How Earthquake Magnitude Actually Works (M4 vs M7 Is Not What You Think)

Direct answer: earthquake magnitude measures the energy released at the source on a logarithmic scale. Going up one whole number — M6 to M7 — releases roughly 32 times more energy, and two whole numbers is about 1,000 times more. But magnitude is only half the story: a shallow M6 directly beneath a city can kill far more people than a deep M7.5 out at sea.

What magnitude actually measures

Every agency you see quoted today — USGS, EMSC, the Japan Meteorological Agency — reports moment magnitude (Mw) for significant earthquakes. Moment magnitude is derived from the seismic moment: the area of fault that broke, multiplied by how far the two sides slipped, multiplied by the stiffness of the rock. In plain terms, it measures how much rock moved, and by how much.

That replaced Charles Richter's 1935 local magnitude scale for large events, because the Richter scale saturates: above roughly M7 it stops distinguishing between earthquakes that are in reality wildly different sizes. The 1960 Chile earthquake and the 2004 Sumatra earthquake would both read as roughly "7-and-a-bit" on Richter, when the first released about four times the energy of the second. News outlets still say "on the Richter scale" out of habit — the number they are quoting is almost always moment magnitude.

The 32x rule, and why it matters

Because the scale is logarithmic, the jumps get brutal quickly:

• M5 to M6 — about 32 times the energy
• M5 to M7 — about 1,000 times
• M5 to M8 — about 32,000 times
• M5 to M9 — about 1,000,000 times

This is why a headline like "a series of magnitude 4 aftershocks are releasing the pressure" is physically wrong. It would take roughly a million M4 earthquakes to release the energy of a single M8. Small earthquakes do not meaningfully prevent large ones.

The fault dimensions scale too. An M5 typically ruptures a patch a couple of kilometres across. An M7 breaks something on the order of 50 km long. An M9 tears a rupture that can run more than 1,000 km — the 2004 Sumatra-Andaman rupture propagated for around ten minutes.

Magnitude vs intensity: the number that actually predicts damage

An earthquake has one magnitude but many intensities. Intensity describes shaking experienced at a specific place, and it is reported on scales like Modified Mercalli (MMI, I to XII) or the JMA Shindo scale in Japan. Intensity falls off with distance and is the number that correlates with buildings falling down.

Four things push intensity up at your location:

• Depth. Earthquakes shallower than about 70 km put their energy into the surface. Deep earthquakes — some occur more than 600 km down inside subducting slabs — can be enormous yet barely felt.
• Distance. Shaking attenuates fast. Being 20 km from an M6 is worse than being 200 km from an M7.
• Ground conditions. Soft sediment and reclaimed land amplify shaking and can liquefy. Mexico City's catastrophic 1985 damage came from an earthquake roughly 350 km away, because the old lake-bed sediment under the city resonated with the incoming waves.
• What is built on top. Unreinforced masonry and soft-storey concrete frames fail at intensities that engineered buildings shrug off.

Why the magnitude changes in the first hour

The first number an agency publishes is automatic — a computer estimate from the first stations to record the P-wave, issued within a minute or two. As data arrives from more distant stations, and as seismologists model the full rupture, the estimate is refined. Revisions of 0.2 to 0.5 are completely normal, and large events are often revised upward because the early estimate had not yet "seen" the whole rupture.

This site shows the current published magnitude from the source agency and the timestamp of that revision, so you can tell an hour-old automatic solution from a reviewed one.

Is a magnitude 10 possible?

Not from a tectonic fault. Magnitude is bounded by the size of the fault that can break at once, and no continuous fault on Earth is long enough to produce an M10 — you would need a rupture running for thousands of kilometres. The largest earthquake ever instrumentally recorded remains the 1960 Valdivia earthquake in Chile at M9.5.

How to read a magnitude number properly

When you see an alert, ask three questions in this order: how deep, how far from people, and what is built there. That trio predicts consequences far better than magnitude alone. It is also why our event pages always print depth beside magnitude, and why GDACS can rate a large earthquake Green — see how GDACS alert colours work.

Track current activity on the live earthquake page, and know what to do before it happens with the earthquake safety guide.

Frequently asked questions

Is a magnitude 10 earthquake possible?

Not from a tectonic fault. Magnitude depends on how much fault area breaks at once, and no fault on Earth is long enough to reach M10. The largest instrumentally recorded earthquake is the 1960 Chile earthquake at M9.5.

Why do magnitude estimates change after an earthquake?

The first figure is an automatic computer estimate from the earliest seismic waves, published within a minute or two. Seismologists refine it as data arrives from more stations, so revisions of 0.2 to 0.5 in the first hours are normal — and large events are often revised upward.

How much stronger is a magnitude 7 than a magnitude 6?

About 32 times more energy released at the source, and roughly 10 times larger wave amplitude on a seismogram. From M5 to M7 is about 1,000 times more energy.

Do small earthquakes prevent big ones?

No. Because the scale is logarithmic, it would take roughly a million magnitude 4 earthquakes to release the energy of one magnitude 8. Small events do not meaningfully relieve the strain stored on a large fault.

Which matters more, magnitude or depth?

For damage, depth and distance usually matter more. Shaking is strongest from earthquakes shallower than about 70 km; a shallow M6 under a city can be deadlier than a deep M7.5 offshore.

Sources

• USGS — Earthquake Magnitude, Energy Release, and Shaking Intensity
• USGS — Moment Magnitude, Richter Scale: what are the different magnitude scales?
• USGS — The Modified Mercalli Intensity Scale
• USGS — Largest earthquakes ever recorded

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