Source Spectral Modeling of Regional P/S Discriminants at Nuclear Test Sites in China and the Former Soviet Union

Theoretical understanding of regional P/S discriminants has been a long- standing research topic with important implications for nuclear explosion monitoring. The observation that discrimination is poor at low frequencies, but becomes significant at higher frequencies, usually at about 3 to 4 Hz, particularly needs understanding. To gain insight, source and attenuation models are used to predict spectra of regional seismic phases ( Pn, Sn , and Lg ) for nuclear explosions and selected earthquakes near the Lop Nor, Semipalatinsk (Balapan and Degelen Mountain), and Novaya Zemlya test sites in China and the former Soviet Union. A modified Brune (1970) model (a la Walter and Taylor, 2002) is used to predict P - and S -source terms for earthquakes. A Mueller/Murphy (1971) model is used for explosions, including a new conjecture that the S waves may be modeled by the same functional form as for P waves, but with corner frequency reduced by the ratio of near-source shear and compressional velocities, vs ( S )/ vs ( P ). Results indicate that the frequency dependence of Pn / Sn and Pn / Lg discrimination performance at all of these hard-rock test sites is primarily due to differences in the corner frequencies of P and S waves for explosions, qualitatively consistent with observations by Xie and Patton (1999) for Lop Nor. P / S discrimination emerges at the explosion S -wave corner frequency and does not improve above the P -wave corner frequency. Scaling of the explosion S -wave corner frequency with elastic radius as vs ( S )/ Re at all of these test sites suggests that major contributions to S -wave generation from explosions may be occurring at the same distance scale (i.e., the elastic radius) as P waves from explosions. The explicit physical mechanism of this near-source S -wave generation is still being investigated. Implications regarding application of regional P / S discriminants are discussed.