Abstract
Abstract In global Earth models such as the preliminary reference Earth model (PREM), seismic attenuation is dominated by shear friction, which is expressed by the relation Q K ≫ Q μ between the bulk and shear Q factors. Weak bulk attenuation appears to be one of the most general observations for Earth materials, which is often used as an assumption for building attenuation models. Nevertheless, we consider three observations suggesting that Q K may generally be comparable to or below Q μ . First, shear-dominated dissipation is a stringent limit requiring “auxetic” behavior of materials in respect to internal friction, that is, analogous to elastic materials with extreme negative Poisson’s ratios. Although possible, auxetic behavior is extremely rare in elasticity, and it is unlikely that it is common for internal friction. To avoid pervasive auxetic effects, the quality factors should generally satisfy Q K Q μ . Second, the relation between Q K and Q μ may depend on the wave modes in which these quantities are measured. Knopoff (1964) observed that in mantle surface waves, Q K may be lower than Q S despite Q K ≫ Q μ in free oscillations. Third, many observations and micromechanics-based models of Q K exist, particularly for rock containing grains, fractures, pores, and fluids. Almost all of these models show Q K values comparable to or below Q μ within the seismic frequency band. Thus, seismic attenuation is broadly variable and likely not shear dominated. The relation Q K ≫ Q μ inverted for the whole Earth could be caused by the use of a constant- Q parameterization, whereas the Q S can be strongly frequency dependent for free oscillations. In addition, key mechanisms of anelasticity (nonlinear viscosity, grain-boundary sliding, effects of partial melts) are not of viscoelastic type and cannot be accurately described by Q factors.
Published Version
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