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Phase and spectral ratio discrimination between NTS earthquakes and explosions. Part I: Empirical observations

Earth Sciences Division Lawrence Livermore National Laboratory, L-205, Livermore, California 94550

Abstract

Seismic discrimination of small-magnitude (m_b < 4) nuclear explosions from earthquakes requires short-period (f > 0.5 Hz) regional measurements. In this study we evaluate the most promising short-period discriminants, P-wave to S-wave phase-amplitude ratios and low- to high-frequency spectral-amplitude ratios, for a data set from the Nevada Test Site. The data consist of 130 underground nuclear explosions, one very large chemical explosion, and 50 earthquakes, ranging from magnitude 2 to 6, recorded at two broadband digital seismic stations operated by Lawrence Livermore National Laboratory. In contrast with much previous discrimination work in the western United States, the common stations and paths for all the events in this study allow the discriminants to be evaluated without applying propagation corrections, and differences between events can be ascribed to very near source effects. The source effects found to have the most influence on the discriminants in this study are depth for the earthquakes, and the source medium properties of gas porosity, density, and velocity for the explosions.

The phase-amplitude ratio discriminants tested are P_n/L_g and P_g/L_g. Both show little dependence on magnitude and work better at higher frequencies and when averaged over the two stations. At 6 to 8 Hz, P_n/L_g discriminates all except the shallowest earthquakes, while P_g/L_g discriminates all but the high gas-porosity explosions.

Low- to high-frequency spectral-amplitude ratio discriminants were tested in each of the phases P_n, P_g, L_g and L_g coda. The explosions show significant dependence on source material properties such as gas porosity. All the spectral ratios show dependence on magnitude and for (1-2 Hz)/(6-8 Hz), none performs well below magnitude 3.5. Of the spectral ratios, L_g-coda performs the best, at least in part because of its extremely small interstation variability, discriminating all but the low gasporosity explosions.

The best discriminant performance is given by combining phase and spectral ratios. A sum of the logarithmic values of P_n/L_g, P_g/L_g, and the L_g coda discriminates all but two of the shallowest earthquakes. The source medium dependence of the discriminants suggests that discrimination performance differences between the western United States and other regions of the world may be due more to source property differences than differences in the regional path. Simply put, western United States discriminant studies differ from those in the rest of the world because of the lack of explosions in very high gas-porosity-low velocity materials outside of the Nevada Test Site.

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