Dislocation Damping and Anisotropic Seismic Wave Attenuation in Earth's Upper Mantle

Abstract

Crystal defects form during tectonic deformation and are reactivated by the shear stress associated with passing seismic waves. Although these defects, known as dislocations, potentially contribute to the attenuation of seismic waves in Earth's upper mantle, evidence for dislocation damping from laboratory studies has been circumstantial. We experimentally determined the shear modulus and associated strain-energy dissipation in pre-deformed synthetic olivine aggregates under high pressures and temperatures. Enhanced high-temperature background dissipation occurred in specimens pre-deformed by dislocation creep in either compression or torsion, the enhancement being greater for prior deformation in torsion. These observations suggest the possibility of anisotropic attenuation in relatively coarse-grained rocks where olivine is or was deformed at relatively high stress by dislocation creep in Earth's upper mantle.

Low‐Frequency Measurements of Seismic Moduli and Attenuation in Antigorite Serpentinite

“…This interpretation is first supported by the overall fine-grained nature of the material, along with its pronounced grain size and shape heterogeneity. Furthermore, laboratory observations suggest that dislocation damping in silicates (e.g., in olivine) is usually only operative at temperatures significantly above the antigorite stability field (Guéguen et al, 1989;Farla et al, 2012). The operation of an intragranular mechanism is unlikely due to the difficulty in activating dislocation slip systems in antigorite without very high differential stresses (Auzende et al, 2015).…”

Dissipation at tidal and seismic frequencies in a melt‐free Moon

“…Very few experimental observations exist on the effect of composition on attenuation, but the effects are probably modest [ Karato , 2006]. Any factors increasing attenuation, such as water [ Karato and Jung , 1998], melt [ Jackson et al , 2004; Faul et al , 2004] or dislocations [ Farla et al , 2012], would require a reduction in temperature in order to match the dissipation constraints. Similarly, the presence of water lowers the effective viscosity [ Hirth and Kohlstedt , 2003] and would make CMB relaxation more likely.…”

Deep Earth Structure: Q of the Earth from Crust to Core

“…Much progress in the measurement of attenuation at seismically relevant frequencies has been achieved in the last 15 years (e.g., Faul et al, 2004;Getting et al, 1997;Jackson, 1993;Jackson and Faul, 2010;Karato and Spetzler, 1990). Research continues into the mechanism of attenuation, including grain boundary sliding (Morris and Jackson, 2009), diffusion creep (Sundberg and Cooper, 2010), and the effect of dislocations (Farla et al, 2012). There is some evidence that attenuation scales with diffusion creep viscosity (e.g., , that hydration of mantle minerals causes significant increases in attenuation (Aizawa et al, 2008), and that water (Shito et al, 2006) and melt can significantly increase attenuation.…”