The Richter Scale
The now-famous Richter scale was developed in 1935 by Charles Richter of the California Institute of Technology. The scale was intended only for use in California to separate the large number of small earthquakes from the small number of larger ones. The scale is derived from astronomy’s stellar magnitude scale.
The scale assigns a number to quantify the size of the earthquake. The number is derived by calculating the logarithm of the combined horizontal amplitude of the largest displacement from zero on a seismogram. It is a logarithmic scale, so an earthquake of five is ten times greater than a four, and an eight is 10,000 times greater than a four.
The energy of the earthquake is proportional to the square root of the cube of the amplitude. So each step of of the scale has an energy 31.6 times that of the previous. A nine, therefore, has 10,000 times the amplitude of a five, but a million times more energy.