What magnitude actually measures
Magnitude is a measure of the size of the earthquake at its source — how much the ground moved where the rock broke, scaled by how far away the instruments were. It is one number per earthquake, and it is the same number everywhere.
That last point is what separates it from intensity, which describes how hard the shaking was at a particular place and is different in every town.
The scale is logarithmic, and the steps are bigger than they look
Each whole step up is ten times the ground motion recorded on a seismogram — and about 32 times the energy released. That second figure is the one that matters for damage, and it is the one people underestimate.
| Step | Ground motion | Energy |
|---|---|---|
| M3 → M4 | 10× | ≈32× |
| M3 → M5 | 100× | ≈1,000× |
| M3 → M6 | 1,000× | ≈32,000× |
| M6 → M7 | 10× | ≈32× |
So an M6 is not "twice" an M3 in any useful sense. It releases roughly thirty thousand times the energy. It is also why QuakeIndex weights its activity score by energy rather than counting events: a week with one M5 and a week with thirty M2s are not comparable, and a count would say they were.
What the letters after the number mean
A magnitude on a USGS page comes with a type: ml, mb,
mww, md. These are different methods, suited to different
sizes and distances, and they agree closely but not exactly. QuakeIndex shows the
type on every event page because "M4.2 ml" and "M4.2 mww" are not quite the same
statement.
| Type | What it is | Typically used for |
|---|---|---|
ml | Local magnitude — the original "Richter" measure | Small, nearby earthquakes |
md | Duration magnitude, from how long the signal lasts | Very small local events |
mb | Body-wave magnitude | Moderate, distant earthquakes |
mww, mw | Moment magnitude, from the physics of the rupture | Large earthquakes — the most reliable at size |
"The Richter scale" is the phrase everyone knows and it is not what modern large magnitudes use. Charles Richter's 1935 local magnitude saturates for big earthquakes — above about M7 it stops distinguishing them. Moment magnitude does not, which is why it is the number you see for anything major.
Why a magnitude changes after it is published
USGS publishes a first automatic solution within about a minute, computed by software from the stations that have reported so far. As more stations report and a seismologist reviews it, the estimate is refined. A change of two or three tenths in the first hour is routine; a larger revision happens when the first solution was based on few stations.
This is not a correction of a mistake. It is how the measurement works. QuakeIndex lists every revision it observes on the event page, with the time, rather than quietly replacing the old number — because somebody read the old one.
What each magnitude usually means for people
Roughly, and with the enormous caveat that depth and local ground conditions change this a great deal:
| Magnitude | Usually |
|---|---|
| below 2.5 | Recorded by instruments; rarely felt |
| 2.5 – 3.9 | Often felt indoors nearby; damage very rare |
| 4.0 – 4.9 | Felt clearly nearby; rattles windows and dishes; damage uncommon |
| 5.0 – 5.9 | Can damage poorly built structures near the epicentre |
| 6.0 – 6.9 | Damage likely near the epicentre; felt over a wide area |
| 7.0 and above | Serious damage over a large area |
These bands are what QuakeIndex uses to write the plain-language sentence on an event page. They are computed from the magnitude, not written per event, so the sentence can never disagree with the number above it.
How many earthquakes there are
Earthquake sizes follow a well-established pattern: for each step down in magnitude there are roughly ten times as many events. Worldwide that means a handful of M7s a year, and hundreds of thousands of events small enough that only instruments notice. This is why a busy-looking map is normal rather than alarming — and why QuakeIndex only offers pages above M2.5 in the US and M4.5 elsewhere for indexing.
Sources
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