Modified Mercalli intensity scale

Italian volcanologist Giuseppe Mercalli formulated his first intensity scale in 1883. It had six degrees or categories, has been described as "merely an adaptation" of the then-standard Rossi–Forel scale of 10 degrees, and is now "more or less forgotten". Mercalli's second scale, published in 1902, was also an adaptation of the Rossi–Forel scale, retaining the 10 degrees and expanding the descriptions of each degree. This version "found favour with the users", and was adopted by the Italian Central Office of Meteorology and Geodynamics. In 1904, Adolfo Cancani proposed adding two additional degrees for very strong earthquakes, "catastrophe" and "enormous catastrophe", thus creating a 12-degree scale. His descriptions being deficient, August Heinrich Sieberg augmented them during 1912 and 1923, and indicated a peak ground acceleration for each degree. and was used extensively in Europe and remains in use in Italy by the National Institute of Geophysics and Volcanology (INGV). When Harry O. Wood and Frank Neumann translated this into English in 1931 (along with modification and condensation of the descriptions, and removal of the acceleration criterion), they named it the "modified Mercalli intensity scale of 1931" (MM31). Some seismologists refer to this version as the "Wood–Neumann scale". Not wanting to have this intensity scale confused with the Richter scale he had developed, he proposed calling it the "modified Mercalli scale of 1956" (MM56). Carl Stover and Jerry Coffman ignored Richter's revision, and assigned intensities according to their slightly modified interpretation of Wood and Neumann's 1931 scale, effectively creating a new, but largely undocumented version of the scale. The basis by which the United States Geological Survey (and other agencies) assigns intensities is nominally Wood and Neumann's MM31. However, this is generally interpreted with the modifications summarised by Stover and Coffman because in the decades since 1931, "some criteria are more reliable than others as indicators of the level of ground shaking". Also, construction codes and methods have evolved, making much of built environment stronger; these make a given intensity of ground shaking seem weaker. Also, some of the original criteria of the most intense degrees (X and above), such as bent rails, ground fissures, landslides, etc., are "related less to the level of ground shaking than to the presence of ground conditions susceptible to spectacular failure". ==Scale values==

The lesser degrees of the MMI scale generally describe the manner in which the earthquake is felt by people. The greater numbers of the scale are based on observed structural damage. This table gives MMIs that are typically observed at locations near the epicentre of the earthquake. Correlation with magnitude Magnitude and intensity, while related, are very different concepts. Magnitude is a function of the energy liberated by an earthquake, while intensity is the degree of shaking experienced at a point on the surface, and varies from some maximum intensity at or near the epicentre, out to zero at distance. It depends upon many factors, including the depth of the hypocentre, terrain, distance from the epicentre, whether the underlying strata there amplify surface shaking, and any directionality due to the earthquake mechanism. For example, a magnitude 8.2 quake between La Paz and Beni Departments, Bolivia, in 1994, that was 631.3 km deep, had a maximum felt intensity of VI, while a magnitude 2.2 event in Barrow-in-Furness, England, in 1865, about 1 km deep, had a maximum felt intensity of VIII, despite the magnitude 8.2 earthquake releasing 1,000,000,000 times more energy than the magnitude 2.2 earthquake. The small table is a rough guide to the degrees of the MMI scale. The colours and descriptive names shown here differ from those used on certain shake maps in other articles. Estimating site intensity and its use in seismic hazard assessment Dozens of 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 related more closely to seismic risk than instrumental strong-motion parameters. Correlation with physical quantities The MMI 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 damage 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 many MMI values may be measured for the same earthquake. These values can be displayed best using a contoured map of equal intensity, known as an isoseismal map. However, each earthquake has only one magnitude. == See also ==