The Modified Mercalli earthquake-intensity scale has 12 levels, numbered I through XII.
Unlike a magnitude scale, the Mercalli scale measures observed effects at a particular place. Reports may describe whether people felt the shaking, whether objects moved, whether buildings were damaged, and whether the ground itself changed. Intensity can therefore vary from neighborhood to neighborhood during the same earthquake.
Level I means the earthquake is generally not felt, while level XII describes total destruction and major changes to the landscape. The scale relies on observations and damage patterns rather than directly calculating the earthquake’s physical energy.
Intensity is often confused with magnitude. One earthquake has a single overall magnitude, although it can produce many intensity values. Building design, distance from the epicenter, local geology, and construction quality all influence the intensity reported at a location. The Modified Mercalli scale is especially useful where instrumental recordings are limited.