What is magnitude? How earthquakes are measured
Published:
When the news says "an earthquake of magnitude 6.2", that number is a magnitude. But what does it actually mean — and why is the difference between 5 and 6 so dramatic?
A logarithmic scale
The magnitude scale is logarithmic: each whole step means roughly 10 times larger ground motion and about 32 times more released energy. An M6 quake releases around 32 times more energy than an M5 — and roughly 1,000 times more than an M4. That is why small differences in the number matter enormously.
Richter or moment magnitude?
The famous Richter scale (ML) from 1935 works best for local, mid-sized quakes. Today seismologists mostly use the moment magnitude (Mw), computed from how large an area of the fault slipped and how far. That is why you see different magnitude types (ml, mb, mw) in EarthquakeIndex's detail view — not errors, just different measuring methods. The number is often revised in the hours after a quake as more stations are included.
What can you feel?
- Below 2: only registered by instruments
- 2–3.9: can be felt near the epicentre, rarely damaging
- 4–4.9: clear shaking, items can fall off shelves
- 5–5.9: can damage weak buildings
- 6+: potentially destructive near the epicentre
Depth matters too: a shallow M5 quake (0–10 km) is felt far more strongly than a deep one (70+ km) of the same magnitude.
In EarthquakeIndex's detail view we convert each quake's energy into tonnes of TNT to make the numbers tangible.