Modified Mercalli intensity scale
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The Modified Mercalli intensity scale (MM or MMI), descended from
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. In not requiring instrumental measurements, they are useful for estimating the magnitude and location of historical (pre-instrumental) earthquakes: the greatest intensities generally correspond to the epicentral area, and their degree and extent (possibly augmented with knowledge of local geological conditions) can be compared with other local earthquakes to estimate the magnitude.
The Italian volcanologist
In 1904 Adolfo Cancani proposed adding two additional degrees for very strong earthquakes, "catastrophe" and "enormous catastrophe", thus creating the 12 degree scale. His descriptions being deficient, Heinrich Sieberg augmented them in 1912 and 1923, and indicated a peak ground acceleration (PGA) for each degree. This became known as the "Mercalli–Cancani scale, formulated by Sieberg", or the "Mercalli–Cancani–Sieberg scale", or simply "MCS", and used extensively in Europe.
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 criteria), they called it the "Modified Mercalli Intensity Scale of 1931". (MM31. Some seismologists prefer to call this version the "Wood–Neumann scale".) Wood and Neumann also had an abridged version, with fewer criteria for assessing the degree of intensity.
The Wood–Neumann scale was revised in 1956 by
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.
The basis by which the
The "catastrophe" and "enormous catastrophe" categories added by Cancani (XI and XII) are used so infrequently that current USGS practice is merge them into a single "Extreme" labeled "X+".