S. G. Kim, Y. T. Chen, Z. L. Wu, and G. F. Panza
A mathematical theorem useful for the direct estimation of seismic source spectra
Bulletin of the Seismological Society of America (October 1997), 87(5):1281-1287

Abstract:
In this note, we prove a mathematical theorem for the direct estimation of the source spectra of earthquakes using a single seismogram. Considering a combination of Fourier spectra, the effects of the "source term" and the "attenuation term" from a single seismogram can be separated. Using signal-processing techniques, such an approach may be expressed in terms of the estimation of the Wigner distribution.