Earthquake intensity describes how strongly the ground shook, or what effects were observed, at a particular place. A single earthquake can therefore have many intensity values. Magnitude is different: it estimates the size of the earthquake source and is reported as one value for the event.
Quick answer
- Modified Mercalli Intensity (MMI) runs from I to XII.
- Lower levels describe weak or unfelt motion; higher levels describe increasingly severe effects.
- Intensity changes with location, distance, local ground conditions and the characteristics of the rupture.
- MMI cannot be converted from magnitude with one reliable formula.
- ShakeMap combines observations and models to map estimated shaking; it is not a map of confirmed damage.
Compare magnitude, depth and intensity in the overview →
What earthquake intensity means
Intensity answers a local question: what shaking or effects occurred here? An observer, instrument or model can assign an intensity for one site while a different site receives another value from the same earthquake.
That variation is expected. Seismic waves lose and redistribute energy as they travel. Distance from the ruptured fault matters, but so do rupture direction, wave frequency, duration, basin geometry, soil and rock. Reports of effects can also reflect the vulnerability of buildings and objects at the observation point.
Magnitude instead estimates source size. An Mw 6.5 earthquake does not become M 5 in a distant city; what decreases and varies is the local shaking. See the focused earthquake magnitude scale guide for the logarithmic source scale.
How intensity is determined
MMI began as a macroseismic scale based on what people observed and how structures and objects responded. Modern products can combine several kinds of evidence:
- reports describing felt motion and effects;
- measured peak ground acceleration and velocity from seismic instruments;
- modeled shaking where observations are sparse;
- information about the fault geometry and local site response.
USGS ShakeMap represents ground motion and shaking intensity following significant earthquakes. A map can change as more station data, felt reports or a revised earthquake solution arrive. Areas without nearby observations depend more heavily on interpolation and ground-motion models.
Other regions use different systems. Japan’s JMA seismic intensity scale has ten steps from 0 through 7, including lower and upper subdivisions at 5 and 6. EMS-98 uses twelve degrees and explicitly considers building vulnerability classes when evaluating observed effects. These systems are related in purpose but are not interchangeable labels.
How to interpret MMI I to XII
The table is a compact reading aid, not a substitute for the full USGS descriptions.
| MMI | Broad description | Typical interpretation |
|---|---|---|
| I | Not felt | Generally not felt; a very small number of people may notice it in exceptional conditions |
| II–III | Weak | Felt by some people, especially indoors |
| IV | Light | Felt by many indoors; hanging objects may move |
| V | Moderate | Felt by nearly everyone; some unstable objects may move |
| VI | Strong | Felt by all; limited damage can occur in vulnerable structures |
| VII | Very strong | Damage varies with construction quality and vulnerability |
| VIII–IX | Severe to violent | Substantial effects can occur across vulnerable building stock |
| X–XII | Extreme | Progressively exceptional and widespread effects |
Roman numerals help prevent MMI from being confused with decimal magnitude. Writing “intensity VI” means a local shaking category; writing “M6.0” means a source magnitude.
Limits and common mistakes
Do not treat intensity as a radius. Equal-intensity contours can be irregular because faults, waves and local geology are irregular. A simple circle around an epicenter cannot reproduce a ShakeMap.
Do not infer damage from magnitude alone. Damage depends on shaking at exposed structures and their vulnerability. A magnitude value does not supply those inputs.
Do not treat an early map as final. Automatic locations, magnitudes and modeled shaking may be revised. Sparse observations increase uncertainty.
Do not compare MMI, JMA and EMS-98 number for number. Each has its own definitions and assignment practice.
Common questions
Is earthquake intensity measured from 1 to 10?
Not in MMI. Modified Mercalli uses twelve degrees, I through XII. The JMA scale has ten named steps, but its numbering and definitions are different.
Can a small earthquake have high intensity?
A smaller, shallow earthquake very close to an observation point can produce noticeable local intensity. Intensity still depends on more than magnitude, so no fixed conversion applies.
Why does one earthquake have several MMI values?
Because intensity belongs to locations, not to the source alone. Each observation or mapped cell describes local shaking or effects.
Does a ShakeMap show actual damage?
No. It maps observed and estimated ground motion and intensity. Damage information requires separate observations and exposure or vulnerability analysis.