Short Period Spectral Discriminants for Explosions

Spectra (0–5 Hz) of the Pg phase at JAS, California (Δ = 250–500 km), for 69 events (M = 2.8–4.5) located within 100 km of NTS are used to investigate source differences between the short period spectra of nuclear explosions and ‘natural’ earthquakes. The Pg spectral ratio (0.6–1.25 Hz)/(1.35–2.0 Hz) of ground displacement satisfactorily discriminates NTS explosions from ‘natural’ earthquakes for a threshold magnitude at least as low as M = 3.2. The explosion spectra are relatively richer in the high frequency band (1.35–2.0 Hz) than are the natural earthquake spectra. An interesting result suggested by this study is that the short period P-wave spectra of ‘afterevents’ of large NTS explosions resemble the spectra of explosions rather than that of ‘natural’ earthquakes.