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Modified Mercalli intensity scale
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<p>The Modified Mercalli intensity scale (MM, MMI, or MCS) measures the effects of an <a href="/facts/Earthquake/NGjIzagw">earthquake</a> at a given location. This is in contrast with the <a href="/facts/Seismic_magnitude_scales/g5uSHyQe">seismic magnitude</a> usually reported for an earthquake. </p><p>Magnitude scales measure the inherent force or strength of an earthquake — an event occurring at greater or lesser depth. (The "<a href="/facts/Seismic_magnitude_scales/g5uSHyQe">Mw</a>" scale is widely used.) The MMI scale measures intensity of shaking, at any particular location, on the surface. It was developed from <a href="/facts/Giuseppe_Mercalli/fLS4KFLt">Giuseppe Mercalli</a>'s Mercalli intensity scale of 1902. </p><p>While shaking experienced at the surface is caused by the <a href="/facts/Seismic_energy/1JTeLfac">seismic energy</a> 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 <a href="/facts/Epicenter/8nwmjncG">epicentre</a>, but it can be amplified in <a href="/facts/Sedimentary_basin/dVkfKMrB">sedimentary basins</a> and in certain kinds of unconsolidated soils. </p><p><a href="/facts/Seismic_intensity_scales/FLQnPGGf">Intensity scales</a> 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. 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. </p>