modified Mercalli intensity scale
This is a scale designed to describe how an earthquake is felt by people (more at the lower end) or how much damage results from it (at the higher end).
- I. Not felt
- II. Very weak: felt only by a few people, mainly at upper floors of buildings
- III. Weak: felt noticeably by people indoors, especially at upper floors of buildings; stationary motor vehicles might rock.
- IV. Light: felt indoors by many, outdoors by few. Things like windows, doors and dishes might be disturbed; walls might make creaking noises. May wake some people up from sleep.
- V. Moderate: felt by nearly everyone; many would be woken at night. Some dishes and windows are broken; objects are overturned.
- VI. Strong: felt by all. Some heavy furniture is moved; there may be fallen plaster. Few instances of serious damage.
- VII. Very strong: damage is minor in “well-designed” structures, slight to moderate in “well-built” structures, but may be extensive in “poorly-built” structures. Chimneys may break.
- VIII. Severe: damage is slight in buildings designed to withstand earthquakes, but may be severe in those that are not. Heavy furniture overturned.
- IX. Violent: considerable damage even to buildings designed to withstand earthquakes. Widespread destruction to structures that were not. Buildings are lifted off foundations; liquefaction occurs.
- X. Extreme: some well-built wooden structures are destroyed; most masonry and frame-based structures are destroyed, including the foundations. Rails are bent.
- XI. Extreme: few structures remain standing. Bridges are destroyed. Fissures erupt in the ground. Underground pipelines are rendered useless. Earth slumps in places. Rails are bent a lot.
- XII. Extreme: damage is total; waves are seen travelling along the ground; lines of sound and sight are distorted. Objects are thrown up into the air.
There is a correlation between the MMI and the seismic magnitude, but it is not absolute. Factors like the depth of an earthquake below the ground can make a difference.