What magnitude measures
Magnitude estimates the energy released at the source of an earthquake. The old Richter scale has largely been replaced by the moment magnitude scale (Mw), which works better for large quakes, but the public still calls it 'the Richter scale'.
Why it's logarithmic
Each whole number up the scale means about 10 times more ground shaking and roughly 32 times more energy. A M6 releases ~32x the energy of a M5, and a M7 about 1,000x a M5. That is why great quakes are in a different league entirely.
Richter scale vs. moment magnitude (Mw)
The original Richter scale (ML), introduced by Charles Richter and Beno Gutenberg in 1935, measured the amplitude of shaking recorded on a specific instrument — the Wood-Anderson seismograph — at a set distance from the epicentre. It worked well for the small-to-moderate Southern California earthquakes it was built for, but it saturates above roughly magnitude 6.5–7: the number stops growing even though the earthquake keeps getting bigger.
The moment magnitude scale (Mw), developed by Hiroo Kanamori and Thomas Hanks in 1979, solved that problem. Instead of shaking amplitude, it calculates the seismic moment — the rigidity of the rock, the size of the fault that ruptured, and how far it slipped — which keeps scaling accurately even for the largest earthquakes. Since the 1990s, the USGS and most seismological agencies report Mw for anything above about magnitude 3.5–4.5. It is Mw, not the old Richter scale, that gives the M9.5 Valdivia 1960 and M9.1 Sumatra 2004 quakes their record-breaking numbers — Richter's original method could never have measured them correctly, because it maxes out long before M9.
Magnitude vs intensity
Magnitude is a single number for the whole quake. Intensity (the Mercalli scale) describes how strongly it was felt at a given place, which depends on distance, depth and local ground.
Magnitude classes, from micro to great
Seismologists group magnitudes into broad classes that describe typical effects. These are the same bands the USGS uses on its live feeds:
| Magnitude | Class | Typical effect |
|---|---|---|
| <3.0 | Micro | Not felt, only recorded by instruments |
| 3.0–3.9 | Minor | Often felt, rarely causes damage |
| 4.0–4.9 | Light | Felt widely; minor damage possible |
| 5.0–5.9 | Moderate | Can damage poorly built structures |
| 6.0–6.9 | Strong | Can be destructive in populated areas |
| 7.0–7.9 | Major | Serious damage over large areas |
| 8.0+ | Great | Severe damage across hundreds of km |
The five largest earthquakes ever measured by Mw
- M9.5 — Valdivia, Chile (1960)
- M9.2 — Alaska, USA (1964)
- M9.1 — Sumatra, Indonesia (2004)
- M9.1 — Tōhoku, Japan (2011)
- M9.0 — Kamchatka, Russia (1952)
All five are within QuakeBeat's full ranking of the biggest earthquakes in history — every one of them measured in moment magnitude, since the old Richter scale could not register numbers that high.
Magnitude on the QuakeBeat live map
Every event on the QuakeBeat live map shows the magnitude reported by the USGS, which is Mw for anything of real consequence and a faster provisional estimate for smaller, more frequent shocks. Remember that magnitude alone does not tell the whole story: a high-magnitude quake far offshore can be far less dangerous than a moderate one shallow and close to a city, as the depth of an earthquake also plays a major role in how destructive it is.