Earthquake Intensity

The first scale to reflect earthquake intensities (damage) was developed by de Rossi of Italy and Forel of Switzerland in the 1880s and is known as the
Rossi-Forel intensity scale.
This scale, with values from I to X, was used for about two decades.
A need for a more refined scale increased with the advancement of the science of seismology.
In 1902 the Italian seismologist, Mercalli, devised a new scale on a I to XII range.
The Mercalli intensity scale was modified in 1931 by American seismologists Harry O. Wood and Frank Neumann to take into account modern structural features.
The Modified Mercalli intensity scale measures the intensity of an earthquake's effects in a given locality, and is perhaps much more meaningful to the layperson because it is based on observations of earthquake effects at specific places.
It should be noted that, because the data used for assigning intensities are obtained from direct accounts of the earthquake's effects at numerous towns, considerable time (weeks to months) is sometimes needed before an intensity map can be assembled for a particular earthquake.

On the Modified Mercalli intensity scale, values range from I to XII.
The most commonly used adaptation covers the range of intensities from the conditions of
"I - not felt except by very few, favorably situated," to
"XII - damage total, lines of sight disturbed, objects thrown into the air"
(see Table 1).
While an earthquake has only one magnitude, it can have many intensities, which decrease with distance from the epicentre.
It is difficult to compare magnitude and intensity because intensity is linked with the particular ground and structural conditions of a given area, as well as distance from the earthquake epicenter, while magnitude depends on the energy released by earthquake faulting.
But there is an approximate relation between magnitude and maximum expected intensity close to the epicenter (see Table 2). The areas shaken at or above a given intensity increase logarithmically with earthquake magnitude.