![]() ![]() This table gives MMIs that are typically observed at locations near the epicenter of the earthquake. The greater numbers of the scale are based on observed structural damage. The lesser degrees of the MMI scale generally describe the manner in which the earthquake is felt by people. The categories "catastrophe" and "enormous catastrophe" added by Cancani (XI and XII) are used so infrequently that current USGS practice is to merge them into a single category "Extreme" abbreviated as "X+". 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". Also, construction codes and methods have evolved, making much of built environment stronger these make a given intensity of ground shaking seem weaker. However, this is generally interpreted with the modifications summarized 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". Geological Survey (and other agencies) assigns intensities is nominally Wood and Neumann's MM31. In their 1993 compendium of historical seismicity in the United States, 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. Not wanting to have this intensity scale confused with the Richter magnitude scale he had developed, he proposed calling it the "modified Mercalli scale of 1956" (MM56). The Wood–Neumann scale was revised in 1956 by Charles Francis Richter and published in his influential textbook Elementary Seismology. Wood and Neumann also had an abridged version, with fewer criteria for assessing the degree of intensity. Some seismologists refer to this version the "Wood–Neumann scale". Wood and Frank Neumann translated this into English in 1931 (along with modification and condensation of the descriptions, and removal of the acceleration criteria), they named it the "modified Mercalli intensity scale of 1931" (MM31). This became known as the "Mercalli–Cancani scale, formulated by Sieberg", or the "Mercalli–Cancani–Sieberg scale", or simply "MCS", and was used extensively in Europe and remains in use in Italy by the National Institute of Geophysics and Volcanology (INGV). His descriptions being deficient, August Heinrich Sieberg augmented them during 19, and indicated a peak ground acceleration for each degree. ![]() ![]() In 1904, Adolfo Cancani proposed adding two additional degrees for very strong earthquakes, "catastrophe" and "enormous catastrophe", thus creating a 12-degree scale. This version "found favour with the users", and was adopted by the Italian Central Office of Meteorology and Geodynamics. 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. 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". Italian volcanologist Giuseppe Mercalli formulated his first intensity scale in 1883. By not requiring instrumental measurements, they are useful for estimating the magnitude and location of historical (preinstrumental) earthquakes: the greatest intensities generally correspond to the epicentral area, and their degree and extent (possibly augmented by knowledge of local geological conditions) can be compared with other local earthquakes to estimate the magnitude. Intensity scales empirically categorize the intensity of shaking based on the effects reported by untrained observers and are adapted for the effects that might be observed in a particular region. Shaking intensity is localized, generally diminishing with distance from the earthquake's epicenter, but can be amplified in sedimentary basins and certain kinds of unconsolidated soils. Deeper earthquakes also have less interaction with the surface, and their energy is spread out across a larger volume. While shaking is caused by the seismic energy released by an earthquake, earthquakes differ in how much of their energy is radiated as seismic waves. It measures the effects of an earthquake at a given location, distinguished from the earthquake's inherent force or strength as measured by seismic magnitude scales (such as the " M w" magnitude usually reported for an earthquake). The modified Mercalli intensity scale ( MM, MMI, or MCS), developed from Giuseppe Mercalli's Mercalli intensity scale of 1902, is a seismic intensity scale used for measuring the intensity of shaking produced by an earthquake. ![]()
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