Source Parameters of Nuclear Explosions MILROW and LONGSHOT from Teleseismic P Waves

Spectral ratios in the frequency range from 0·5 Hz to 3·0 Hz between underground nuclear explosions MILROW and LONGSHOT have been computed from teleseismic P waves recorded at seven LRSM stations. These ratios are interpreted on the basis of a theoretical model by Sharpe and Blake to estimate source parameters for the two explosions. The adopted model has a spherical source exerting a pressure, which is a step function of time, to the surrounding elastic medium. The results indicate that MILROW has a source radius of 3/4 km and LONGSHOT, 1/2 km. The source pressure of MILROW is estimated to be about twice as large as that of LONGSHOT. Spectral modulation, presumably caused by the pP waves, indicates a pP-P time interval of 0·86 s for MILROW and 0·55 s for LONGSHOT. These intervals imply an apparent average overburden P-wave velocity of 29 km s−1 for MILROW and 2·6 km s−1 for LONGSHOT. These velocities are significantly lower than the values obtained from on-site measurements, probably due mainly to a difference between the P-wave and the pP-wave velocities in the source region. The pP-wave amplitudes are apparently 0·6 times the corresponding P-wave amplitudes. Our results support the suggestions that the pP interference is partly responsible for the low excitation of long-period P waves by underground nuclear explosions and that the resultant spectral modulation may be utilized for estimating detonation depths and as a diagnostic aid for explosion identification.