The Modified Mercalli Intensity scale
The Modified Mercalli Intensity (MMI) scale describes earthquake shaking at one place by its effects on people, objects and buildings, from I (not felt) to X (extreme), the range USGS reports. Unlike magnitude, which is one number per earthquake, intensity differs from place to place with distance, local ground and the kind of building.
The ten levels
| Level | What happens | Damage | Peak acceleration | Peak velocity | Clips |
|---|---|---|---|---|---|
| INot Felt | Not felt except by a very few under especially favorable conditions. | None | under 0.05% g | under 0.1 cm/s | – |
| IIWeak | Felt only by a few persons at rest, especially on upper floors of buildings. | None | 0.05–0.29% g | 0.1–1.1 cm/s | – |
| IIIWeak | 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. | None | 0.3–1.4% g | 1.1–3.4 cm/s | – |
| IVLight | 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. | None | 1.4–3.9% g | 3.4–8.1 cm/s | – |
| VModerate | Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop. | Very light | 3.9–9.2% g | 8.1–16 cm/s | 7 |
| VIStrong | Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight. | Light | 9.2–18% g | 16–31 cm/s | 18 |
| VIIVery 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. | Moderate | 18–34% g | 31–60 cm/s | 7 |
| VIIISevere | 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. | Moderate to heavy | 34–65% g | 60–116 cm/s | 4 |
| IXViolent | 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. | Heavy | 65–124% g | 116–219 cm/s | – |
| XExtreme | Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent. | Very heavy | over 124% g | over 219 cm/s | 2 |
Descriptions: the USGS abridged Modified Mercalli Intensity scale (Wood and Neumann, 1931; Richter, 1958). Ground-motion ranges: the USGS ShakeMap documentation (Worden et al., 2012), approximate and overlapping between neighbouring levels. Intensity Lab reports MMI interpolated from USGS ShakeMap.
You are the sensor
The scale rests on observation: you do not need a seismometer to estimate the intensity where you are. Like the Beaufort scale for wind, which grades conditions by smoke drift and tree movement, MMI relies on what you see, hear and feel. If you felt an earthquake, find the level that matches your experience, then report it to USGS Did You Feel It? A camera can be a sensor too: watch clips rated at each level.
Perception at low levels, damage at high ones
From I to IV, intensity rests on what people notice: whether a sleeper wakes, whether dishes rattle, whether it feels like a passing truck. From VII to X everyone feels it, so the scale turns to damage: cracked plaster against toppled chimneys, partial collapse against destruction. ShakeMap also estimates intensity from recorded ground motion; that is instrumental intensity, on the same scale and colours.
Common questions
What does the Modified Mercalli scale measure?
It measures the strength of shaking at one place, judged by its effects: what people feel, how objects move and what happens to buildings. It does not measure the earthquake itself. One earthquake produces many intensities, strongest near the fault and weaker with distance.
How is intensity different from magnitude?
Magnitude measures the energy an earthquake releases and is one number per earthquake. Intensity describes the shaking at one place. A magnitude 6 earthquake can produce MMI VIII near the fault and MMI III a hundred kilometres away.
How are Mercalli intensities estimated?
In three ways. People report what they felt and saw to USGS Did You Feel It?, which turns the answers into an intensity for each place. Seismic stations record ground acceleration and velocity, which USGS ShakeMap converts to intensity and maps between stations. After a damaging earthquake, field teams survey buildings and assign intensities from what they find. Intensity Lab reads each town's value from ShakeMap.
Why does the scale use Roman numerals?
To keep intensity apart from magnitude. Each numeral names a category of effects rather than a measured quantity, so MMI VI is not twice MMI III.
At what intensity does damage begin?
Slight damage, such as fallen plaster, begins at about MMI VI. At VII, poorly built structures are damaged considerably while well-built ones fare well; from VIII up, damage spreads to ordinary buildings.
Does the scale go above X?
The scale published in 1931 runs to XII. At XI few masonry structures remain standing and bridges are destroyed; at XII damage is total and objects are thrown into the air. Those top levels depend more on ground failure and construction than on shaking, and are rarely assigned. USGS ShakeMap stops at X, shown as X+ on its maps, and so does this site.
Who created the Modified Mercalli scale?
Giuseppe Mercalli published a ten-level scale in 1902; Adolfo Cancani and August Sieberg extended it to twelve. Harry Wood and Frank Neumann adapted it to North American buildings in 1931 as the Modified Mercalli scale, and Charles Richter revised it in 1956. USGS uses an abridged form of that scale.