The Modified Mercalli intensity scale (MM, MMI, or MCS) measures the effects of an earthquake at a given location. This is in contrast with the seismic magnitude usually reported for an earthquake.

Magnitude scales measure the inherent force or strength of an earthquake – an event occurring at greater or lesser depth. (The "M_w" scale is widely used.) The MM scale measures intensity of shaking, at any particular location, on the surface. It was developed from Giuseppe Mercalli's Mercalli intensity scale of 1902.

While shaking experienced at the surface is caused by the seismic energy released by an earthquake, earthquakes differ in how much of their energy is radiated as seismic waves. They also differ in the depth at which they occur; deeper earthquakes have less interaction with the surface, their energy is spread throughout a larger volume, and the energy reaching the surface is spread across a larger area. Shaking intensity is localized. It generally diminishes with distance from the earthquake's epicenter, but it can be amplified in sedimentary basins and in certain kinds of unconsolidated soils.

Intensity scales categorize intensity empirically, based on the effects reported by untrained observers, and are adapted for the effects that might be observed in a particular region.^[1] 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.

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 10 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 10 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 a 12-degree scale.^[6] His descriptions being deficient, August Heinrich Sieberg augmented them during 1912 and 1923, and indicated a peak ground acceleration for each degree.^[7][8] This became known as the "Mercalli–Cancani scale, formulated by Sieberg", or the "Mercalli–Cancani–Sieberg scale", or simply "MCS",^[8] and was used extensively in Europe and remains in use in Italy by the National Institute of Geophysics and Volcanology (INGV).^[9]

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 named it the "modified Mercalli intensity scale of 1931" (MM31).^[10] Some seismologists refer to this version the "Wood–Neumann scale".^[8] 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 Richter magnitude scale he had developed, he proposed calling it the "modified Mercalli scale of 1956" (MM56).^[8]

In their 1993 compendium of historical seismicity in the United States,^[12] 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,^{[lower-alpha 1]} effectively creating a new, but largely undocumented version of the scale.^[13]

The basis by which the U.S. Geological Survey (and other agencies) assigns intensities is nominally Wood and Neumann's MM31. 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".^[14] Also, construction codes and methods have evolved, making much of built environment stronger; these make a given intensity of ground shaking seem weaker.^[15] 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".^[14]

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+".^[16]

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 epicenter of the earthquake.^[17]

• European macroseismic scale
• Japan Meteorological Agency seismic intensity scale (Shindo scale)
• Rohn emergency scale
• Seismic intensity scales
• Seismic magnitude scales
• Spectral acceleration
• Strong ground motion