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Are L_g spectra from NTS explosions self-similar?

Earth and Environmental Sciences Division Los Alamos National Laboratory University of California, Los Alamos, NM 87545

Abstract

Network-averaged L_g spectra for 106 underground Nevada Test Site (NTS) nuclear explosions (recorded at the four broadband seismic stations operated by Lawrence Livermore National Laboratory) are shown to be approximately self-similar and to scale with cavity radius and scaled-cavity radius. Composite spectra produced by superimposing the spectra of individual tests of nearly identical yields conducted in a single geologic unit (e.g., Tunnel Beds tuff, Yucca Flat, below the water table) have amplitude variability of about a factor of 2. If a master spectrum is chosen that resembles the shape of a particular subgroup of nearly identical NTS explosions, the spectra of all other tests in the database can be superimposed on the master spectrum through the application of a shot-specific amplitude shift and a shot-specific frequency shift. The amplitude shift basically measures the difference between the low-frequency asymptotes of the individual spectrum and the master spectrum, and the frequency shift measures the difference in corner frequencies. We show that, for explosions producing cavities with a narrow range of measured scaled-cavity radii, the amplitude shifts scale with cavity volume. Further, if cavity volume scaling is applied to the amplitude shifts, the residuals are shown to depend on scaled-cavity radius or gas-filled porosity. The residuals (a measure of seismic coupling) appear to show slightly reduced scatter when plotted versus scaled-cavity radius as opposed to gas-filled porosity. The frequency shifts scale approximately as cavity radius to the 3/4 power. Although the NTS L_g spectra are approximately self-similar, differences in high-frequency decay and other shape parameters exist. For example, the rate of high-frequency decay is observed to progressively decrease as explosions approach the static water level from above. These differences do not have a significant effect on the connections established between the amplitude and frequency shifts with cavity radius and other source parameters but do suggest source differences of potential interest in discrimination studies.