Charles Francis Richter in the context of "Seismicity"

The moment magnitude scale (MMS; denoted explicitly with M_w  or Mwg and generally implied with use of a single M for magnitude) is a measure of an earthquake's magnitude ("size" or strength) based on its seismic moment. M_w  was defined in a 1979 paper by Thomas C. Hanks and Hiroo Kanamori. Similar to the local magnitude/Richter scale (M_L ) defined by Charles Francis Richter in 1935, it uses a logarithmic scale; small earthquakes have approximately the same magnitudes on both scales. Despite the difference, news media often use the term "Richter scale" when referring to the moment magnitude scale.

Moment magnitude (M_w ) is considered the authoritative magnitude scale for ranking earthquakes by size. It is more directly related to the energy of an earthquake than other scales, and does not saturate – that is, it does not underestimate magnitudes as other scales do in certain conditions. It has become the standard scale used by seismological authorities like the United States Geological Survey for reporting large earthquakes (typically M > 4), replacing the local magnitude (M_L ) and surface-wave magnitude (M_s ) scales. Subtypes of the moment magnitude scale (M_ww , etc.) reflect different ways of estimating the seismic moment.

The surface wave magnitude (${\displaystyle M_{s}}$) scale is one of the magnitude scales used in seismology to describe the size of an earthquake. It is based on measurements of Rayleigh surface waves that travel along the uppermost layers of the Earth. This magnitude scale is related to the local magnitude scale proposed by Charles Francis Richter in 1935, with modifications from both Richter and Beno Gutenberg throughout the 1940s and 1950s. It is currently used in People's Republic of China as a national standard (GB 17740-1999) for categorising earthquakes.

Recorded magnitudes of earthquakes through the mid 20th century, commonly attributed to Richter, could be either ${\displaystyle M_{s}}$ or ${\displaystyle M_{L}}$.

Beno Gutenberg (/ˈɡuːtənbɜːrɡ/; June 4, 1889 – January 25, 1960) was a German-American seismologist who made several important contributions to the science. He was a colleague and mentor of Charles Francis Richter at the California Institute of Technology and Richter's collaborator in developing the Richter scale for measuring an earthquake's magnitude.