The Mercalli seismological scale is a scale of 12 degrees, where 1 is very weak and 12 catastrophic.
This scale owes its name to the Italian physicist Giuseppe Mercalli and was developed to assess the intensity of earthquakes through the effects and damages caused to different structures.
This means the Mercalli scale measures how strong a earthquake has been by its consequences and not by its magnitude, therefore it is based on empirical observations.