Source Characteristics of Two Underground Nuclear Explosions

Abstract

SUMMARY Broadband seismic waveform data recorded in the distance range of 1-11 km from two underground explosions detonated at Pahute Mesa of the Nevada Test Site have been used to estimate the source time functions for the second-order moment tensors of these events. Data were available from eight three-component stations for the explosion Harzer and from 11 stations for the explosion Chancellor. Stable moment tensor estimates with a good signal-to-noise ratio were obtained in the frequency range of 0.2-5.0 Hz. The moment tensors are dominated by the diagonal elements, but a significant asymmetry is present, with vertical force couples being larger, having additional low-frequency energy in the 0.3-0.4 Hz range, and having a different time dependence in the 0.5-2.5s range than the horizontal force couples. Considerable evidence points to the possibility that this asymmetry is related to spall processes taking place in the depth interval between the explosion and the free surface. When the isotropic parts of the moment tensors are interpreted as reduced displacement potentials, the results are in reasonable agreement with theoretical and empirical models, except that the characteristic times are about three times those predicted by published scaling relationships. This difference is most likely due to the effects of spall, but the fact that the explosions were detonated in unsaturated materials may also be a contributing factor.