Slide from Supercourse Library - http://www.pitt.edu/~super1/lecture/lec32361/010.htm

“The intensity is assigned for a particular location on the basis of the visible consequences left by the earthquake and from subjective reports by people who experience the shaking.

Intensity scales also allow comparisons with earthquakes that occurred before the development of seismic monitoring instruments.  The destruction that an earthquake causes is a function of its intensity and the resistance of structures to seismic damage. “