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Frequency Dependence of Coda Q, Part I: Numerical Modeling and Examples from Peaceful Nuclear Explosions

Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK S7N 5E2, Canada igor.morozov{at}usask.ca

Joel N. Duenow,^* Elena A. Morozova, and Scott B. Smithson

Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071

^* Present address: Colorado School of Mines, Golden, Colorado 80401.

Frequency-dependent coda attenuation values are often reported; however, such measurements usually depend on the types of the Q(f) models employed. We use numerical modeling of peaceful nuclear explosion (PNE) coda at far-regional to teleseismic distances to compare two such models, namely, the conventional frequency-dependent quality factor (Q_coda(f)=Q₀f) and frequency-independent coda attenuation (Q_c) with geometrical attenuation (). The results favor strongly the (,Q_c) model and illustrate the mechanisms leading to apparent Q_coda(f) dependencies. Tests for variations of the crustal velocity structures show that the values of are stable and related to lithospheric structural types, and the inverted Q_c-values can be systematically mapped into the true S-wave attenuation factors within the crust. Modeling also shows that could increase in areas where relatively thin attenuating layers are present within the crust; such areas could likely be related to younger and active tectonics. By contrast, when interpreted by using the traditional (Q₀,) approach, the synthetic coda shows a strong and spurious frequency dependence with 0.5, which is also similar to the observations.

Observed Lg coda from two PNEs located in different areas in Russia show similar values of 0.75x10^-2 sec^-1, which are also remarkably close to the independent numerical predictions. At the same time, coda Q_c-values vary strongly, from 850 in the East-European platform to 2500 within the Siberian Craton. This suggests that parameters and Q_c could provide stable and transportable discriminants for differentiating between the lithospheric tectonic types and ages and also for seismic coda regionalization in nuclear test monitoring research.