Earthquake Scales Decoded
Two numbers, two completely different questions
When a news headline says "magnitude 7.6 earthquake", and a damage report says "MMI IX in the city centre", they are measuring different things. Magnitude is a single number assigned to the earthquake itself, measuring the energy it releases at the source. It does not change with distance. Intensity is a local measure of how hard the ground shook at a specific place, and it changes from one street to the next. One earthquake has one magnitude. The same earthquake has dozens of intensities, highest near the epicentre and fading with distance. This reference explains both scales, how they relate, and why the magnitude alone rarely predicts how deadly an earthquake will be.
Richter is obsolete: use Moment Magnitude
The scale journalists still call "the Richter scale" has not been standard seismology since the 1970s. Charles Richter devised his local magnitude scale (ML) in 1935 for small earthquakes in California, using a specific type of seismograph. It saturates above about M6.5: two earthquakes that differ enormously in energy both register as "6.8" because the instrument and formula run out of resolution. The Moment Magnitude Scale (Mw), developed by Kanamori and Hanks, replaced it. Mw measures the total seismic moment: the area of the fault that ruptured multiplied by how far it slipped, multiplied by the rock's rigidity. It works at all sizes, from micro-tremors to M9+ megathrust events. When USGS or any national seismic network reports a magnitude today, it is Mw. The numbers are calibrated to be numerically similar to Richter up to about M6.5, which is why the press still uses "Richter" and gets away with it for most common earthquakes.
The logarithmic scale: what each step actually means
Both scales are logarithmic. Each whole number step is not "one more unit of shaking" but a fixed multiplier. Specifically: each whole number increase means ground motion (amplitude) increases ten times, and energy released increases 31.6 times (because 10 to the power of 1.5 equals 31.6). The table below makes the gap concrete.
What each magnitude step multiplies
| Difference | Ground motion (amplitude) | Energy released |
|---|---|---|
| +0.5 | 3.2 times | 5.6 times |
| +1.0 | 10 times | 31.6 times |
| +2.0 | 100 times | 1,000 times |
| +3.0 | 1,000 times | 31,600 times |
| +4.0 | 10,000 times | 1,000,000 times |
Moment Magnitude: the full scale in one table
Below M2.0 earthquakes are micro-tremors, almost always undetectable without instruments. M5.0 is roughly where structural damage begins in poorly built areas. Above M8.0 events are rare: only a few per decade globally. The largest ever measured was the 1960 Valdivia earthquake in Chile at M9.5.
Moment Magnitude (Mw) reference scale
| Mw range | Category | Typical effects | Global frequency (approx) |
|---|---|---|---|
| Below 2.0 | Micro | Detected by instruments only | Millions per year |
| 2.0 to 3.9 | Minor | Felt by sensitive people indoors; no damage | ~130,000 per year |
| 4.0 to 4.9 | Light | Felt by most indoors; minor objects move; rare damage | ~13,000 per year |
| 5.0 to 5.9 | Moderate | Felt widely; damage to poorly built structures | ~1,300 per year |
| 6.0 to 6.9 | Strong | Damage in populated areas; poorly built structures collapse | ~130 per year |
| 7.0 to 7.9 | Major | Widespread damage; casualties in populated areas | ~15 per year |
| 8.0 to 8.9 | Great | Serious damage over large regions | ~1 per year |
| 9.0 and above | Extreme | Devastating over very large areas; can trigger tsunamis | Very rare |
The Modified Mercalli Intensity Scale (MMI): I to XII
Where magnitude describes the earthquake, the Modified Mercalli Intensity Scale (MMI) describes what people and structures experience at a specific location. It runs from I (not felt) to XII (total destruction) and is assigned after the event using reports from residents and engineers. The same earthquake can be MMI II in a distant city and MMI IX at the epicentre. At lower levels (I to V) the scale relies on human perception. At higher levels (VII to XII) it grades structural damage, so engineers typically contribute the high-end assessments.
Modified Mercalli Intensity (MMI) Scale I to XII
| MMI | Shaking | What people and structures experience |
|---|---|---|
| I | Not felt | Detected only by sensitive instruments |
| II | Weak | Felt by very few people at rest, especially on upper floors |
| III | Weak | Felt indoors by some; hanging objects swing; like a passing truck |
| IV | Light | Felt indoors by many, outdoors by few; dishes rattle; walls creak |
| V | Moderate | Felt by almost everyone; small objects shift; some windows break |
| VI | Strong | Felt by all; difficult to stand; some heavy furniture moves; minor plaster damage |
| VII | Very strong | Negligible damage in well-built structures; moderate in ordinary buildings; chimneys crack |
| VIII | Severe | Considerable damage in ordinary buildings; partial collapse in poorly built ones |
| IX | Violent | Considerable damage in well-built structures; frames thrown out of plumb |
| X | Extreme | Most masonry and frame structures destroyed; ground cracks badly |
| XI | Extreme | Rails bent; underground pipes out of service; near-total structural collapse |
| XII | Extreme | Total damage; ground waves visible; objects thrown into the air |
Why magnitude alone does not predict casualties
Three factors matter at least as much as magnitude when estimating damage and deaths. Depth: a shallow earthquake (under 30 km) hits the surface with far less energy dissipated than a deep one at 300 km. Proximity: a M6.0 directly under a city is more destructive than a M7.5 in open ocean. Building quality: unreinforced masonry collapses at MMI VII; a modern seismically designed building may survive MMI IX. The 2010 Haiti earthquake was M7.0 and killed over 100,000 people. The 2010 Chile earthquake was M8.8 and killed 525. Chile had depth, distance from population centres, and enforced building codes. Haiti had none of those things. The practical test: look at the magnitude for a sense of scale, the MMI map for where damage actually fell, and the depth and population density for why.
Magnitude vs. intensity in one sentence: magnitude is the size of the event, the same number wherever you are; intensity is what the ground does at your specific address, and it decreases with distance. A single earthquake has one magnitude and many intensities.
Notable benchmarks to anchor the scale
The numbers below appear frequently in news coverage. They are worth memorising as
anchors so that any reported magnitude lands somewhere concrete.
M9.5, Valdivia, Chile, 1960: the largest ever recorded; triggered tsunamis across
the Pacific and altered the Chilean coastline permanently.
M9.1, Sumatra, Indonesia, 2004: triggered the Indian Ocean tsunami that killed
roughly 228,000 people across 14 countries.
M7.9, San Francisco, California, 1906: 3,000 deaths; helped establish modern
seismology and building codes in California.
M7.0, Haiti, 2010: shallow, directly under Port-au-Prince, poorly built city;
over 100,000 deaths despite moderate magnitude.
M6.9, Loma Prieta, California, 1989: 63 deaths, collapsed the Bay Bridge section
and the Cypress Viaduct; a modern infrastructure benchmark.
M6.0, Napa Valley, California, 2014: the largest in the San Francisco Bay Area
since 1989; $1 billion in damage, 1 death, due to building improvements.