Amplitude spectra of surface waves from small earthquakes and underground nuclear explosions

Theoretical amplitude spectra of surface waves in a layered half-space are compared with the observed spectra for 13 underground nuclear explosions in the Nevada Test Site and for 12 earthquakes in the Gulf of California and in the United States. The amplitude spectral shape of Rayleigh waves for periods 10–50 sec from explosions does not vary much with the shot medium and the shot yield. Neither the source time function nor the finite source size appears to be the primary cause of the difference in spectral shape between underground nuclear explosions and small earthquakes. Rather, it is due to the small difference in focal depth and the different depth dependence of spectrum for different source mechanisms. Our result gives a theoretical basis for the use of amplitude spectra of Rayleigh waves at long periods as seismic discriminants between underground explosions and earthquakes.

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