The moment magnitude scale (MMS) was created in 1979 as a means of measuring medium to large earthquakes because of problems and inability to give reliable results (when applied to earthquakes of magnitudes of 7and above) using the Richter Scale, which was developed by Charles Richter and Beno Gutenberg in the 1930’s.
In 1979, because the Richter Scale had problems in measuring medium-sized to large quakes; two Cal Tech seismologists; Thomas C. Hanks and Hiroo Kanamori, came up with the moment magnitude scale, successor to the Richter Scale (or local magnitude). Their goal was to quantify medium-sized earthquakes between 3.0 and 7.0 in Southern California.
The moment magnitude scale enables seismologists to compare the energy released by different earthquakes on the basis of the area of the geological fault that ruptured in the quake. The MMS symbol is M with W written as superscript which means “mechanical work.” The “W” indicates the work accomplished; the magnitude is based on the moment of the earthquake, which is equal to the rigidity of the earth multiplied by the average amount of skip on the fault and size of the area slipped. The seismic moment M and O written superscript is a measure of the total amount of energy that is transformed during an earthquake. It is now the most common measure for medium to large earthquake magnitudes but it does break down for smaller earthquakes. The US Geological Survey does not use this scale for earthquakes with a magnitude of less than 3.5, which is the majority of earthquakes.
The moment magnitude scale assigns a single number to quantify the size of an earthquake. The magnitude is based on the moment of earthquake which is equal to the rigidity of the earth multiplied by the average amount of slip on the fault and size of the area that slipped. The formulae is different than the Richter scale; however, the moment magnitude scale still has a continuum of magnitude values defined by the older one. The moment magnitude scale is used to estimate magnitudes for all modern and large earthquakes by the US Geological Survey. It is logarithmic like the Richter.
Seismologists often no longer follow Richter’s original methodology because it was not designed to use data from earthquakes recorded at epicenter distances greater than about 600 km. According to USGS Geologiocal Survey, the moment is the preferred magnitude for all earthquakes listed in their catalog. The least complicated and probably most accurate terminology is to use the term “magnitude” and to use the symbol “M” (without any subscripts or superscripts).
For very large earthquakes, the moment magnitude gives the most reliable estimate of earthquake size. The recent earthquake that hit Haiti measured 7.0 on the moment magnitude scale, according to the U.S. Geological Survey, thought to have killed between 100,000 and 200,000 people. Eight days later, an aftershock was also measured by the moment magnitude scale as 6.0.
Inventors.about.com/od/qrstartventors/a/charles_richter.htm (By Mary Bellis)