to the magnitude 8.8 earthquake that struck Chile in February 2010 where less than 1,000 people died.

Chile has a long history of strong earthquakes. The largest recorded earthquake took place there in 1960. It measured 9.5 and was also followed by tsunamis.

About 1,655 people were killed - it's thought the casualties were comparatively light because there were a number of warning shocks which sent people running out of their homes before the main quake.

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