The Earthquake Magnitude Scale Explained

Magnitude is a single number describing the size of an earthquake at its source. Unlike intensity, which varies from place to place, an earthquake has exactly one magnitude no matter where you measure it from.

What magnitude actually measures

Modern magnitude is derived from seismic moment, a physical quantity equal to three things multiplied together: the rigidity of the rock, the area of the fault surface that slipped, and the average distance it slipped. In other words, magnitude is fundamentally about how much rock moved, and how far.

This is why very large earthquakes require very long faults. A magnitude 9 event needs a rupture hundreds of kilometres long, which in practice means a subduction zone. A magnitude 6 might rupture only 10–15 km of fault. There is no way to produce a magnitude 9 on a short fault regardless of how much stress has accumulated.

Why the Richter scale was replaced

Charles Richter devised his scale in 1935 to compare earthquakes in southern California using a specific instrument at a specific distance. It worked well for moderate local events, but it has a fatal limitation: it saturates. Above roughly magnitude 7, the instrument response stops increasing meaningfully even as earthquakes get genuinely larger, so a magnitude 8.5 and a magnitude 9.5 look almost identical on Richter's scale despite differing by a factor of about 32 in energy.

The moment magnitude scale (Mw), introduced in the 1970s by Hiroo Kanamori and Thomas Hanks, solved this by calculating magnitude directly from seismic moment. It was deliberately calibrated to line up with Richter values in the mid range, so the numbers feel familiar, but it keeps working all the way up.

When you see a magnitude reported today it is almost certainly moment magnitude, even when the report calls it "the Richter scale". Smaller events are sometimes still reported using local magnitude (ML) or body-wave magnitude (mb), because for small earthquakes those are quicker to compute and the difference is negligible.

The energy behind each step

The magnitude scale is logarithmic, and it is easy to underestimate what that means.

Each whole magnitude step represents approximately 10 times the ground-motion amplitude recorded on a seismograph, but approximately 31.6 times the energy released. Energy is what determines destructive potential.

The practical consequence: a single magnitude 8 releases more energy than roughly a thousand magnitude 6 earthquakes combined. Small earthquakes do not meaningfully relieve stress on a fault, because it would take an implausible number of them to release the energy a single large event does.

What each band means in practice

Below 2.0 — micro. Recorded by instruments only. Not felt.

2.0–3.9 — minor. Sometimes felt very close to the epicentre as a brief jolt or a passing truck. No damage. These are the most common earthquakes people ever experience.

4.0–4.9 — light. Widely felt nearby. Rattles windows, dishes and hanging objects, and can wake sleeping residents. Damage is uncommon and typically limited to cracked plaster or items falling from shelves.

5.0–5.9 — moderate. Can damage poorly built or unreinforced structures near the epicentre. Well-engineered buildings generally survive with little or no structural damage. This is roughly where damage becomes a genuine concern.

6.0–6.9 — strong. Destructive in populated areas within about 100 km of the epicentre, especially where construction standards are poor. Landslides and liquefaction become significant risks in susceptible ground.

7.0–7.9 — major. Serious damage over wide areas. Even well-built structures can be damaged. Long aftershock sequences are typical.

8.0 and above — great. Devastating across regions hundreds of kilometres wide. Offshore events of this size are the main cause of ocean-crossing tsunamis.

Why reported magnitudes change

It is completely normal for an earthquake's magnitude to be revised in the hours after it occurs, often by a few tenths. The first figure is computed automatically within minutes from a handful of nearby stations. As more stations report, and as analysts review the waveforms manually, the solution improves.

For very large earthquakes the revision can be substantial, because the full extent of a long rupture takes time to characterise. The 2004 Sumatra–Andaman earthquake was initially reported around magnitude 8.0 and was eventually assessed at magnitude 9.1–9.3 once the complete rupture was modelled.

Magnitude is not the whole story

A magnitude number alone tells you very little about consequences. Focal depth, distance from population centres, local soil conditions and above all building construction quality determine whether an earthquake is a curiosity or a catastrophe. A shallow magnitude 6.3 beneath a city of unreinforced masonry can be far deadlier than a magnitude 7.8 in a remote area or at great depth.