A seismic-spectrum scaling model based on an analytic approximation to the nuclear seismic-source function is described and evaluated in terms of the behavior of a large body of measured spectral data. The results of the scaling analysis indicate that the yield, depth of burial and emplacement medium of the nuclear device play significant roles in defining the seismic-spectrum scaling of underground nuclear detonations. Results are presented which indicate that cube-root-yield scaling of the nuclear seismic-source function is not appropriate. For events buried sufficiently deep for containment, the scaling theory predicts frequency-dependent yield exponents ranging from 0.90 at low frequencies to 0.45 at high frequencies. These yield-scaling exponents are in good agreement with empirical relations statistically derived from measured data.