In simple terms
The moment magnitude scale is the standard way seismologists state how big an earthquake was. It is written Mw, and it has replaced the Richter scale for most serious work.
It describes the earthquake at its source. One earthquake has one magnitude, wherever you happened to be standing.
How it works
The scale is built on seismic moment. The US Geological Survey describes moment as a physical quantity proportional to the slip on the fault multiplied by the area of fault surface that slipped. Britannica adds the third ingredient, the rigidity of the rock that broke.
A standard formula turns that moment into a magnitude number, so modern readings can sit alongside older ones.
The older method worked differently. Charles Richter built local magnitude in the 1930s from high-frequency recordings at seismograph stations near Southern California earthquakes. The USGS notes it was strictly valid only for certain frequency and distance ranges.
For very large earthquakes that method flattens out, a problem called saturation, and it stops telling big quakes apart. Britannica describes the amplitude readings plateauing regardless of the true size of the event.
Moment magnitude does not have that ceiling. The USGS calls it valid over the complete range of magnitudes. Hiroo Kanamori and Thomas C. Hanks developed it in the 1970s.
The scale is logarithmic. A ten-fold drop in recorded amplitude lowers the magnitude by one, so the gap between a 6 and a 7 is far wider than the numbers look.
Why it matters
Magnitude is what makes earthquakes comparable across countries and across decades. It feeds hazard maps, building codes and tsunami warning thresholds.
It is also the number most often misreported. Coverage still says Richter scale for figures that were never calculated that way.
Where you’ll see it
- Earthquake bulletins from national agencies, written as Mw or M
- Seismic hazard maps and engineering codes
- Tsunami warning criteria
- News coverage of major earthquakes
Example
An earthquake reported at Mw 7.1 shook the ground about ten times harder than one reported at Mw 6.1, even though the two numbers look close together.
Often confused with
Magnitude is not intensity. Magnitude describes the earthquake at its source, and there is one value. Intensity, on the Modified Mercalli scale, describes the shaking felt at a particular place, and it changes with distance from the epicenter.
Key facts
- Moment magnitude is based on seismic moment, a quantity proportional to the slip on the fault multiplied by the area of fault surface that slips.1
- Britannica adds the rigidity of the rock as the third component of seismic moment, and dates the scale to 1970s work by Hiroo Kanamori and Thomas C. Hanks.2
- The USGS describes moment magnitude as valid over the complete range of magnitudes, unlike the earlier scales.1
- Richter's local magnitude, devised in the 1930s for Southern California, was strictly valid only for certain frequency and distance ranges, so it saturates for large earthquakes.1
- The scale is logarithmic: a ten-fold drop in recorded amplitude lowers the magnitude by one.3
Related concepts
In the news
Quick checkWhy did moment magnitude replace the Richter scale for large earthquakes?Show answer
The Richter method saturates, so it cannot separate very large quakes. Moment magnitude stays valid across the whole range.
Sources
- U.S. Geological Survey. Moment magnitude, Richter scale: what are the different magnitude scales, and why are there so many?. 10 September 2024 (accessed 17 September 2026)
- Encyclopaedia Britannica, John P. Rafferty. Moment magnitude. Undated (accessed 17 September 2026)
- U.S. Geological Survey. What is the difference between earthquake magnitude and earthquake intensity? What is the Modified Mercalli Intensity Scale?. Undated (accessed 17 September 2026)
Editorially reviewed by Specialty Digest Editorial TeamLast reviewed September 17, 2026Researched and drafted with AI assistanceReport an issue