The spatial attenuation characteristics of seismic waves can be significantly different in different geographic regions. Attenuation of ground motion in regions other than the western United States has to be determined principally using the intensity observations because of the paucity of instrumental strong-motion data. Isoseismals of ten earthquakes of epicentral intensity, Io equal to or greater than VI on the Modified Mercalli (MM) scale, all occurring in the central United states have been investigated. By measuring the areas occupied by isoseismals of various intensities and applying the least squares method, the following attenuation relationship is obtained  
I(R)=Io+2.35-0.0316R-1.79logR (for R>20km)
where I(R) is the site intensity at an epicentral distance, R. This relationship can be useful in providing estimates of maximum probable site intensity and seismic risk due to a large earthquake at a known distance from the site.

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