Richter vs Moment Magnitude: Why M7 Is Not Twice M3.5

Quick Answer: Richter vs Moment Magnitude

Almost every earthquake magnitude you read today is moment magnitude (Mw), not Richter, even when a news report says "on the Richter scale." Richter's original scale was built in 1935 for small Southern California earthquakes and stops working properly above about magnitude 7.

	Richter Scale (ML)	Moment Magnitude (Mw)
Introduced	1935, Charles Richter	1979, Hanks and Kanamori
Measures	Peak wiggle height on a seismogram	Physical size of the fault rupture
Built for	Southern California, one instrument type	Any earthquake, anywhere
Breaks down above	~M7 (saturates)	Does not saturate
Used today for	Some small local quakes	Essentially all significant earthquakes

And the number itself is logarithmic, which is where the intuition fails. M7 is not twice M3.5. It is roughly 3,000 times more ground motion and nearly 180,000 times more energy.

The Scale Is Logarithmic, and That Changes Everything

Every whole number step on the magnitude scale means:

• About 10 times more ground motion on a seismogram
• About 32 times more energy released at the source

The gap between those two numbers trips people up, so it is worth being precise. Magnitude was defined from wave amplitude, which scales by 10 per unit. Radiated energy climbs faster than that, because a bigger rupture also shakes for longer and at longer periods. The measured relationship between the two, log10 E = 4.8 + 1.5M, puts one magnitude unit at 10^1.5, which is about 31.6 times the energy.

Compounding that over a few steps gets extreme fast:

Magnitude Difference	Ground Motion	Energy Released
0.5	~3 times	~5.6 times
1.0	10 times	~32 times
2.0	100 times	~1,000 times
3.0	1,000 times	~32,000 times
3.5	~3,000 times	~178,000 times
4.0	10,000 times	~1,000,000 times

So a M7.0 releases roughly 178,000 times the energy of a M3.5. Put another way, you would need well over a hundred thousand M3.5 earthquakes to equal one M7.0, which is why small earthquakes do not meaningfully "release pressure" and prevent large ones. This is a genuinely common misconception, and the arithmetic is what kills it: a M5.0 releases about 1/1000th the energy of a M7.0, so a fault would need roughly a thousand M5s to substitute for a single M7.

Why the Richter Scale Was Retired

Charles Richter, working with Beno Gutenberg at Caltech, needed a way to compare Southern California earthquakes objectively. His 1935 solution was elegant for the problem in front of him: take the peak amplitude recorded on a specific instrument, the Wood-Anderson torsion seismograph, correct it for distance from the epicenter, and take the base-10 logarithm.

That design carried three limits that eventually made it unusable as a global standard:

1. It was calibrated for one region. Seismic waves lose energy differently depending on the rock they pass through. Richter's distance corrections were tuned to Southern California crust and are wrong elsewhere.
2. It was tied to one obsolete instrument. Modern broadband seismometers do not behave like a Wood-Anderson torsion instrument, so ML has to be simulated rather than measured.
3. It saturates. This is the fatal one.

What saturation actually means

The Richter scale measures the height of relatively short-period waves. Those waves are generated efficiently by small ruptures. But once a rupture becomes very large, it keeps breaking for tens of seconds along hundreds of kilometers of fault, and that additional size shows up as longer-period energy, not taller short-period wiggles.

The short-period amplitude effectively maxes out. Feed a M8.0 and a M9.0 through the Richter calculation and you get nearly identical answers, somewhere below M7. The scale simply cannot see the difference.

That mattered enormously in practice. It meant the largest and most destructive earthquakes on Earth, exactly the events where an accurate number matters most, were the ones the scale could not measure.

What Moment Magnitude Measures Instead

Thomas Hanks and Hiroo Kanamori proposed moment magnitude in 1979, built on a physical quantity called seismic moment. Instead of asking how big the wiggle was, it asks how big the earthquake was:

Seismic moment = rupture area × average slip × rigidity of the rock

That is, how much fault surface broke, how far the two sides slid past each other, and how stiff the rock is. Multiply those together and you get a direct measure of the work the earthquake did. Moment magnitude is then derived from seismic moment through a two-thirds logarithmic relationship, deliberately scaled so that the resulting numbers line up with familiar Richter values in the mid-range where Richter still worked.

That backward compatibility is why the switch was invisible to the public. A M5.5 means roughly the same thing it always did. Only the top of the scale changed, and it changed by finally becoming accurate.

Because seismic moment describes the rupture itself, there is no ceiling. A M9.5 (Chile, 1960, the largest ever instrumentally recorded) is measurable on the same scale as a M2.0.

The Alphabet Soup on USGS Event Pages

Open any earthquake on the USGS site or on our live tracker and you will see a small magnitude type code next to the number. It tells you which method produced the figure, which in turn tells you how much to trust it:

Code	Name	When It Is Used	Notes
mww	W-phase moment magnitude	Most events above ~M5.5	The preferred solution for significant earthquakes
mw, mwb, mwc, mwr	Moment magnitude variants	Moderate to large events	Different wave types, same underlying physics
mb	Body-wave magnitude	Global events roughly M4 to M6.5	Saturates above ~M6.5
ml	Local magnitude (Richter)	Small regional earthquakes	The original scale, still useful at small sizes
md or mc	Duration or coda magnitude	Very small local events	Based on how long shaking persists
ms	Surface-wave magnitude	Large shallow events	Largely superseded by moment magnitude

The practical rule: if you see mww on a large earthquake, you are looking at a moment magnitude solution rather than a saturated early estimate. If you see mb or ml on a large earthquake, you are looking at an early figure that will probably rise. For the very largest events the mww value itself keeps being refined for hours, so read it as much better rather than final. We cover that process in detail in why earthquake magnitudes change after the first report.

Magnitude Is Not Damage

This is the most consequential thing to understand about magnitude, and it is the reason a M6.2 sometimes kills thousands while a M7.8 elsewhere kills nobody.

Magnitude describes the earthquake at its source. What happens to you depends on:

Factor	Why It Matters
Depth	A shallow quake concentrates energy at the surface. A M6 at 10 km can be devastating; the same M6 at 300 km may barely be felt. See earthquake depth explained.
Distance	Shaking falls off quickly with distance from the rupture.
Local soil	Soft sediment and landfill amplify shaking dramatically and can liquefy. Bedrock does not.
Building stock	Unreinforced masonry fails catastrophically. Seismically engineered structures survive shaking that would flatten it.
Time of day	Occupancy patterns change casualty numbers enormously.
Rupture direction	Energy focuses in the direction the rupture travels, an effect called directivity.

The scale that describes what actually happened at a given place is Modified Mercalli Intensity, running from I (not felt) to XII (total destruction). One earthquake has one magnitude but many intensities, one for every location. USGS ShakeMap products map exactly that, and they are far more useful than magnitude for judging where damage is likely.

A Practical Magnitude Reference

Roughly how often each size occurs worldwide, and what it typically means:

Magnitude	Per Year (Global, Approx.)	What It Is Like
2.0 to 2.9	~1,300,000	Recorded but almost never felt
3.0 to 3.9	~130,000	Often felt nearby, no damage
4.0 to 4.9	~13,000	Noticeable indoor shaking, rattling objects
5.0 to 5.9	~1,300	Can damage poorly built structures
6.0 to 6.9	~130	Destructive in populated areas up to ~100 km across
7.0 to 7.9	~15	Major. Serious damage over large areas
8.0 and above	~1	Great. Devastating hundreds of kilometers from the source

Notice the shape of that distribution: drop one whole magnitude and earthquakes become roughly 10 times more common. That relationship is the Gutenberg-Richter law. Two caveats on the table above. The M8 row is open-ended rather than a decade band, so the step into it is not a like-for-like comparison, and at the very top the pattern flattens because there is a hard limit on how much fault can break at once.

FAQ

Is the Richter scale still used?

Rarely, and only for small local earthquakes where it is reported as local magnitude (ML). Every significant earthquake today is measured with moment magnitude. News coverage still says "Richter" out of habit, but the figure quoted is almost always moment magnitude.

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

A M7 produces about 10 times more ground motion and releases about 32 times more energy than a M6. Between M6 and M8, the energy difference is roughly 1,000 times.

Why is there no upper limit to the magnitude scale?

The scale is open-ended by design, but physics limits it in practice. Magnitude depends on how much fault can break at once, and USGS is blunt about the ceiling: no fault long enough to generate a magnitude 10 is known to exist, and one that could would have to extend around most of the planet. The largest instrumentally recorded earthquake remains the M9.5 in Chile in 1960, which ruptured roughly 1,000 km of fault, about 620 miles.

Can small earthquakes prevent a big one?

No. A M7 releases roughly 32,000 times the energy of a M4, so you would need tens of thousands of small earthquakes to release the equivalent strain. The energy arithmetic makes the idea impossible in practice.

Why does a smaller earthquake sometimes cause more damage?

Because damage depends on depth, distance, local soil, and building construction, not just magnitude. A shallow M6 beneath a city with unreinforced masonry can be far deadlier than a deep M7.5 offshore.

What does the magnitude type code mean?

It identifies which method produced the number. Codes starting with mw are moment magnitude solutions and are the most reliable for large events. Codes like mb and ml come from older amplitude-based scales that saturate, so on a large earthquake they usually indicate a preliminary figure.

Always Verify with Official Sources

Magnitude values are revised as analysis continues. For authoritative earthquake parameters and shaking estimates, use the USGS Earthquake Hazards Program, and follow your local emergency management agency for protective action.

Track Earthquakes with WeatherAI

WeatherAI puts seismic activity next to your weather on iPhone, iPad, Mac, and Vision Pro:

• Magnitude and distance filtering so you see the events that matter to you
• Nearby and global earthquake notifications with thresholds you set
• USGS or EMSC as your data source, whichever suits your region
• Full event detail and maps, plus tsunami bulletins
• Earthquake widgets for Home Screen and Lock Screen with your own minimum magnitude

WeatherAI reports earthquakes that have already been detected, so it is monitoring rather than earthquake early warning, and it does not replace official emergency alerts.

Download WeatherAI for iPhone →

---

Keep reading: why earthquake magnitudes change after the first report, why shallow earthquakes do more damage, and foreshocks, mainshocks and aftershocks. You can also browse the live earthquake tracker or look up terms in our earthquake glossary.