The
moment magnitude scale (abbreviated as
MMS; denoted as
MW or
M) is used by
seismologists to measure the size of
earthquakes in terms of the energy released. The magnitude is based on the
seismic moment of the earthquake, which is equal to the rigidity of the
Earth multiplied by the average amount of slip on the
fault and the size of the area that slipped. The scale was developed in the 1970s to succeed the 1930s-era
Richter magnitude scale (M
L). Even though the formulae are different, the new scale retains a similar continuum of magnitude values to that defined by the older one. The MMS is now the scale used to estimate magnitudes for all modern large earthquakes by the
United States Geological Survey.