Source characterization of Nevada Test Site explosions and western US earthquakes using Lg waves: Implications for regional source discrimination

An investigation is made of the Lg waves from 15 nuclear tests at the Nevad a Test Site (NTS) and 25 shallow western United States earthquakes to obtai n a scheme for discriminating between possible source mechanisms. The data were recorded at four broadband stations, operated by Lawrence Livermore Na tional Laboratory (LLNL), which encircle the NTS. The Lg wave spectra are m odeled through a genetic algorithm search to find optimal values for (1) th e seismic moment (M-O), (2) the corner frequency of the amplitude spectrum (f(C)), (3) the 1-Hz attenuation quality factors (Q(O)) for all of the path s, and (4) the frequency dependence (eta) of attenuation along the paths. B ecause the inversion solves for the path attenuation characteristics, littl e a priori information is needed for the analysis. The resulting M-O and f( C) values are used in conjunction with the rn, values as seismic discrimina nts. The populations of earthquakes and explosions have nearly identical re lationships between their moments and corner frequenciles. For earthquakes, the relationship is log M-O = 16.07 - 2.93 logf(C). For the set of NTS exp losions (using an earthquake source model), the relationship is log M-O = 1 5.87 - 2.76 logf(C). A source discriminant is found, however, through a com parison of the moments and m(b) values. This gives a line, log M-O = 10.20 + 1.16m(b), which cleanly separates the earthquake and explosion population s, with earthquakes falling above and explosions below the line. Reliabilit y tests examine the inverse trade-off between moments and Q(O) values. Nois e in the Lg spectra has a limited effect on the scaling relations.