How does seismic attenuation correlate to rheology of crustal rocks? Results from a numerical approach

Abstract

Most natural resources are distributed within the uppermost layer of the lithosphere and their exploitation is limited by the transition from brittle to ductile rocks' deformation (BDT), which coincides with a strong reduction in rocks permeability. Therefore, knowledge of the physical and mechanical crustal properties is crucial for improving our understanding of the exploitable potential. Previous studies have showcased the existence of a relation between rocks' seismic attenuation and their viscous modes of deformations, considering that both depend on intrinsic rocks characteristics (e.g., grain size, fluid content) and background P-T conditions. In this study, we investigate such quantitative relationships between seismic attenuation and viscous rocks' rheology across the domain where rocks transition from a dominant brittle to a more ductile deformation mode. We rely on the Burgers and Gassmann mechanical model to derive shear wave attenuation (1/Qs), for several dry and wet crustal rheology, thermal conditions, and different strain rates. This allows us to establish geothermal and mechanical conditions at which the BDT occurs and cross-correlate this transition to computed shear seismic wave attenuation values. In particular, we observe that Qs variation with depth is more sensitive to the input strain rate than to the adopted rock‘s rheology and thermal conditions, so that a fixed amount of the Qs reduction can be used to identify the average BDT depths for each strain rate used. Below the BDT depth, we observe a significant drop in Qs (up to 10-4 % of the surface value), being also influenced by the background temperature and rock rheology. Since the greatest Qs reduction is estimated for the highest input strain rate (10-13 s-1), our results have implications for tectonically active/geothermal areas. Ongoing and future works will focus on a further validation of the modelling implications by systematic analyses of observations derived from rocks’ laboratory experiments. The last ones can add constraints on the relationship found in this study between seismic attenuation and adopted rheological flow law.