The Modified Mercalli intensity scale (MM, MMI, or MCS), developed 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 "M_w" magnitude usually reported for an earthquake). While shaking is caused 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 volume. 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. 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.