What the Richter Scale actually means

A US expert studies a seismogram displaying the 7.5 magnitude earthquake that struck the Mexican coast in 1999. Photo / Reuters

The Richter Scale assigns a single number to quantify the amount of seismic energy released by an earthquake. It is measured using a seismograph.

And although a seismograph is a serious piece of scientific equipment, can be made in relatively simple form using these instructions.

It is a logarithmic scale, which means that each full number on the scale is multiplied by a factor of ten - meaning a 5.0 quake is 10 times larger than one which measures 4.0.

To find out exactly where an earthquake occurred, here are instructions on finding its epicentre.

THE RICHTER SCALE

2.5 or less Quakes this small are usually not felt, but can be recorded by and there are an estimated 900,000 of these little shakes annually. Under 2.5 is considered a 'microquake'.

2.5 to 5.4 Earthquakes in this lower-end range can often be felt, but only cause minor damage at worst. Experts estimate about 30,000 of these shakes a year.

5.5 to 6.0 Things get slightly more serious as the scale
heads towards 6.0, and can cause slight damage to buildings and other structures. There are approximately 500 each year.

6.1 to 6.9 Up to seven on the Richter scale, quakes may cause a lot of damage in highly populated areas. One measuring 6.5 hit Indonesia's Nusa Tenggara island chain earlier this year. Estimates put the number of earthquakes of this size around 100.

7.0 to 7.9 Only 20 take place a year, and are considered a very major earthquake causing serious damage.

8.0 or greater The most terrifying of earthquakes, which will totally decimate communities near the epicentre, these
destructive forces are only unleashed every five to ten years.

MAGNITUDE CLASSES

Aside from the commonly-used Richter Scale, earthquakes are classified in categories ranging from 'minor to 'great', depending on their magnitude.

'Great' means magnitude of 8 or more; 'major': 7 - 7.9; 'strong': 6 - 6.9; 'moderate': 5 - 5.9; 'light': 4 - 4.9; 'minor': 3 - 3.9.

- NZ HERALD STAFF