A study of the Lg radiation from 17 southeastern U.S. earthquakes shows the attenuation of that phase to be at a 0.07°−1 rate for epicentral distances of 100 to 700 km. At longer distances, it is also at that rate for some of the earthquakes, but for five of the events it was at a somewhat greater (0.10°−1) rate. These different Lg attenuation rates are clearly a distance-related effect, but they are not caused by differences in source area, propagation path, radiation pattern, wave period, or group velocity. A possible explanation for the different attenuation rates is a shift from a normal-mode form of propagation to a leaking-mode form, brought about by slight phase-velocity variations in the crustal wave guide and/or in the underlying layer. Also, the influence of lateral heterogeneity and variations in the thickness of the crustal wave guide offer alternative explanations.
Nuttli's (1973) mb (Lg) formulas, determined for central U.S. events, were found to be appropriate for use on southeastern U.S. shocks if their application is restricted to epicentral distances less than 2,000 km. This distance-restriction result agrees with that determined for the northeastern U.S. earthquakes by Street (1976).