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It’s 26 April, and today we celebrate Richter Scale Day. It is the commemoration of the birth of Charles Francis Richter (26 April 1900 – 30 September 1985), the American seismologist who is famous as creator, in 1935, of the ‘Richter magnitude scale’, commonly known as the Richter scale.

The Richter scale was developed as an attempt to quantify, in a single number, the energy released during an earthquake. It is a base-10 logarithmic scale, meaning that a magnitude 4 earthquake, for example, would be 10 times that of a magnitude 3 earthquake. Since the mid 20th century, the ‘moment magnitude scale’ (MMS) has replaced the Richter scale as the preferred measure of the strength of earthquakes, particularly for larger earthquakes. Despite this, and despite the fact that many earthquakes these days are actually measured according to the MMS, the magnitude values are commonly still referred to as Richter scale values by the general public.

(© All Rights Reserved)

(© All Rights Reserved)

For low magnitude/weak earthquakes, the Richter and MMS scales are very closely related. However, for values above 5, the Richter classification is considered ‘at risk’, while Richter values above 6 are essentially meaningless. Both the Richter and MMS scales are open-ended, meaning they have no maximum value, but to date the highest value recorded has been the Valdivia earthquake that took place in Chile in 1960, measuring a magnitude of 9.5 on the MMS scale.

Both the Richter and MMS scales measure the energy release of an earthquake, not the damage done. Since quakes can manifest in many different ways – jolts, wobbles, shakes, vertical movement, horizontal movement etc – two earthquakes of the same magnitude can have greatly varying impacts. The ‘Mercalli intensity scale’, on the other hand, classifies earthquakes based on their effect/impact – it provides an indication of the effects of an earthquake on humans, natural objects and man-made structures, on a scale from I (‘not felt’) to XII (‘total destruction’).

As recent comparative examples, the 2011 Christchurch earthquake here in New Zealand, and 2011 Japan earthquake, which measured 6.3 and 8.9 on the MMS scale respectively, were both categorised as IX (‘violent’) on the Mercalli scale. The 2011 Van earthquake in Turkey, on the other hand, which measured 7.2 MMS, was categorised one level higher as an X (‘intense’) according to the Mercalli scale.

Living in the very seismically active New Zealand, one cannot help but have an appreciation for people like Charles Richter, and the work he has done in advancing the understanding and categorisation of earthquakes. So here’s to Charles Richter, and here’s to a seismically calm Richter Scale Day.