Spectra of more than three hundred short-period, vertical component L, waves of 78 earthquakes in the central United States were determined. From these an empirical relationship between the long-period level of the L, spectrum and the seismic moment was developed. The shape of the reduced spectrum shares some characteristics of Aki's revised model A.There is a uniform relationship between the corner period and the seismic moment, which implies a uniformity of earthquake processes in the central United States over a wide range of event sizes, as opposed to California where wide variations in the corner frequency are observed for source spectra having the same seismic moment. Using the reduced spectra, a method for relating magnitude observations at periods other than 1 s to mb is outlined. 4 > J :: *< a 7 ,I c cd +-I n comer between the wo and w -~ asymptotic trends of the spectra. (b) Event 6, 1962 July 23, showing the To, comer between the wo and w -l trends and the Ti2 comer between the UJ-' and w -~ trends.

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