Intensity: what the shaking felt like
Magnitude is one number for the whole earthquake. Intensity is a different number for every place — how hard the ground actually shook where you were standing, and what that did to the building around you.
One earthquake therefore has many intensities: strong near the epicentre, weaker further away, and sometimes anomalously strong a long way off where soft ground amplifies the shaking.
Why Roman numerals
The Modified Mercalli scale is written I to XII in Roman numerals precisely so nobody confuses it with a magnitude. It is not measured by an instrument in the first instance; it is a description of observed effects, which is why the scale reads like a set of sentences rather than a formula.
| Intensity | What people experience |
|---|---|
| I | Not felt. |
| II–III | Felt by a few people at rest, or on upper floors. Standing cars may rock slightly. |
| IV | Felt indoors by many. Dishes and windows rattle; a parked car rocks noticeably. |
| V | Felt by nearly everyone. Some dishes and windows break; unstable objects overturn. |
| VI | Felt by all, many frightened. Some heavy furniture moves; slight damage. |
| VII | Considerable damage to poorly built structures; negligible damage to well-built ones. |
| VIII | Considerable damage to ordinary buildings; chimneys and walls fall. |
| IX | Considerable damage even to well-designed structures; buildings shifted off foundations. |
| X–XII | Most masonry destroyed; rails bent; total destruction at the extreme. |
Abbreviated from the USGS description of the Modified Mercalli Intensity scale. The full wording is on the USGS page linked below and is worth reading if you are trying to place what you felt.
The two intensities on a QuakeIndex event page
You may see two different intensity figures, and they come from different places:
- Reported intensity (CDI) — computed from Did You Feel It? reports submitted by the public. It exists only where people filled in the form, and it can appear within minutes of a felt event.
- Measured intensity (MMI) — from ShakeMap, which USGS computes from instrument recordings combined with a model of how shaking travels through local ground. It does not need anybody to report anything.
They often differ, and neither is wrong. A reported intensity reflects what people noticed; a measured one reflects what the ground did.
Why the same earthquake feels different across one city
Soft sediment amplifies shaking, and deep soft basins can trap and prolong it. Two neighbourhoods a few kilometres apart, at the same distance from the same epicentre, can experience intensities a whole step or two apart. This is the single biggest reason a magnitude alone does not tell you what happened to a given building, and it is why ground conditions are part of how earthquake insurance is priced.
Reporting what you felt actually matters
Did You Feel It? is not a comment box. The reports are aggregated into the official intensity maps used by emergency managers, and for small and moderate earthquakes they are often the best information available about what the shaking was like. Reports go to USGS, not to us.
Report an earthquake you felt to USGS →
Sources
Last checked:
- The Modified Mercalli Intensity scale — USGS
- What is the difference between magnitude and intensity? — USGS
- Did You Feel It? — USGS
- ShakeMap — USGS