The Mercalli intensity scale is a scale used for measuring the intensity of an earthquake. The scale quantifies the effects of an earthquake on the Earth's surface, humans, objects of nature, and man-made structures on a scale of I through XII, with I denoting not felt, and XII total destruction.[1][2] The values will differ based on the distance to the earthquake, with the highest intensities being around the epicentral area. Data are gathered from individuals who have experienced the quake, and an intensity value will be given to their location.
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The Mercalli (Intensity) scale originated with the widely used simple ten-degree Rossi-Forel scale, which was revised by Italian vulcanologist Giuseppe Mercalli in 1884 and 1906.
In 1902 the ten-degree Mercalli scale was expanded to twelve degrees by Italian physicist Adolfo Cancani. It was later completely re-written by the German geophysicist August Heinrich Sieberg and became known as the Mercalli-Cancani-Sieberg (MCS) scale. The Mercalli-Cancani-Sieberg scale was later modified and published in English by Harry O. Wood and Frank Neumann in 1931 as the Mercalli-Wood-Neumann (MWN) scale. It was later improved by Charles Richter, the father of the Richter magnitude scale. The scale is known today as the Modified Mercalli scale or Modified Mercalli Intensity scale, and abbreviated MM or MMI.
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 table on the right is a rough guide to the degrees of the Modified Mercalli Intensity scale.[1][2] The colors and descriptive names shown here differ from those used on certain shake maps in other articles. The table on the left gives Modified Mercalli scale intensities that are typically observed at locations near the epicenter of the earthquake.[1]
| Richter 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 | Not felt by many people unless in favourable 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 awakened. 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-favourable 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 damage - Everything is destroyed. Total destruction. 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 route of rivers is changed. |
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.[3]
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