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 ( Q coda( f )= Q f η ) and frequency-independent coda attenuation ( Q c ) with geometrical attenuation ( γ ). The results favor strongly the ( γ , 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 γ 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 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 ( Q , 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 Q c -values vary strongly, from 850 in the East-European platform to 2500 within the Siberian Craton. This suggests that parameters γ 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.