Moment magnitude scale 
Part of





Causes 
Characteristics 

Measurement 

Other topics 

The moment magnitude scale (MMS; denoted as M_{w} or M) is used by
The scale was developed in the 1970s to succeed the 1930sera
The moment magnitude is based on the
Since January 2002, the MMS has been the scale used by the
Popular press reports of earthquake magnitude usually fail to distinguish between magnitude scales, and are often reported as "Richter magnitudes" when the reported magnitude is a moment magnitude (or a surfacewave or bodywave magnitude). Because the scales are intended to report the same results within their applicable conditions, the confusion is minor.
In 1935,
The Richter scale was not effective for characterizing some classes of quakes. As a result,
The Richter scale, as modified, was successfully applied to characterize localities. This enabled local building codes to establish standards for buildings which were earthquake resistant. However a series of quakes were poorly handled by the modified Richter scale. This series of "great earthquakes", included faults that broke along a line of up to 1000 km. Examples include the
The difficulties with use of M_{s} in characterizing the quake resulted from the size of these earthquakes. Great quakes produced 20 s waves such that M_{s} was comparable to normal quakes, but also produced very long period waves (more than 200 s) which carried large amounts of energy. As a result, use of the modified Richter scale methodology to estimate earthquake energy was deficient at high energies.^{ [5]}
The concept of
Most earthquake magnitude scales suffered from the fact that they only provided a comparison of the amplitude of waves produced at a standard distance and frequency band; it was difficult to relate these magnitudes to a physical property of the earthquake. Gutenberg and Richter suggested that radiated energy E_{s} could be estimated as
(in Joules). Unfortunately, the duration of many very large earthquakes was longer than 20 seconds, the period of the surface waves used in the measurement of M_{s}. This meant that giant earthquakes such as the 1960 Chilean earthquake (M 9.5) were only assigned an M_{s} 8.2.
Kanamori recognized that measurement of radiated energy is technically difficult since it involves integration of wave energy over the entire frequency band. To simplify this calculation, he noted that the lowest frequency parts of the spectrum can often be used to estimate the rest of the spectrum. The lowest frequency
(where E is in Joules and M_{0} is in Nm), Kanamori approximated M_{w} by
The formula above made it much easier to estimate the energybased magnitude M_{w}, but it changed the fundamental nature of the scale into a moment magnitude scale.
Hanks & Kanamori (1979) combined their work to define a new magnitude scale based on estimates of seismic moment
Although the formal definition of moment magnitude is given by this paper and is designated by M, it has been common for many authors to refer to M_{w} as moment magnitude. In most of these cases, they are actually referring to moment magnitude M as defined above.
Moment magnitude is now the most common measure of earthquake size for medium to large earthquake magnitudes,^{
[11]} but in practice seismic moment, the seismological parameter it is based on, is not measured routinely for smaller quakes. For example, the
Current practice in official earthquake reports is to adopt moment magnitude as the preferred magnitude, i.e. M_{w} is the official magnitude reported whenever it can be computed. Because seismic moment (M_{0}, the quantity needed to compute M_{w}) is not measured if the earthquake is too small, the reported magnitude for earthquakes smaller than M 4 is often Richter's M_{L}.
Popular press reports most often deal with significant earthquakes larger than M ~ 4. For these events, the official magnitude is the moment magnitude M_{w}, not Richter's local magnitude M_{L}.