Geology And Geophysics

# An Overview on the Richter Scale

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The Richter Scale is still the most widely recognised earthquake measuring scale, and is the scale used when earthquakes are being reported by the media. This is despite the fact that for a number of years seismologists have been making use of the Moment Magnitude Scale. The Richter Scale though serves a purpose and ensures that the general public recognise the severity of the event.

The Richter Scale is named after its developer, Charles Richter, who in 1934-1935 was working alongside Beno Gutenberg, at the California Institute of Technology. After intensive study, Charles Richter discovered that through making use of a seismograph, the amplitude or ground movement could be measured and the magnitude of the earthquake could then be calculated.

Charles Richter designed his scale to be logarithmic, meaning that for each increase of one, the amplitude went up by a factor of ten. Thus an earthquake that is 5.0 on the Richter Scale is ten times greater than a 4.0. At the same time the Richter Scale is also used to distinguish the amount of energy released by the earthquake, although this increases by a factor of just under 32. An earthquake that measures 6.0 on the Richter Scale is said to release the equivalent of a million tons of TNT, whilst a 7.0 earthquake releases 32 million tons.

There are no real lower up upper limits of the Richter Scale, and whilst no earthquakes above 10.0 have been recorded, micro-earthquakes can go into a negative figure on the scale.

The Richter Scale makes use of descriptive terms as well as numerals to describe earthquakes. Micro-earthquakes relate to those events that are below 2.0 on the scale; Minor earthquakes range from 2.0 through to 3.9; Light earthquakes measure from 4.0 to 4.9; Moderate ones are recorded between 5.0 and 5.9; Strong earthquakes relate to those between 6.0 and 6.9; Major earthquakes, the ones that most often hit the headlines, are between 7.0 and 7.0; Great earthquakes measure from 8.0 to 9.9; and Epic earthquakes are 10.0 and above.

There are thousands of earthquakes every single day, and although most go unfelt, there are normally over a hundred each day that shake the ground enough to be felt by local populations. Major earthquakes, those measuring 7.0 or more on the Richter Scale, are really not that uncommon either, with twenty occurring each year.

The 2010 Haiti earthquake measured 7.0, and in this millennium there have been four Great Earthquakes; the 2001 Indian Gujaret earthquake; the 2007 Peruvian Chincha Alta earthquake; the Chinese 2008 Sichuan earthquake; and the 2004 Indian Ocean earthquake. There are of course no certainties about when the next Great Earthquakes will occur, and on average an earthquake on par with the Indian Ocean one, occur only once every twenty years. In the 1960s though, the Anchorage earthquake (measuring 9.2) and the Chilean Valdivia (9.5) both occurred within four years of each other.

The Richter Scale is of course only a measure of magnitude rather than severity. The location of the earthquake will depend whether it has a serious impact or a minor one. In general terms an earthquake that hits in a populated area will have a greater impact than one that hits in an unpopulated one, although resulting tsunamis need to be taken into account.

There are a number of reasons as to why the Richter Scale has been replaced in scientific circles. Charles Richter based his work on the earthquakes detected and observed in California, and was dependent upon local conditions. An earthquake that struck in another part of the world would be operating in different conditions which would impact upon the actual magnitude of the earthquake. The Richter Scale is also primarily based upon the comparison of one earthquake against another, although most scientists prefer to deal in absolutes.

To overcome the shortcomings of the Richter Scale the Moment Magnitude Scale was invented. The Moment Magnitude Scale though was calibrated to the Richter Scale and as a result the figures produced for the magnitude of an earthquake are very close.

The Richter Scale has for generations been an important guide to the severity or magnitude of an earthquake, and when used in predictions has also ensured that people are adequately prepared.

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