Mercalli intensity scale
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The Mercalli intensity scale is a seismic scale used for measuring the intensity of an earthquake. It measures the effects of an earthquake, and is distinct from the moment magnitude usually reported for an earthquake (sometimes misreported as the Richter magnitude), which is a measure of the energy released. The intensity of an earthquake is not totally determined by its magnitude.
The scale quantifies the effects of an earthquake on the Earth's surface, humans, objects of nature, and man-made structures on a scale from I (not felt) to XII (total destruction). Values depend upon the distance to the earthquake, with the highest intensities being around the epicentral area. Data gathered from people who have experienced the quake are used to determine an intensity value for their location. The Mercalli (Intensity) scale originated with the widely used simple ten-degree Rossi-Forel scale which was revised by Italian volcanologist, 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 (MM) or Modified Mercalli Intensity scale (MMI).
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 the depth of the earthquake, terrain, population density, and damage. For example, on May 19, 2011, an earthquake of magnitude 0.7 in Central California, United States 4 km deep was classified as of intensity III by the United States Geological Survey (USGS) over 100 miles (160 km) away from the epicenter (and II intensity almost 300 miles (480 km) from the epicenter), while a 4.5 magnitude quake in Salta, Argentina 164 km deep was of intensity I.
Modified Mercalli Intensity
|2.0 – 2.9||I – II|
|3.0 – 3.9||II – IV|
|4.0 – 4.9||IV – VI|
|5.0 – 5.9||VI – VIII|
|6.0 – 6.9||VII – X|
|7.0 – 7.9||VIII or higher|
|8.0 or higher||IX or higher|
|I. Instrumental||Generally not felt by people unless in favorable conditions.|
|II. Weak||Felt only by a couple people that are sensitive, especially on the upper floors of buildings. Delicately suspended objects (including chandeliers) may swing slightly.|
|III. Slight||Felt quite noticeably by people indoors, especially on the upper floors of buildings. Many do not recognize it as an earthquake. Standing automobiles may rock slightly. Vibration similar to the passing of a truck. Duration can be estimated. Indoor objects (including chandeliers) may shake.|
|IV. Moderate||Felt indoors by many to all people, and outdoors by few people. Some awakened. Dishes, windows, and doors disturbed, and walls make cracking sounds. Chandeliers and indoor objects shake noticeably. The sensation is more like a heavy truck striking building. Standing automobiles rock noticeably. Dishes and windows rattle alarmingly. Damage none.|
|V. Rather Strong||Felt inside by most or all, and outside. Dishes and windows may break and bells will ring. Vibrations are more like a large train passing close to a house. Possible slight damage to buildings. Liquids may spill out of glasses or open containers. None to a few people are frightened and run outdoors.|
|VI. Strong||Felt by everyone, outside or inside; 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 to moderate to poorly designed buildings, all others receive none to slight damage.|
|VII. Very Strong||Difficult to stand. Furniture broken. Damage light in building of good design and construction; slight to moderate in ordinarily built structures; considerable damage in poorly built or badly designed structures; some chimneys broken or heavily damaged. Noticed by people driving automobiles.|
|VIII. Destructive||Damage slight in structures of good design, considerable in normal buildings with a possible partial collapse. Damage great in poorly built structures. Brick buildings easily receive moderate to extremely heavy damage. Possible fall of chimneys, factory stacks, columns, monuments, walls, etc. Heavy furniture moved.|
|IX. Violent||General panic. Damage slight to moderate (possibly heavy) in well-designed structures. Well-designed structures thrown out of plumb. Damage moderate to great in substantial buildings, with a possible partial collapse. Some buildings may be shifted off foundations. Walls can fall down or collapse.|
|X. Intense||Many well-built structures destroyed, collapsed, or moderately to severely damaged. Most other structures destroyed, possibly shifted off foundation. Large landslides.|
|XI. Extreme||Few, if any structures remain standing. Numerous landslides, cracks and deformation of the ground.|
|XII. Catastrophic||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. Even the routes of rivers can be 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 shaking 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.
- Other seismic scales
- Japan Meteorological Agency seismic intensity scale
- Hayward Fault Zone for seismic shake maps using the Modified Mercalli scale
- Isoseismal map
- Strong ground motion
- Spectral acceleration
- Rohn Emergency Scale for measuring the magnitude (intensity) of any emergency
- "Magnitude / Intensity Comparison". USGS.
- "Modified Mercalli Intensity Scale". Association of Bay Area Governments (ABAG).
- USGS: Did you feel it? for 20 May 2011
- "ShakeMap Scientific Background". USGS.
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- National Earthquake Information Center (U.S.)
- Mercalli Scale Simulator, Northern Illinois University
- John N. Louie, Associate Professor of Seismology at the University of Nevada