Editing Modified Mercalli intensity scale (section)

{{Short description|Seismic intensity scale used to quantify the degree of shaking during earthquakes}}
{{Redirect|Mercalli|the scientist whom the scale is named after|Giuseppe Mercalli}}
{{Earthquakes}}
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 scales|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|link=y}}" scale is widely used.) The MMI 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 localised. It generally diminishes with distance from the earthquake's [[Epicenter|epicentre]], but it can be amplified in [[sedimentary basin]]s and in certain kinds of unconsolidated soils.

[[Seismic intensity scales|Intensity scales]] categorise intensity empirically, based on the effects reported by untrained observers, and are adapted for the effects that might be observed in a particular region.<ref>{{cite web|title=The Severity of an Earthquake|url=https://pubs.usgs.gov/gip/earthq4/severitygip.html|publisher=United States Geological Survey|date= November 5, 2021}}</ref> By 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 by knowledge of local geological conditions) can be compared with other local earthquakes to estimate the magnitude.