|On the other hand, intensity is
a measure of the felt or perceived effects of an earthquake rather than the strength of
the earthquake itself. It is a measure of how severe the shaking was at a particular
location. Thus, whereas magnitude refers to the force of the earthquake as a whole (i.e.,
an earthquake can have just one magnitude), intensity refers to the effects of an
earthquake at a particular site. The intensity is usually strongest close to the epicenter
and is weaker the farther a site is from the epicenter. Intensity is determined by
classifying the degree of shaking severity as measured by an intensity scale. 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.
There are many intensity scales in use today around the world. The most commonly used
scale for intensity in the United States is the Modified Mercalli (MM) scale, a 12-point
scale that ranges from barely perceptible earthquakes at MM I to near total destruction at
MM XII (Table 8--3).
The intensity of an earthquake is more germane to its public
health consequences than its magnitude. 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.