Modified Mercalli Intensity (MMI) Scale

An effects-based measure of shaking at a specific location — what people feel, hear, and see.

The Modified Mercalli Intensity scale measures the effects of shaking at a specific location: what people feel and hear, what they observe moving or breaking, and what structural damage occurs. Unlike magnitude — a single number describing energy released at the source — MMI varies from place to place depending on distance from the epicenter, local geology, and building construction.

Instrumental Intensity is the term used by USGS ShakeMap for MMI values derived from recorded ground motion rather than direct human reports. ShakeMap translates peak ground acceleration (PGA) and peak ground velocity (PGV) into the MMI scale using Ground-Motion–to–Intensity Conversion Equations (GMICEs). Intensity Lab uses the Worden et al. (2012) GMICE. These are instrument-derived estimates that draw on the same observational tradition as the original scale, extended to locations where seismographs are present but human observers may not be.

ShakeMap also integrates Did You Feel It? (DYFI) macroseismic data — crowdsourced shaking reports from people who experienced the earthquake — as additional constraints on the intensity map. ShakeMap values are approximate and typically improve over the hours and days following an event as more seismic station data and DYFI reports become available.

You are the sensor

The MMI scale is unique because it is based on human observation. You don't need a multi-million dollar seismometer to estimate the intensity at your location — your own experience is the measurement. Like the Beaufort Scale for wind — which grades conditions from "calm" to "hurricane" based on observable effects such as smoke drift, tree movement, and wave height — or the Douglas Sea Scale for ocean waves — which describes sea state from "glassy" to "phenomenal" using observations like whitecaps visible, spray, and foam streaks — MMI relies on what you see, hear, and feel.

If you felt an earthquake, select the level below that best matches your experience. Then report it to USGS Did You Feel It? to contribute to the scientific record.

Select a Level

Color Scale

I II III IV V VI VII VIII IX X

Not felt Moderate Extreme

How the scale works at different levels

At lower levels (I–IV), MMI is based almost entirely on human perception: whether people feel shaking at all, and how strongly. The descriptions center on sensory experience — whether someone asleep is awakened, whether dishes rattle, whether the shaking feels like a passing truck.

At higher levels (VII–X), human perception saturates — everyone is aware of severe shaking — and the descriptions shift to structural damage as the distinguishing criterion: the difference between cracked plaster and toppled chimneys, or between partial collapse and complete destruction. This dual nature reflects the original intent of the scale.

Quick Reference

Intensity	Shaking	Key Observations
I	Not felt	Not felt except by a very few under especially favorable conditions.	Details →
II	Weak	Felt only by a few persons at rest, especially on upper floors of buildings.	Details →
III	Weak	Felt quite noticeably by persons indoors. Many people do not recognize it as an earthquake. Vibrations similar to the passing of a truck.	Details →
IV	Light	Felt by many indoors, few outdoors. At night some awakened. Sensation like a heavy truck striking the building.	Details →
V	Moderate	Felt indoors by practically all, outdoors by many. Many people awakened; a few run outdoors. Buildings tremble throughout.	Details →
VI	Strong	Felt by all; many frightened. Some run outdoors. Walking feels unsteady. Damage slight.	Details →
VII	Very strong	Difficult to stand. Noticed by drivers. Damage negligible in well-built structures; considerable in poorly built ones.	Details →
VIII	Severe	Damage slight in specially designed structures; considerable with partial collapse in ordinary buildings; great in poorly built structures.	Details →
IX	Violent	General panic. Well-designed structures thrown out of plumb; great damage in substantial buildings with partial collapse. Buildings shifted off foundations.	Details →
X	Extreme	Most masonry and frame structures destroyed with foundations. Wooden structures and bridges destroyed. Large landslides. Rails bent greatly.	Details →

Descriptions are verbatim from the USGS MMI Scale, based on Wood & Neumann (1931) and Richter (1958). Source: USGS Modified Mercalli Intensity Scale .

Instrumental intensity values and PGA/PGV ranges from USGS ShakeMap V4 Technical Documentation (Worden et al.). Intensity Lab reports MMI derived via the Worden et al. (2012) GMICE, interpolated from USGS ShakeMap. PGA/PGV ranges are approximate; actual values overlap between adjacent levels.