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Corner Frequency Scaling of Regional Seismic Phases for Underground Nuclear Explosions at the Nevada Test Site

¹ ATK Mission Research
8560 Cinderbed Road, Suite 700
Newington, Virginia 22122
mark.fisk{at}atk.com

Model fits are used to estimate source spectral corner frequencies of regional seismic phases Pn, Pg, and Lg from underground nuclear explosions (UNEs) at the Nevada Test Site (NTS), based on recordings by Lawrence Livermore National Laboratory (LLNL) NTS Network stations. The spectra are corrected for instrument response, distance, and station effects, and then network averaged. Explicit information regarding geophysical working-point properties, depth of burial, and some announced yields for NTS explosions (Springer et al., 2002) is utilized for the spectral modeling. Scaling of Pn, Pg, and Lg corner frequencies with source yield is examined for sets of explosions at Yucca Flat, Pahute Mesa, and Rainier Mesa with similar material properties (medium type, density, velocities, and gas porosity). The UNEs are limited to those in media with gas porosity of 10% or less so that a source spectral model with a rolloff of f^–2 is applicable. Model comparisons are also examined for regional P/Lg discriminants. A key result is that Lg corner frequencies for NTS UNEs exhibit similar scaling with source size as for P waves, but shifted lower, analogous to observations by Fisk (2006) for the Lop Nor, Semipalatinsk, and Novaya Zemlya test sites. This appears to corroborate an important effect, that is, that major contributions to S-wave generation by explosions occur near the source with a similar length scale, comparable to the elastic radius, as for P waves from explosions. Although the explicit physical mechanism is not yet understood, the implications are important regarding where and perhaps how S waves are predominantly generated by explosions. A related, key result is that the increasing separation of P/Lg at higher frequencies between NTS explosions and earthquakes has a consistent model-based explanation, as at other nuclear test sites, in terms of the difference between explosion P and S corner frequencies.