The Scale You Know vs. The Scale Scientists Use
When a major earthquake strikes, news reports often cite a magnitude number. Many people assume this comes from the "Richter scale" — but in most cases today, it doesn't. Modern seismologists use the Moment Magnitude Scale (Mw), which replaced the Richter scale for most applications decades ago. Understanding the difference tells you a lot about how earthquake science has evolved.
The Original Richter Scale (ML)
Charles Richter and Beno Gutenberg developed the local magnitude scale (ML) in 1935 at Caltech. It was designed specifically for earthquakes in Southern California and used readings from a particular type of seismograph — the Wood-Anderson torsion seismograph — at distances up to about 600 km from the epicenter.
The scale is logarithmic: each whole number increase represents a tenfold increase in measured wave amplitude, and roughly 31.6 times more energy released. A magnitude 6.0 earthquake releases about 31 times more energy than a 5.0.
Limitations of the Richter Scale
- Only reliable for earthquakes in a specific region (originally Southern California).
- Becomes inaccurate for very large earthquakes — it "saturates" above about magnitude 6.5.
- Tied to a specific instrument type that is no longer standard.
- Does not account for the geometry or duration of fault rupture.
The Moment Magnitude Scale (Mw)
Developed by Tom Hanks and Hiroo Kanamori in 1979, the Moment Magnitude Scale calculates seismic moment — a measure of the total energy released based on the area of the fault that ruptured, the average amount of slip, and the rigidity of the rock. The formula is:
Mw = (2/3) log₁₀(M₀) − 10.7
where M₀ is the seismic moment in dyne-centimeters.
Why Mw Is Superior
- Works for earthquakes of any size, anywhere on the planet.
- Does not saturate at high magnitudes — accurately distinguishes between giant earthquakes.
- Based on physical properties of the rupture, not just ground motion at one station.
- Consistent and reproducible across different seismograph networks.
Comparison at a Glance
| Feature | Richter Scale (ML) | Moment Magnitude (Mw) |
|---|---|---|
| Developed | 1935 | 1979 |
| Basis | Seismograph amplitude | Seismic moment (fault physics) |
| Useful range | Up to ~6.5 | All magnitudes |
| Geographic scope | Regional (Southern California) | Global |
| Used today? | Rarely | Standard worldwide |
What About the Numbers You See in Headlines?
When USGS or other agencies report a "magnitude 7.8 earthquake," they are almost always referring to Mw. The two scales produce similar numbers for moderate earthquakes (between about 3.0 and 6.5), which is why media continues to use "Richter scale" loosely — the numbers feel familiar. But for the largest earthquakes in history, the distinction matters enormously.
The 2004 Indian Ocean earthquake, for example, was initially reported around Mw 9.0 before being revised to Mw 9.1–9.3 as more data came in — a level of precision the Richter scale simply couldn't provide.