On the attenuation of Modified Mercalli Intensity with distance in the United States

A large number of Modified Mercalli Intensity isoseismal maps for earthquakes which have occurred in the United States have been analyzed to find the distribution of distances ( R ) between the epicenter and isoseismal contours. The data include most of the maps in the series “United States Earthquakes,” and a smaller number of maps from other sources. Where the data permit, best fitting parameters have been found in a Gaussian distribution function to fit the observed distributions of log 10 R for each combination of epicentral intensity and the intensity bounded by the contour; the data do not contradict this assumed Gaussian shape. The standard deviation of log 10 R is at best 0.2 for well-determined distributions; it is typically 0.3, implying that over an order of magnitude range of distances is present in the data.