Frequency dependence of coda Q, Part I: Numerical modeling and examples from peaceful nuclear explosions

Abstract

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 (Qcoda(f) = Q0fη) and frequency-independent coda attenuation (Qc) with geometrical attenuation (γ). The results favor strongly the (γ, ≤c) model and illustrate the mechanisms leading to apparent Qcoda(f) dependencies. Tests for variations of the crustal velocity structures show that the values of γ are stable and related to lithospheric structural types, and the inverted Qc-values can be systematically mapped into the true S-wave attenuation factors within the crust. Modeling also shows that γ 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, η) approach, the synthetic coda shows a strong and spurious frequency dependence with η ≈ 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 γ ≈ 0:75 × 10-2 sec-1, which are also remarkably close to the independent numerical predictions. At the same time, coda Qc-values vary strongly, from 850 in the East-European platform to 2500 within the Siberian Craton. This suggests that parameters γ and Qc 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.