Microstructural Heterogeneity May Explain Missing Seismic Signature in Upper-Mantle Grain-Boundary Sliding
A new computational study suggests that grain-boundary viscosity variations can suppress and broaden the seismic signature of elastically accommodated grain-boundary sliding (EAGBS) in olivine, potentially explaining why laboratory experiments show weaker signals than classical theory predicts. EAGBS is considered a candidate mechanism for seismic attenuation and dispersion in Earth's upper mantle. The findings reconcile a long-standing discrepancy between theoretical predictions and experimental observations, suggesting the mechanism may still be geophysically relevant even when not clearly visible in measurements.
Researchers used finite-element simulations on realistic grain geometries to investigate why elastically accommodated grain-boundary sliding (EAGBS)—a theoretically important source of seismic energy loss in the upper mantle—produces a weak or absent spectral peak in dry olivine experiments, contrary to classical predictions. The simulations tested two sources of microstructural heterogeneity: irregular grain shapes and variations in grain-boundary viscosity. While irregular grain geometry alone produced only modest effects, a broad distribution of grain-boundary viscosities progressively suppressed and broadened the characteristic Debye-like loss peak into a weak, wide-frequency background. This broadening results from the superposition of many relaxation processes with different timescales. The authors propose a reduced-order model to describe this aggregate behavior and conclude that EAGBS may remain geophysically significant for upper-mantle seismic attenuation and velocity dispersion even when not clearly detectable as a pronounced peak in laboratory measurements.
What's missing
The study does not discuss how its predictions might be validated through new experimental designs, nor does it address whether other competing mechanisms for upper-mantle attenuation might produce similar broadened spectra, making discrimination between mechanisms difficult.
What different sources said
- arXiv physicsCenter
Effects of microstructural heterogeneity on the macroscopic spectrum of elastically accommodated grain-boundary sliding
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