Earthquakes guide
Earthquake Magnitude Explained: What M4, M6 and M8 Really Mean
Every earthquake report starts with one number. Here is what it measures, why a magnitude 7 is so much bigger than a 6, and why two people can feel the same quake very differently.
Updated 4 November 2026
What magnitude measures
Magnitude is a measure of the energy an earthquake releases at its source. It is worked out from seismometer recordings around the world, and it is one number for the whole earthquake, wherever you are.
The famous Richter scale was devised by Charles Richter in 1935 for earthquakes in southern California. Today agencies such as the US Geological Survey (USGS) and the European-Mediterranean Seismological Centre (EMSC) mostly use the moment magnitude (Mw), which works for earthquakes of every size and anywhere on Earth. For everyday purposes the numbers are similar, so people still say "Richter scale".
Why each step matters so much
The scale is logarithmic. Each whole step up means the ground waves on a seismogram are about 10 times larger, and the energy released is about 32 times greater.
- A magnitude 6 releases about 32 times more energy than a 5.
- A magnitude 7 releases about 1,000 times more than a 5.
- A magnitude 9, like Japan in 2011, releases about a million times more than a 5.
The largest earthquake ever recorded was the magnitude 9.5 in Chile in 1960.
How often they happen
Small earthquakes happen all the time; big ones are rare. The USGS gives these long-term yearly averages for the whole world:
| Magnitude | Per year (average) | What it can do |
|---|---|---|
| 8 and above | about 1 | Great earthquake: severe damage over a large region, can cause an ocean-wide tsunami |
| 7 to 7.9 | about 15 | Major: serious damage over a wide area |
| 6 to 6.9 | about 130 | Strong: damage in populated areas near the epicentre |
| 5 to 5.9 | about 1,300 | Moderate: felt widely, slight damage to weaker buildings |
| 4 to 4.9 | about 13,000 | Light: felt by many, rarely causes damage |
Some years have more, some fewer. A run of big quakes in one year is usually chance, not a sign that the Earth is becoming more active.
Magnitude is not how hard you felt it
How strongly the ground shakes where you are is called intensity. It depends on the magnitude, but also on how far away the quake was, how deep it was, and the ground under you: soft soil and filled land shake more than solid rock.
Intensity is measured on the Modified Mercalli Intensity scale, written in Roman numerals: I is not felt, IV is felt indoors by many, VI is felt by everyone with some damage, and VIII and above means considerable damage, even to well-built houses at the top of the scale. The USGS publishes intensity estimates (ShakeMap) and collects reports from people who felt the quake ("Did You Feel It?").
Depth
- Shallow earthquakes, from 0 to 70 km deep, cause most of the damage, because the energy is released close to the surface.
- Intermediate ones, 70 to 300 km deep, are felt over a wider area but less strongly.
- Deep earthquakes, down to about 700 km, are often felt only lightly even when they are large.
Why the number changes
The first magnitude is a quick automatic estimate, published within minutes. As more stations report, seismologists review it, and it often moves by a few tenths in the first hour. Different agencies can also give slightly different figures. Quake shows which agency each number comes from and how old it is.
Not an official warning service. In an emergency, follow your local authorities.