A measure of the strength of an earthquake or strain energy released by it, as determined by seismographic observations. The amplitude on a seismogram, the magnitude and the energy released are related through a log- linear relationship, which was originally defined by Charles Richter in 1935. An increase of one unit of magnitude (for example, from 4.6 to 5.6) represents a 10-fold increase in wave amplitude on a seismogram or approximately a 30-fold increase in the energy released.

            In other words, a magnitude 6.7 earthquake releases over 900 times (30 times 30) the energy of a 4.7 earthquake - or it takes about 900 M:4.7 earthquakes to equal the energy released in a single 6.7 earthquake! This is an open-ended scale and hence there is no beginning or end to this scale.

            However, rock mechanics seems to preclude earthquakes smaller than about -1 or larger than about 9.5. An earthquake of magnitude -1.0 releases about 900 times less energy than a M:1.0 quake. Except in special circumstances, earthquakes below M:2.5 are not generally felt by humans. Depending upon the range of magnitude, epicentral distance and the type of seismic waves considered in the computation, there are several magnitude scales in use as: Local magnitude (Ml or ML), commonly referred to as "Richter magnitude", Surface-wave magnitude (Ms), Body-wave magnitude (mb), and Moment magnitude (Mw).

            The first three magnitude scales Ml, Ms and mb make use of amplitudes and time periods of seismic wave and suffer from the saturation effect. They have some or other limitation with regard to their applicability uniformly to all magnitude ranges, epicentral distances and focal depths. To avoid the saturation effect and standardize the magnitude scales, a magnitude scale based on seismic moment (Mo) was proposed by Kanamori (1977).

            The moment magnitude (Mw) scale is estimated using the formula, Mw=(log Mo –16)/1.5, where Mo, is the seismic moment in dyne-cm. Since seismic moment is a measure of strain energy released from the entire rupture surface, a magnitude scale based on seismic moment most accurately describes the size of large earthquakes. Since Mo does not saturate, so also Mw. The moment magnitude scale is the most preferred magnitude scale in case of large earthquakes.