Earthquake magnitude is a logarithmic estimate of the size of the source. Each earthquake has one source magnitude at a given stage of review, although the value and magnitude type can change as scientists receive more data. Magnitude does not directly state local shaking, damage or risk.
Quick answer
- One magnitude unit represents about ten times the traditional measured wave amplitude and roughly 32 times the released energy.
- Mw, moment magnitude, is preferred for comparing large earthquakes because it is based on seismic moment.
- ML, local magnitude, is the original Richter-type scale and remains useful for many local events.
- Magnitudes can be zero or negative because the scale is logarithmic relative to a reference signal.
- Labels such as “minor” or “major” are descriptive conventions, not impact forecasts.
What earthquake magnitude means
Magnitude compresses a very wide range of earthquake source sizes into manageable numbers. A linear scale would make the largest and smallest recorded events awkward to compare, so seismology uses logarithmic relations.
The number belongs to the earthquake source, not to a city. Local effects are described with earthquake intensity, ground-motion measurements and products such as ShakeMap. Depth, distance, rupture direction, local ground and buildings all influence what people experience.
The familiar phrase “Richter scale” refers most precisely to local magnitude, ML. Modern catalogs publish several types, including ML, mb, Ms and Mw. The focused Richter versus moment magnitude guide explains why those labels should not be collapsed into one name.
How magnitude is determined
Seismic stations record ground motion as waveforms. Different magnitude methods use different parts of those recordings and different distance ranges.
| Type | Main input | Typical use |
|---|---|---|
| ML | Calibrated local wave amplitude | Small and regional events |
| mb | Short-period body waves | Rapid estimates at teleseismic distances |
| Ms | Surface-wave amplitude | Shallow, larger events within its useful range |
| Mw | Seismic moment from waveform or source analysis | Large earthquakes and global comparison |
| Md | Signal duration | Some local networks and small events |
Seismic moment combines rock rigidity, the fault area that slipped and average slip. Mw converts that physical estimate to a magnitude scale broadly compatible with older scales within their useful ranges.
An early automatic result may use the information available first. Later station data, improved phase picks or a more complete source model can change both the number and its type. A revision does not mean the earthquake itself changed.
How to read magnitude classes
Class names vary among organizations. The chart below is a communication aid, not an official damage scale.
| Magnitude | Common descriptive class | Source-size interpretation |
|---|---|---|
| Below 2.0 | Micro | Often recorded only by nearby instruments |
| 2.0–3.9 | Minor | Commonly detected; local felt reports vary |
| 4.0–4.9 | Light | Clearly recorded regionally and globally in many settings |
| 5.0–5.9 | Moderate | Substantially larger source than M4 |
| 6.0–6.9 | Strong | Large source; consequences still depend on exposure and shaking |
| 7.0–7.9 | Major | Very large rupture potential |
| 8.0+ | Great | Exceptional source size |
Moving from M5 to M6 is not a 20 percent increase. The standard approximation gives about 10 times the wave amplitude and 32 times the energy. M5 to M7 is about 100 times the amplitude and roughly 1,000 times the energy.
Limits and common mistakes
Magnitude is not intensity. It does not vary from neighborhood to neighborhood.
A class name does not predict damage. A deep offshore event and a shallow event beneath a populated area can share a magnitude yet produce very different observations.
Decimal precision is not certainty. A displayed M6.1 remains an estimate with method and data limitations.
Different types are not guaranteed to match exactly. They sample different wave periods and use different calibrations. The type code is meaningful metadata.
Common questions
What is the highest earthquake magnitude?
The largest instrumentally recorded earthquake was M9.5 in Chile on May 22, 1960. The scale has no fixed numerical ceiling, but possible fault dimensions and slip impose physical limits. USGS explains the record and physical limits.
Is a magnitude 10 earthquake possible?
USGS says no: no known fault is long enough to generate a magnitude 10 earthquake. A mathematically open scale does not mean every number is physically possible.
Why can magnitude be negative?
The logarithmic scale has a reference level. Signals smaller than that reference can produce negative values; the earthquake still released positive energy.
Which magnitude should I compare?
Use the source’s reported type and review status. Mw is generally best for comparing large global events, while ML and other types can be appropriate in their calibrated ranges.