P-wave Spectra from Underground Nuclear Explosions

This paper reports observations of P-wave spectra from underground nuclear explosions for periods greater than about 1 s. The spectra have a maximum at about 2–3 s and decrease rapidly at longer periods. Two phenomena could cause such a decrease: (1) The surface reflection (pP), with its polarity opposite to that of the direct P wave, might differentiate the long-period signal and cause a modulation of 2|sin ωt0/2| where t0 is the interval of time between P and pP; or (2) the source time function for the pressure on the boundary of the elastic zone surrounding the explosion might more nearly resemble an impulse than a step function as often assumed. The data are consistent with the simultaneous occurrence of both phenomena, but they are of insufficient quality or quantity to establish which is more important. If other information shows that the reflection does not have a large effect on the observed spectrum, then the data show that the source time function is primarily an impulse. In addition, the pronounced difference in spectral content of P waves from earthquakes and underground nuclear explosions suggests a method for identification that is applicable for explosions large enough to be recorded by long-period seismographs at teleseismic distances. This method may be particularly important when long-period instruments are disturbed by other events and methods of identification that rely on analysis of surface waves become inapplicable.