Regional and teleseismic analysis of underground nuclear explosion waveforms and constraints for shear wave excitation mechanisms

Regional and teleseismic waveforms of underground nuclear explosions (UNEs) at the Balapan and the Nevada test sites are analyzed to understand the nature of shear waves excited from UNEs. The phase composition of wavefields in the source and receiver regions is examined using a source array frequency-wave-number analysis and polarization analyses. It is observed that regional mantle-lid shear waves, Sn, often develop strongly. On the other hand, observed teleseismic shear waves from UNEs are much weaker than those from earthquakes. Source array analyses of the shear waves from UNEs display coherent energy that is polarized in a certain azimuthal direction. The observation of coherent shear energy in different UNEs suggests that the shear energy is excited in a consistent manner at every UNE. We constrain the dominant shear wave excitation mechanism from the observed shear wave features. The observations appear to be consistent with the tectonic release model with conical rock cracking.