Modified Mercalli intensity scale

The Modified Mercalli intensity scale (MM or MMI), descended 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. 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 "Mw" magnitude usually reported for an earthquake). While shaking is driven by the seismic energy released by an earthquake, earthquakes differ in how much of their energy is radiated as seismic waves. Deeper earthquakes also have less interaction with the surface, and their energy is spread out across a larger area. 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.

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.[1] 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.

History

The Italian volcanologist Giuseppe Mercalli formulated his first intensity scale in 1883.[2] It had six degrees or categories, has been described as "merely an adaptation" of the then standard Rossi–Forel scale of ten degrees, and is now "more or less forgotten."[3] Mercalli's second scale, published in 1902, was also an adaptation of the Rossi‒Forel scale, retaining the ten degrees and expanding the descriptions of each degree.[4] This version "found favour with the users", and was adopted by the Italian Central Office of Meteorology and Geodynamics.[5]

In 1904 Adolfo Cancani proposed adding two additional degrees for very strong earthquakes, "catastrophe" and "enormous catastrophe", thus creating the 12 degree scale.[6] His descriptions being deficient, August Heinrich Sieberg augmented them in 1912 and 1923, and indicated a peak ground acceleration (PGA) for each degree.[7] This became known as the "Mercalli–Cancani scale, formulated by Sieberg", or the "Mercalli–Cancani–Sieberg scale", or simply "MCS",[8] 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".[9] (MM31. Some seismologists prefer to call this version the "Wood–Neumann scale".[10]) 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 Charles Francis Richter and published in his influential textbook Elementary Seismology.[11] Not wanting to have this intensity scale confused with the magnitude scale he had developed, he proposed calling it the "Modified Mercalli scale of 1956" (MM56).[12]

In their 1993 compendium of historical seismicity in the United States,[13] 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,[14] effectively creating a new but largely undocumented version of the scale.[15]

The basis by which the U. S. Geological Survey (and other agencies) assigns intensities is nominally Wood and Neumann's "Modified Mercalli Intensity Scale of 1931". However, this is generally interpreted with the modifications summarized by Stover and Coffman because in the decades since 1931 it has been found that "some criteria are more reliable than others as indicators of the level of ground shaking."[16] Also, construction codes and methods have evolved, making much of built environment stronger; these make a given intensity of ground shaking seem weaker.[17] And it is now recognized that some of the original criteria of the higher 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...."[18]

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+".[19]

Other Languages
brezhoneg: Skeul Mercalli
Deutsch: Mercalliskala
Bahasa Indonesia: Skala mercalli
italiano: Scala Mercalli
latviešu: Merkalli skala
македонски: Меркалиева скала
Bahasa Melayu: Skala Mercalli
norsk nynorsk: Mercalliskalaen
português: Escala de Mercalli
sicilianu: Scala Mercalli
Simple English: Mercalli intensity scale
slovenščina: Mercallijeva lestvica
српски / srpski: Меркалијева скала
srpskohrvatski / српскохрватски: Mercallijeva ljestvica
Basa Sunda: Skala Mercalli
українська: Шкала Меркаллі
Tiếng Việt: Thang đo Mercalli