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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. It is distinct from the moment magnitude (Mw) usually reported for an earthquake, which is a measure of the energy released (sometimes misreported as the Richter magnitude, ML). The intensity of an earthquake is not entirely determined by its magnitude. It is not based on first physical principles, but is, instead, empirically based on observed effects.
The Mercalli 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 from 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 Italian volcanologist Giuseppe Mercalli revised the widely used simple ten-degree Rossi–Forel scale between 1884 and 1906, creating the Mercalli Intensity scale which is still used nowadays.
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 by Harry O. Wood and Frank Neumann, and published in English 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).
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.
This table gives Modified Mercalli scale intensities that are typically observed at locations near the epicenter of the earthquake.
|I. Not felt||Not felt except by very few under especially favorable conditions.|
|II. Weak||Felt only by a few people at rest, especially on upper floors of buildings.|
|III. Weak||Felt quite noticeably by people 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. Light||Felt 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. Moderate||Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.|
|VI. Strong||Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.|
|VII. Very strong||Damage 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. Severe||Damage 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. Violent||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. Liquefaction.|
|X. Extreme||Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.|
|XI. Extreme||Few, 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. Extreme||Damage total. Waves seen on ground surfaces. Lines of sight and level distorted. Objects thrown upward into the air.|
|Magnitude||Magnitude / intensity comparison|
|1.0 – 2.9||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 and higher||VIII or higher|
The correlation between magnitude and intensity is far from total, depending upon several factors including the depth of the hypocenter, terrain, distance from the epicenter. 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.
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.
Dozens of so-called intensity prediction equations have been published to estimate the macroseismic intensity at a location given the magnitude, source-to-site distance and, perhaps, other parameters (e.g. local site conditions). These are similar to ground motion prediction equations for the estimation of instrumental strong-motion parameters such as peak ground acceleration. A summary of intensity prediction equations is available. Such equations can be used to estimate the seismic hazard in terms of macroseismic intensity, which has the advantage of being more closely related to seismic risk than instrumental strong-motion parameters.
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.
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. However, each earthquake has only one magnitude.