What is the difference between the Richter scale and the Mercalli scale?
The Richter scale is a magnitude scale: it measures the size of an earthquake at its source, and one earthquake has one magnitude. The Mercalli scale is an intensity scale: it measures the shaking and observable damage at a particular place, so the same earthquake produces many different intensity values.
Two numbers get quoted after nearly every earthquake, and they answer different questions. Magnitude answers how big the quake was at its source. Intensity answers how strongly a specific place shook. The Richter scale belongs to the first family. The Modified Mercalli Intensity scale, used in the United States, belongs to the second. Confusing the two is common, and it leads people to expect the same damage everywhere from a single number.
[2]What the Richter scale actually measures
The logarithmic magnitude scale was first developed by Charles Richter in the 1930s for measuring earthquakes in southern California, using relatively high-frequency data from nearby seismograph stations. That original scale is referred to as ML. It was based on the maximum amplitude of a seismogram recorded on a Wood-Anderson torsion seismograph. Those instruments are no longer widely in use, but ML values are still calculated using modern instrumentation with appropriate adjustments.
[3]A key point: an earthquake has one magnitude. The magnitude does not depend on where the measurement is made. That said, slightly different magnitudes are often reported for the same event, because the relationship between seismic measurements and magnitude is complex and different procedures can give slightly different results. So a small spread between reported figures is normal, not a contradiction.
[2]Today, ML is only authoritative for smaller events, typically below magnitude 4.0, where no other magnitude type is available. For larger events, agencies such as the USGS rely on moment magnitude methods. So when you see a modern magnitude for a large earthquake, it is usually not a Richter number at all, even though the public still uses Richter's name for it.
[3]What the Mercalli scale measures
The Modified Mercalli Intensity scale measures the amount of shaking at a particular location. An earthquake causes many different intensities of shaking across the area near the epicenter, so the intensity varies depending on where you are. Sometimes earthquakes are referred to by the maximum intensity they produce, which can make it sound like a single value when it is really the top of a range.
[2]Mercalli is based on observable earthquake damage. At levels I through V, the observable effects are small: doors rattle, dishes break, and weak or poor plaster cracks. As the level rises, damage increases considerably. Intensity X (10) is the highest value on the MMI scale. Because it rests on observation, the USGS notes that Mercalli data can be subjective, while magnitude, being based on seismic records, is considered scientifically more objective and therefore more accurate.
[2]A concrete comparison
Imagine a moderately large earthquake with its epicentre in a rural area. A news report gives one magnitude figure for the whole event. Then reports come in from different towns. Near the epicentre, plaster cracks and dishes break, which sits in the lower Mercalli range. Further away, people report only rattling doors, a lower intensity still. One earthquake, one magnitude, several intensities. The reverse also happens: a moderate quake directly under a sedimentary valley can produce longer, stronger shaking at one spot than a larger quake farther away, because shaking duration and strength at any point depend on how the waves move through the ground there.
[2][4]| Aspect | Richter (magnitude, ML) | Modified Mercalli (MMI) |
|---|---|---|
| What it measures | Size of the earthquake at its source | Amount of shaking at a particular location |
| How many values per quake | One magnitude | Many intensities across the affected area |
| Basis | Seismic records, originally Wood-Anderson seismograph amplitudes | Observable damage and effects, which can be subjective |
| Range of use today | Authoritative mainly for smaller events, typically under M4.0 | Used in the United States; highest value is intensity X (10) |
Limits worth knowing
- A magnitude number alone does not tell you the damage at any given place; that depends on distance, ground conditions and other factors.
- Damage at a site depends on both the amplitude of shaking and its duration, and how best to combine those into a damage estimate is ongoing research.
- Because Mercalli relies on observation, two people at the same location can describe effects differently.
When you next see an earthquake listed somewhere, check which number is being quoted. If it is a magnitude, remember it describes the source, not your street. If it is an intensity, remember it describes one place, not the whole event. Comparing magnitudes across records also means checking that the same scale and source context apply, since for very old earthquakes magnitudes have sometimes been estimated from physical effects rather than instrument readings.
[2]Sources and dates
- https://www.usgs.gov/programs/earthquake-hazards/determining-depth-earthquake · accessed 2026-10-18
- https://www.usgs.gov/faqs/what-difference-between-earthquake-magnitude-and-earthquake-intensity-what-modified-mercalli · accessed 2026-10-18
- https://www.usgs.gov/programs/earthquake-hazards/magnitude-types · accessed 2026-10-18
- https://www.usgs.gov/faqs/what-was-duration-earthquake-why-dont-you-report-duration-each-earthquake-how-does-duration · accessed 2026-10-18