Modified Mercalli Intensity scale
The lower degrees of the Modified Mercalli Intensity scale generally deal with the manner in which the earthquake is felt by people. The higher numbers of the scale are based on observed structural damage.
The small table is a rough guide to the degrees of the Modified Mercalli Intensity scale. The colors and descriptive names shown here differ from those used on certain shake maps in other articles.
The large table gives Modified Mercalli scale intensities that are typically observed at locations near the epicenter of the earthquake.
The correlation between magnitude and intensity is far from total, depending upon several factors including depth of the earthquake, terrain, and population density. For example, on 19 May 2011 an earthquake of magnitude 0.7 in Southern California, USA 4 km deep was classified as of intensity III by the United States Geological Survey (USGS), while a 4.5 magnitude quake in Salta, Argentina 164 km deep was of intensity I.
|Moment Magnitude||Typical Maximum
Modified Mercalli Intensity
|1.0 – 3.0||I|
|3.0 – 3.9||II – III|
|4.0 – 4.9||IV – V|
|5.0 – 5.9||VI – VII|
|6.0 – 6.9||VII – IX|
|7.0+||VIII or higher|
|I. Instrumental||Generally not felt by people unless in favorable conditions.|
|II. Weak||Felt only by a few people at best, especially on the upper floors of buildings. Delicately suspended objects may swing.|
|III. Slight||Felt quite noticeably by people indoors, especially on the upper floors of buildings. Many do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of a truck. Duration estimated.|
|IV. Moderate||Felt indoors by many people, outdoors by few people during the day. At night, some awaken. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rock noticeably. Dishes and windows rattle alarmingly.|
|V. Rather Strong||Felt outside by most, may not be felt by some outside in non-favorable conditions. Dishes and windows may break and large bells will ring. Vibrations like large train passing close to house.|
|VI. Strong||Felt by all; many frightened and run outdoors, walk unsteadily. Windows, dishes, glassware broken; books fall off shelves; some heavy furniture moved or overturned; a few instances of fallen plaster. Damage slight.|
|VII. Very Strong||Difficult to stand; furniture broken; damage negligible in building 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. Noticed by people driving motor cars.|
|VIII. Destructive||Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture moved.|
|IX. Violent||General panic; 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.|
|X. Intense||Some well built wooden structures destroyed; most masonry and frame structures destroyed with foundation. Rails bent.|
|XI. Extreme||Few, if any masonry structures remain standing. Bridges destroyed. Rails bent greatly.|
|XII. Cataclysmic||Total destruction – Everything is destroyed. Lines of sight and level distorted. Objects thrown into the air. The ground moves in waves or ripples. Large amounts of rock move position. Landscape altered, or leveled by several meters. In some cases, even the routes of rivers are changed.|
Correlation with physical quantities
The Mercalli scale is not defined in terms of more rigorous, objectively quantifiable measurements such as shake amplitude, shake frequency, peak velocity, or peak acceleration. Human perceived shakings and building damages are best correlated with peak acceleration for lower-intensity events, and with peak velocity for higher-intensity events.
Comparison to the moment magnitude scale
The effects of any one earthquake can vary greatly from place to place, so there may be many Mercalli intensity values measured for the same earthquake. These values can be best displayed using a contoured map. Each earthquake, on the other hand, has only one magnitude.