The earthquake magnitude scale
Earthquake magnitude is measured on the moment magnitude scale, written Mw or just M. It is logarithmic, so each whole number is ten times the ground motion and roughly 32 times the energy release of the number below it. A magnitude 6 does not shake twice as hard as a magnitude 3; it shakes a thousand times harder and releases around 32,000 times the energy.
What each magnitude means
These are the descriptive classes USGS uses, with the number of events the world sees in an average year. The felt effects assume a shallow event under populated ground; the same magnitude 300 km down is barely noticed.
| Magnitude | Class | What it does | Per year, worldwide |
|---|---|---|---|
| Below 2.5 | Micro | Recorded by instruments; not usually felt | Millions |
| 2.5 to 3.9 | Minor | Often felt indoors; no damage | Over 100,000 |
| 4.0 to 4.9 | Light | Rattles dishes and windows; damage unlikely | About 10,000 |
| 5.0 to 5.9 | Moderate | Can damage weak or poorly built structures | About 1,000 |
| 6.0 to 6.9 | Strong | Damaging in populated areas up to about 100 km across | About 130 |
| 7.0 to 7.9 | Major | Serious damage over large areas | About 15 |
| 8.0 and above | Great | Devastating; felt thousands of kilometres away | About 1 |
The frequencies are averages over decades, not a schedule. A year with three magnitude 8 events and a year with none are both ordinary.
Magnitude describes the earthquake. It says nothing about how hard the ground moved where you were standing, which depends on distance, depth and the rock and soil underneath you. That second quantity is intensity, and it is measured separately.
Why nobody uses the Richter scale any more
News reports still say "on the Richter scale", but seismologists stopped using it decades ago. The number you see today is almost always moment magnitude.
Charles Richter published his local magnitude scale in 1935. It was designed for southern California, for one instrument type, and for events within a few hundred kilometres. Past about magnitude 7 it saturates: genuinely enormous earthquakes all come out around the same number, because the instrument it was calibrated against cannot distinguish them.
Moment magnitude, introduced in 1979, is computed from the physical size of the rupture: the area of fault that moved, how far it moved, and the rigidity of the rock. It does not saturate, so it can describe a magnitude 9.5 honestly. The two scales were deliberately matched in the middle of the range, which is why a moderate quake gets a similar number on either, and why the old name survived in headlines.
Is the Richter scale the same as magnitude?
- Not since 1979. The Richter scale is one specific way of measuring magnitude, now largely retired because it saturates above about magnitude 7. Modern figures, including every number on this site, are moment magnitude.
How much stronger is a 6.0 than a 5.0?
- Ten times the ground motion and about 32 times the energy. The scale is logarithmic, so a single decimal point matters: a 5.8 releases roughly twice the energy of a 5.6.
What magnitude earthquake can you feel?
- Around magnitude 2.5 if you are close to it and standing still. Below that, only instruments notice. Depth matters as much as magnitude: a magnitude 4 five kilometres down is far more noticeable than a magnitude 5 two hundred kilometres down.
What is the highest possible earthquake magnitude?
- There is no ceiling in the mathematics, but there is one in the geology. Magnitude depends on how much fault can rupture at once, and the longest faults on Earth cap that at somewhere around magnitude 10. The largest ever measured is the 1960 Valdivia earthquake in Chile at magnitude 9.5.