Modeling Anelastic Effects Within the Near Surface

Date: 4/24/2019

Time: 04:45 PM

Room: Pine

In order to understand seismic site responses and wave propagation at various frequencies and scales, it is important to model them by using the same physical laws and consistent physical properties. In most current models, the elastic properties of the medium are generally well understood, but anelastic effects are only described by empirical parameters such as the damping ratio (ksi) spectral decay (kappa), or coda quality Q_c. In this paper, I discuss the medium properties and equations needed for ab initio modeling of these empirical parameters. Unfortunately, current models (including most seismic modeling codes) describe medium anelasticity also by empirical concepts such as the dynamic moduli, quality factor (Q) and the phenomenon of “material memory.” Although these concepts work in material-science applications in which no spatial gradients are considered, they are insufficient for seismic waves and lead to overly complex but inaccurate or incorrect integro-differential equations for the medium.

As an alternative approach, a rigorous and physically-consistent description of Earth’s media can be obtained by extending Biot’s poroelasticity. Two general observations follow from realizing this analogy with poroelasticity. First, pore-fluid flow friction is indeed the dominant anelastic mechanism in many near-surface, sedimentary, hydrothermal, and partial-melt environments. Second, poroelasticity shows that in blocky or layered media, the attenuation is often dominated not by the “dynamic moduli” or Q-factors of the blocks but by boundary conditions between these blocks, such as allowed or prevented pore flows. These boundary conditions are poorly known but can be highly variable. Consequently, the popular “intrinsic” and “scattering” components of attenuation may in fact be impossible to consistently define and separate. Nevertheless, direct modeling by using the extended Biot’s model can be used to model all types of seismic phenomena, as illustrated here by modeling laboratory experiments and wave transmission and reflections in layered media.

Presenting Author: Igor Morozov

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