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.