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

Intrinsic anelasticity of crustal rocks causes energy dissipation of seismic waves when they propagate through the media. This dissipated energy can be quantified in terms of a seismic quality factor (𝑄), which can be used as a potential attribute for subsurface characterization. This parameter depends on the seismic frequency, as well as on temperature, water content, and grain size of the rocks (e.g., Karato & Spetzler, 1990; Jackson & Faul, 2010). On the other hand, the viscous deformation of crustal rocks occurs through different anelastic mechanisms including diffusion creep, numerous mechanisms of the dislocation creep, pressure solution that exhibits dependency on their structure, composition, and fluid content, as well as on their P-T conditions (e.g., Burov, 2011). Therefore, it is likely that seismic attenuation and the viscous modes of deformations of rocks can be correlated, based on their dependency on the aforementioned conditions, as expressed by an Arrhenius-type equation (Farina et al., 2019). In this study, we investigate the quantitative relationships between seismic attenuation and viscous rocks' rheology, especially across the domain where rocks transition from a dominant brittle to a more ductile deformation mode (Brittle Ductile Transition, BDT). We rely on a Burgers mechanical model to derive shear wave attenuation (1/𝑄𝑠 ), for several dry and wet crustal rheology, thermal conditions, and different strain rates values. This allows us to establish geothermal and mechanical conditions at which the BDT occurs and to cross-correlate this transition to computed shear seismic wave attenuation values. In particular, we observe a relatively significant 𝑄𝑠 reduction for strain rates of 10-13 s -1 , despite the assumed rock‘s rheology and thermal conditions. These first results confirm our hypothesis that variations in the 𝑄𝑠 factor can be effectively used to identify the BDT’s depths in tectonically active 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, which can add constraints on the relationship between seismic attenuation and rheological flow laws.