Two basically different methods are used for describing the severity of an earthquake. One is Intensity and the other is Magnitude. Intensity is an estimation of the earthquake’s effect on people, damages inflicted to structures and the changes caused to the earth’s surface etc. In 1905, an Italian Seismologist Giuseppe Mercalli devised an Intensity Scale, based on evidences such as human reactions, damages to structures, fissures in the earth’s surface, landslides, floods etc. This Scale had 10 divisions – Roman Numbers I to X – and was known as the Mercalli Scale. In 1931, two Seismologists, Wood and Newman modified this Scale and extended it to 12 divisions, i.e. up to XII. After this, the Mercalli Scale came to be known as Modified Mercalli Scale or simply, the MM Scale.
Of far more importance and of greater scientific value is to describe the severity of earthquakes by Magnitude. Magnitude is related to the amount of strain energy released at the focus or epicenter of the earthquake, as recorded by the Seismographs. This is where the Richter scale comes into relevance. American Physicist and Seismologist, Charles Francis Richter (1900-1985), while with the Carnegie Institute, USA, embarked on the project for developing a suitable scale to describe the degree of intensity of earthquakes in terms of Magnitude, by numerical values.
The culmination of his efforts was the evolvement of a Scale in 1935. This was called the Richter scale, after his name. In this endeavour, Richter was supported by his collaborator, Professor Beno Guternerg of the California Institute of Technology. The Richter scale is an open-ended numerical Scale that describes an earthquake independently of its effects on people, buildings or other objects. It begins with Zero, in which the greatest wave-amplitude registered on a seismograph at a distance of 100 kms. From the epicenter of the earthquake does not exceed one Micron (one thousandth of a millimetre).
