Earthquake Intensity Of the two ways to measure earthquake size, magnitude based on instrumental readings and intensity based on qualitative effects of earthquakes, only intensity can be applied to pre-instrumental earthquakes. The 1931 Modified Mercalli scale used in the United States assigns a Roman numeral in the range I - XII to each earthquake effect. The methodology is simple. At each ...

Article from The National Geophysical Data Center. Although the history of the development of earthquake intensity scales is long and interesting, only a brief summary will be presented here.