High-frequency P wave spectra from explosions and earthquakes

Two explosion P wave spectral models and two earthquake P wave spectral models are reviewed to assess their implications for high frequency (greater than 1 Hz) seismic discrimination between earthquakes and explosions. The importance of the corner frequency scaling, particularly for models with the same high frequency spectral decay rate, is demonstrated by calculating source spectral ratios (a potentially important regional discriminant) for these models. We compare North American events and a limited data set of Central Asian events with these spectral models. We find North American earthquakes are consistent with a constant stress drop modified Brune model between 10 and 30 Hz. Shallow (less than 700 m depth) Pahute Mesa explosions at the Nevada Test Site have a high frequency spectral decay between 10 and 30 Hz greater than the omega (exp -2) predicted by the explosion models. Near regional recordings of the Soviet Joint Verification Experiment (JVE) explosion show a higher corner frequency and lower 1 to 4 Hz spectral ratios than predicted by either explosion model. The higher corner frequency of the Soviet JVE appears not to be due to attenuation, or receiver effects, and may represent a need for different corner frequency scaling, or result from source complications such as spall and tectonic release. A regional recording of the Soviet JVE (NEIC m sub b equals 6.1) is shown to have a lower 1 to 4 Hz spectral ratio than a smaller earthquake (NEIC m sub b equals 4.6) recorded on a nearly reciprocal path.