ANISOTROPIC ELASTIC STRESSES, CRITICALITY, AND ROCK FAILURE

Heterogeneous grain-scale stresses in elastically inhomogeneous, anisotropic rocks have been inferred to modulate rock failure in brittle, thermal-elastic, and numerical experiments. A quantitative understanding of grain-scale stresses is necessary to validate conceptual and mechanical models of rock damage, the role of heterogenous stress distributions in the seismic cycle, and the strength of Earth’s lithosphere in general. Despite their importance, no published studies precisely calculate the grain-scale distributions and magnitudes of these stresses in real anisotropic polycrystalline rocks under geologically constrained macroscale loading conditions. Here we apply numerical methods to real rock microstructures derived from electron backscatter diffraction to calculate grain-scale stresses arising from macroscale loading consistent with 7.5 km depth adjacent to the San Andreas fault. Using five different rock types we show that rocks containing more than one mineral phase experience grain-scale tensile principal stresses large enough to cause tensile microfracturing even though all principal components of macroscale loading are compressive. These tensile stresses arise owing to the elastic interactions of individual anisotropic grains. In addition, local differential stresses can exceed macroscale values by a factor of up to ~2.3, and relatively small stress perturbations can cause cascading failure. Our results provide a quantitative microstructural view of criticality in the continental crust with applications to earthquake nucleation and the progressive weakening of rocks through damage.