Abstract

A new method for studying the source characteristics of regional phases including explosion-generated S waves is developed and utilizes differences between spectrograms of two closely located explosions recorded at a common station. Relative source effects of a normal-buried explosion with respect to an overburied explosion are isolated in the resultant difference spectrogram because path and receiver site effects cancel. Difference spectrograms provide a global view of the relative frequency content, while the spectral ratio method is specific to the time window selected for Fourier analysis. Difference spectrograms for Nevada test site (NTS) explosions are characterized by the presence of amplitude modulations in Pg coda, in time windows predicted for Sn and Lg, and in long-duration Lg codas. Previous studies have modeled similar features in Lg spectral ratios by invoking the Rg-to-S scattering hypothesis and the notion of Rg imprinting, where the modulations arise due to interference of Rg waves from explosion and induced tensile failure sources. We propose that spectral peaking at low frequencies in Lg spectra is related to resonances in the source medium that affect the excitation and propagation of Rg waves, adding further support to Rg imprinting and the Rg-to-S scattering hypothesis. Difference spectrograms for explosion pairs Rousanne/Techado and Baseball/Borrego are interpreted to have a spectral null related to a pulse of Rg-derived energy from a common scatterer. Using the relative timing of this null, we estimate an Rg velocity (∼0.8 km/sec) and locate a candidate scattering source at the Yucca Basin boundary near Climax Stock. Difference spectrograms provide a wealth of information about the spectral content of regional phases related to source effects, which can be used to gain insights into source generation processes.

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