Reading earthquake data
Earthquake magnitude vs intensity
Magnitude and intensity are related, but they do not measure the same thing. One describes the earthquake; the other describes its effects at a particular place.
Magnitude is one estimate of the earthquake's overall size. Intensity varies from place to place and describes observed shaking and effects. One earthquake has one preferred magnitude but can produce many intensity values.
Magnitude measures the size of the earthquake
Magnitude is calculated from seismic recordings. Modern catalogs may report several magnitude types before selecting a preferred value. For large earthquakes, moment magnitude (Mw) is commonly used because it relates to the physical size of the fault rupture and does not saturate as quickly as some older scales.
Magnitude scales are logarithmic. According to the USGS, an increase of one whole magnitude corresponds to about ten times greater recorded wave amplitude and roughly 32 times more energy release. An M7 is therefore not simply “one unit stronger” than an M6.
Intensity measures shaking at a specific location
Intensity describes what people feel and what happens to objects, buildings and the natural environment at a location. In the United States it is commonly expressed with the Modified Mercalli Intensity scale; Europe commonly uses the European Macroseismic Scale. Both use Roman-numeral levels.
Intensity normally decreases with distance from the rupture, but distance is not the only factor. Earthquake depth, rupture direction, soft soil, basin effects and building vulnerability can all change the shaking experienced locally.
| Question | Magnitude | Intensity |
|---|---|---|
| What does it describe? | Overall earthquake size | Shaking and effects at one place |
| How many values? | One preferred magnitude for the event | Many values across affected locations |
| Based on | Instrumental seismic measurements | Observed effects, reports and instrumental shaking data |
| Common examples | Mw, ML, mb | Modified Mercalli, EMS-98 |
Why similar-magnitude earthquakes can feel different
A shallow earthquake directly beneath a city may cause stronger local shaking than a slightly larger but deeper or more distant event. Soft sediments may amplify motion compared with nearby bedrock. Construction quality also affects damage, so damage alone is not a reliable measurement of magnitude.
This is why an earthquake map's magnitude badge is useful for comparison but cannot tell you exactly how strongly every community shook. For local impact, look for official intensity maps or ShakeMap products in addition to magnitude and depth.
Does “Richter scale” mean magnitude?
The original local magnitude scale developed by Charles Richter was designed for a particular region and instrument range. The phrase remains common in everyday language, but agencies now use several magnitude calculations. A catalog entry labeled Mw, ML or mb is more precise than calling every value a “Richter” magnitude.
Frequently asked questions
Can an earthquake have more than one reported magnitude?
Yes. Different methods and additional data can produce revised values. Agencies normally identify one preferred magnitude while retaining other calculations in the event record.
Does a higher magnitude always mean more damage?
No. Damage also depends on depth, distance, local geology, duration, construction and preparedness. Magnitude is important, but it is not a complete damage forecast.
Can intensity be measured before people submit reports?
Instrument networks can estimate shaking rapidly. Public “Did You Feel It?” reports then add observations about how shaking was experienced across different locations.