The Modified Mercalli intensity scale explained

Magnitude answers one question: how big was the earthquake at its source. It says nothing about how hard the ground shook where you were standing. That is what an intensity scale is for. The Modified Mercalli Intensity scale, written in Roman numerals from I to XII, rates the strength of shaking and its effects at a specific place. A single earthquake has one magnitude but many intensities, strongest near the epicentre and fading with distance.

Intensity versus magnitude

The distinction is easy to blur but important. Magnitude is measured by instruments and reduced to one number for the whole event. Intensity is a description of what actually happened at a location: what people felt, how objects and buildings responded, and what damage occurred. A magnitude 7 earthquake might produce intensity IX shaking directly above the rupture, intensity VI in a city 50 kilometres away, and intensity III in a town beyond the hills. The magnitude is the same everywhere; the intensity is not. For more on the source side of this, see the guide to earthquake magnitude scales.

Where the scale comes from

The scale descends from work by the Italian scientist Giuseppe Mercalli around 1900 and was revised for conditions in the United States in 1931, which is why it is called Modified Mercalli. Intensity is assigned after an earthquake from many sources: questionnaires and online reports from people who felt it, damage surveys by engineers, and instrument recordings. Monitoring agencies combine these into shaking maps that colour each area by its estimated intensity, giving a picture of where the strong shaking landed rather than just where the epicentre sat.

The twelve levels in plain language

The levels run from shaking that only instruments detect up to total destruction. The lower half, roughly I to V, is about what people feel. The upper half, VI to XII, is about damage to buildings and the ground.

Intensity	Shaking	What people notice and typical effects
I	Not felt	Detected only by instruments. No one feels it.
II	Weak	Felt by a few people at rest, mostly on upper floors of buildings.
III	Weak	Felt indoors, especially upstairs, like a light truck passing. Hanging objects sway. Many do not recognise it as an earthquake.
IV	Light	Felt by many indoors and some outdoors. Dishes, windows, and doors rattle; parked cars rock. Feels like a heavy truck striking the building.
V	Moderate	Felt by nearly everyone; sleepers wake. Some dishes and windows break, unstable objects overturn, and doors swing.
VI	Strong	Felt by all. People walk unsteadily, some heavy furniture moves, plaster falls, and there is slight damage to weak structures.
VII	Very strong	Hard to stand. Slight to moderate damage in ordinary buildings, considerable damage in poorly built ones; chimneys and loose bricks fall.
VIII	Severe	Considerable damage to ordinary buildings with partial collapse. Chimneys, walls, and monuments fall; heavy furniture overturns.
IX	Violent	Serious damage even to well-designed structures. Buildings shift off foundations, the ground cracks, and underground pipes break.
X	Extreme	Most masonry and frame structures destroyed. Rails bend, large landslides occur, and water sloshes over banks.
XI	Extreme	Few structures remain standing. Bridges collapse, broad fissures open in the ground, and rails bend greatly.
XII	Extreme	Total destruction. Objects are thrown into the air and the ground surface is visibly distorted.

Reading intensity in the real world

Because intensity captures local conditions, it explains things magnitude cannot. Soft soils can push an area up a level or two compared with a neighbouring district on bedrock. Older, unreinforced buildings show higher damage, and therefore higher assessed intensity, than modern engineered ones in the same shaking. This is why two places the same distance from an earthquake can report very different intensities, and why intensity maps, not the magnitude alone, guide emergency response.

The upper intensities, X to XII, are rare and describe near-total destruction; most damaging earthquakes people experience fall in the VI to IX range. If you felt shaking recently, you can compare it against the descriptions above, and check the magnitude and distance of what was recorded near you or across the world on the live globe. For a feel of how magnitudes translate into experience, see the guide to what a given magnitude feels like.