Modified Mercalli Intensity (MMI) Scale

An effects-based measure of shaking at one place: 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 the energy released at the source, intensity varies from place to place with distance from the earthquake, local geology, and building construction.

USGS ShakeMap also estimates intensity from recorded ground motion; that is instrumental intensity, on the same scale with the same colors.

You are the sensor

The MMI scale is based on human observation. You do not need a 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" by 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" by whitecaps, 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

How the scale works at different levels

At lower levels (I–IV), intensity rests 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), perception saturates, since everyone is aware of severe shaking, and the descriptions turn to structural damage: 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	Description and damage
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, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated.	Details →
IV	Light	Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.	Details →
V	Moderate	Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.	Details →
VI	Strong	Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.	Details →
VII	Very strong	Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.	Details →
VIII	Severe	Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.	Details →
IX	Violent	Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.	Details →
X	Extreme	Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.	Details →

Descriptions are verbatim from the USGS abridged Modified Mercalli Intensity table, based on Wood & Neumann (1931) and Richter (1958). Source: USGS Modified Mercalli Intensity Scale. Each level's page lists its effects in more detail.

Instrumental intensity and PGA/PGV ranges follow the USGS ShakeMap documentation (Worden et al.). Intensity Lab reports MMI interpolated from USGS ShakeMap.