A magnitude 7 releases 32 times more energy than a magnitude 6 — and 1,000 times more than a magnitude 5. The number on the news is deceptively simple. Behind it lies a century of seismological refinement, from Richter's original scale to the moment magnitude standard used by every agency on Earth today.
📊 OPEN LIVE EQ DASHBOARDWhen an earthquake strikes, the first number reported is magnitude. Within minutes of a major event, seismic networks around the world have computed an estimate — and within hours, that number is refined using data from hundreds of stations. Yet magnitude is one of the most misunderstood metrics in all of geoscience. It is not a measure of destruction. It is not linear. And "the Richter scale" — the phrase used by virtually every news broadcast — has not been the scientific standard since the 1970s. This guide explains what magnitude actually measures, how it is calculated, and how to read it correctly.
Magnitude is a logarithmic scale — each whole number increase represents a tenfold increase in ground motion amplitude measured on a seismogram, and approximately a 32-fold increase in energy released. This means the difference between a M5 and a M7 is not a factor of two — it is a factor of 1,000 in energy. The 2011 Tōhoku M9.0 earthquake released more energy than all earthquakes recorded globally in the previous decade combined. The scale compresses an almost incomprehensible range of energies into a handful of digits.
When an earthquake occurs, seismic waves radiate outward at 6–8 km/s through the crust. Within seconds, the nearest seismic stations record the ground motion. Automated algorithms compute a first magnitude estimate — typically using P-wave amplitude — within 1–3 minutes of origin time. This rapid estimate, often denoted Mwp or Mww, drives the initial tsunami advisory decision. Over the following hours, as more stations contribute data and longer-period waves are analysed, agencies compute a refined centroid moment tensor (CMT) solution that yields the definitive Mw and also describes the fault geometry.
Magnitude measures energy at the source. Intensity measures the effect at a specific location — and it varies enormously with distance, depth, local geology, and building stock. A M6.0 earthquake at 5 km depth directly below a city on soft sediment can cause catastrophic damage. The same M6.0 at 200 km depth beneath the same city may go unfelt. The Modified Mercalli Intensity (MMI) scale, running from I (imperceptible) to XII (total destruction), captures this spatial variation. USGS ShakeMap products combine magnitude, depth, fault geometry, and site amplification to produce real-time MMI maps within minutes of a significant event.
| MMI | DESCRIPTION | TYPICAL Mw AT EPICENTRE |
|---|---|---|
| I–II | Not felt / barely felt by some | M2–M3 |
| III–IV | Felt indoors; hanging objects swing | M3–M4 |
| V | Felt by most; dishes rattle, liquids spill | M4–M5 |
| VI | Felt by all; slight damage to poorly built structures | M5–M5.5 |
| VII | Damage to ordinary structures; chimneys fall | M5.5–M6 |
| VIII–IX | Considerable damage; buildings collapse | M6–M7 |
| X–XII | Most structures destroyed; ground rupture visible | M7+ |
A single magnitude number freezes one moment in a dynamic fault system. The time simulation below shows how magnitude events evolve across a seismic sequence — the mainshock spike followed by the power-law decay of aftershocks (Omori's Law), each dot scaled by its magnitude. Watching the sequence unfold makes the logarithmic nature of the scale viscerally clear: the aftershocks that persist for weeks are releasing a vanishing fraction of the mainshock's energy, even when they are themselves damaging M5–M6 events.
| RANK | LOCATION | DATE | Mw | NOTES |
|---|---|---|---|---|
| 1 | Valdivia, Chile | 1960-05-22 | 9.5 | Largest ever; triggered Pacific-wide tsunami |
| 2 | Prince William Sound, Alaska | 1964-03-28 | 9.2 | Good Friday earthquake; 139 deaths, major tsunami |
| 3 | Sumatra–Andaman | 2004-12-26 | 9.1 | Indian Ocean tsunami; ~230,000 deaths |
| 3 | Tōhoku, Japan | 2011-03-11 | 9.0 | Fukushima; largest in Japanese recorded history |
| 5 | Kamchatka, Russia | 1952-11-04 | 9.0 | Pacific-wide tsunami, no major casualties |
| 6 | Maule, Chile | 2010-02-27 | 8.8 | 525 deaths; significant infrastructure damage |
Magnitude is a measure of source energy — not of what will happen to buildings and people. The 2010 Haiti earthquake (M7.0) killed over 200,000 people. The 2011 Christchurch earthquake (M6.3) killed 185. A M9.5 in the middle of the Pacific Ocean causes no casualties. Damage depends on depth, distance, local soil conditions, building quality, and population density. Always read magnitude alongside depth and location — never in isolation.
The magnitude scale has no lower bound. Modern sensitive networks routinely detect events down to M−1 or even M−2 — tiny stress releases along fractures that release less energy than a hammer blow. Mines, quarry blasts, and even large trucks can register as M1–M2 events on local networks. At the top end, the scale is also open — though practical limits exist because fault area and slip cannot grow indefinitely. The largest theoretically possible earthquake on Earth is estimated around M10, which would require a rupture spanning an entire tectonic plate boundary simultaneously.
In the 3D simulations above, earthquake events are rendered as spheres scaled by magnitude — but because the scale is logarithmic, the visual sizing uses a cube-root transform so differences remain perceptible across the full M2–M9 range. A raw linear mapping would make M7+ events so large they would obscure the entire globe.
The dashboard breaks down the current catalog by magnitude class in real time, showing the characteristic Gutenberg-Richter distribution: for every M7 event, there are roughly 10 M6 events, 100 M5 events, and 1,000 M4 events — the power-law fingerprint of a self-organised critical system.