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. It does not have a mathematical basis but instead it is an arbitrary ranking based on observed effects.[1]

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).[2][3] 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.[2][3] 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.[2]

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.[4]

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. Not feltNot felt except by a very few under especially favorable conditions.
II. WeakFelt only by a few persons at rest, especially on upper floors of buildings.
III. WeakFelt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated.
IV. LightFelt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.
V. ModerateFelt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.
VI. StrongFelt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.
VII. Very StrongDamage negligible in buildings 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.
VIII. SevereDamage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.
IX. ViolentDamage 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. ExtremeSome well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.
XI. ExtremeFew, if any (masonry), structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipe lines completely out of service. Earth slumps and land slips in soft ground. Rails bent greatly.
XII. ExtremeDamage total. Waves seen on ground surfaces. Lines of sight and level distorted. Objects thrown upward into the air.

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.[5]

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[6]–8.5[7]XII
1976 Tangshan earthquake242,769[8]7.8–8.2XI[9]
2008 Sichuan earthquake69,1957.9[10]XI
1556 Shaanxi earthquake830,0007.9–8.0XII
1960 Valdivia earthquake6,0009.5XI[11]
1908 Messina earthquake200,0007.2XI
1964 Alaska earthquake1439.2XI
Great Hanshin earthquake6,4346.8–7.3X–XI[9]
1906 San Francisco earthquake3,4257.8X
1755 Cape Ann Earthquake05.9–6.3VIII
1977 Vrancea earthquake1,5787.2X–XI
2010 Canterbury earthquake07.1X
2010 Haiti earthquake100,0007.0X
1980 Irpinia earthquake2,9146.9X
2011 Van earthquake4327.2X
2004 Indian Ocean earthquake and tsunami280,0009.3IX[12]
2011 Japan earthquake and tsunami18,5009.0IX[13]
2011 Van earthquake4327.2X
1989 Loma Prieta earthquake636.9IX
2011 Christchurch earthquake1856.3IX[14]

See also[edit]


External links[edit]