The Modified Mercalli intensity scale (MM, MMI, or MCS) measures the effects of an earthquake at a given location.
Magnitude scales measure the inherent force or strength of an earthquake – an event occurring at greater or lesser depth.
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.
[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.
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).
[13] 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 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".
It depends upon many factors, including the depth of the hypocenter, terrain, distance from the epicenter, whether the underlying strata there amplify surface shaking, and any directionality due to the earthquake mechanism.
Dozens of intensity-prediction equations[22] 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).