Richter vs. Moment Magnitude
February 16, 2026
When a big earthquake is in the news, the report almost always says it measured some number “on the Richter scale.” That phrase has been out of date since the 1970s. The figure being quoted is nearly always moment magnitude — a different measurement built for exactly the earthquakes the Richter scale handles worst.
What Richter actually built
In 1935 Charles Richter, working with Beno Gutenberg at Caltech, needed a simple way to compare the many small earthquakes recorded in southern California. His local magnitude (ML) is the base-10 logarithm of the largest movement recorded on a particular instrument — the Wood–Anderson torsion seismograph — adjusted for how far away the earthquake was.
It was a brilliant, practical tool for its purpose. But it was tied to one instrument, one region, and one type of seismic wave, and it was never meant for great earthquakes on the other side of the planet.
The saturation problem
Every magnitude scale based on the amplitude of a particular wave eventually saturates. The waves the Richter scale measures have periods of about one second. A magnitude 7 rupture is already tens of kilometres long and takes many seconds to unfold; a magnitude 9 rupture can be over 1,000 km long and last several minutes. Once the rupture is much bigger than the wavelength being measured, adding more rupture barely changes that wave — so the number stops climbing.
In practice, the Richter scale flattens out above about magnitude 7. The surface-wave scale (Ms) used for distant earthquakes pushes that to roughly 8. Neither can tell an 8.5 from a 9.5 — a difference of more than thirty times in energy.
What moment magnitude measures instead
Moment magnitude (Mw) starts from the seismic moment: the rigidity of the rock, times the area of fault that slipped, times how far it slipped. That is a direct physical description of the earthquake, and it has no upper limit. Thomas Hanks and Hiroo Kanamori defined the Mw scale in 1979 so that its numbers line up with the familiar Richter values in the range where Richter works.
So a “magnitude 4.8” means about the same thing whether it is ML or Mw. The scales only diverge for large earthquakes — and there, moment magnitude is the one that stays accurate.
This is why some historical earthquakes have been re-rated. The 1906 San Francisco earthquake was long quoted around 8.3 on older scales; its modern moment magnitude estimate is about 7.9.
Why early magnitude reports change
For a large earthquake, the first automatic magnitude is released within minutes, before the full rupture has even been recorded everywhere. As more stations report and analysts model the whole waveform, the number is revised — usually upward for a great earthquake, because the largest, slowest part of the rupture takes time to measure. A shift of 0.2–0.3 in the first hour is normal and is not a mistake.
On the map, all magnitudes are the reviewed values from the USGS catalogue, so they reflect the best current estimate rather than the first headline.
Sources
- U.S. Geological Survey — “Moment magnitude, Richter scale — what are the different magnitude scales, and why are there so many?”
- Hanks, T. C. & Kanamori, H. (1979). “A moment magnitude scale.” Journal of Geophysical Research 84(B5), 2348–2350.
- Richter, C. F. (1935). “An instrumental earthquake magnitude scale.” BSSA 25(1), 1–32.