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 M_w 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).[1][2] 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[edit]

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.[1][2] 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.[1]

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.[3]

MagnitudeTypical Maximum
Modified Mercalli Intensity
Under 2.0I
2.0 – 2.9II – III
3.0 – 3.9IIIIV
4.0 – 4.9IVV
5.0 – 5.9VVI
6.0 – 6.9VIVII
7.0 – 7.9VII or higher
8.0 or higherVIII or higher

I. InstrumentalGenerally not felt by people unless in favorable conditions.
II. WeakFelt only by a couple people that are sensitive, especially on the upper floors of buildings. Delicately suspended objects (including chandeliers) may swing slightly.
III. SlightFelt 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. ModerateFelt 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 StrongFelt 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. StrongFelt 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 StrongDifficult 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. DestructiveDamage 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. ViolentGeneral 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. IntenseMany well-built structures destroyed, collapsed, or moderately to severely damaged. Most other structures destroyed, possibly shifted off foundation. Large landslides.
XI. ExtremeFew, if any structures remain standing. Numerous landslides, cracks and deformation of the ground.
XII. CatastrophicTotal 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[edit]

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.[4]

Comparison to the moment magnitude scale[edit]

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 of equal intensity, known as an isoseismal map. Each earthquake, on the other hand, has only one magnitude.

Historical analysis[edit]

EarthquakeDeath TollMoment MagnitudeMercalli intensity
1920 Haiyuan earthquake235,5027.8[5]–8.5[6]XII
1976 Tangshan earthquake242,769-650,000[7]7.8–8.2XI[8]
2008 Sichuan earthquake69,1957.9[9]XI
1556 Shaanxi earthquake830,0007.9–8.0XI
1960 Valdivia earthquake2,230–6,0009.5XI[10]
1908 Messina earthquake150,000–200,0007.2XI
1964 Good Friday/Anchorage/Alaska earthquake143. Totals include tsunami.9.2XI
1995 Kobe/Great Hanshin earthquake6,4346.8–7.3X–XI[8]
1906 San Francisco earthquake3,4257.8X
1755 Boston/Massachusetts/New England earthquake05.9–6.3VIII
1977 Vrancea earthquake1,5787.2X–XI
2010 Canterbury earthquake07.1X
2010 Haiti earthquake316,0007.0X
1980 Irpinia earthquake2,9146.9X
2011 Van earthquake4327.2X
2004 Indian Ocean earthquake and tsunami230,000–280,000. Totals included tsunami.9.1–9.3IX[11]
2011 Japan earthquake and tsunami18,500. Totals include tsunami.9.0IX[12]
1994 Northridge/Los Angeles earthquake606.7IX
2011 Christchurch earthquake1856.3IX[13]

See also[edit]


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