Evaluating the intensity of U.S. earthquakes

The intensity scale based on the Modified Mercalli Scale of 1931 (MM scale) measures the effects of seismic shaking. Intensity estimations are often the only representation of the size of an earthquake when small shocks occur in areas far removed from seismograph stations that can record them instrumentally. The impossibility of setting up and maintaining instruments to measure ground motion everywhere that earthquakes occur makes the intensity scale a necessary and useful method to estimate earthquake effects.

The effects of seismic shaking are objective. All observers can agree these are real occurences and not subjective speculation. Reliable intensity evaluations are based not on a single factor on any scale but on consistent combinations.