abstract

The Lg phases on seismograms of earthquakes which have occurred and were recorded in the southeastern United States are used to define the Lg phase amplitude of a zero-magnitude event as a function of distance in the range of 150 to 850 km. A segmented zero-magnitude function is fitted in the least-squares sense to the logarithm of the Lg phase amplitudes following the method of Richter. The resulting zero-magnitude function exhibits a nonuniform decrease with distance which includes zones of slight amplification at multiples of 200 to 250 km. The northeast regional strike of the geologic structures is observed to correlate with increases in the efficiency of propagation of the Lg phase. However, when the data are analyzed for azimuthal dependence within distinct regions, this correlation is observed only for waves propagating through the Folded Appalachians. Propagation in the Coastal Plain is most efficient in a direction perpendicular to the trend of the Folded Appalachians. Also, the Piedmont Province propagation is only slightly more efficient perpendicular to the trend of the Folded Appalachians.

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