Quantitative characterization of evolving strain and microstructure during brittle faulting

Abstract:

Quantitative earthquake assessment using laboratory-derived constitutive laws requires better understanding of the heterogeneous distribution of stress on faults, and the evolution of fault surfaces during slip. Experimental characterization of the coupling between the evolving internal stress-strain and the evolving microstructure is extremely challenging with conventional experimental configurations. Synchrotron imaging technologies are poised to transform the study of rock deformation. Recent dynamic microtomography experiments have shown great potential in providing quantitative information of evolving strain distribution during fault growth at in-situ pressure and temperature conditions. Using an X-ray transparent deformation apparatus that operates at crustal stress conditions, we have imaged the process of fault nucleation and propagation in natural rocks undergoing brittle faulting. Applying the digital volume correlation technique to time-resolved 3-dimensional microtomographic datasets, we documented, for the first time, the evolution of strain distribution within a deforming rock. These results elucidate how fractures open, slide, coalesce, and propagate in rock samples responding to increasing shear stress.

If documented only after an experiment has concluded, microstructure constitutes an integrated record of many different processes and feedbacks between them. To unravel this record, experiments have traditionally been designed to isolate a given process, limiting access to the interplay between various processes that takes place in nature. Dynamic microtomography provides an alternative, more satisfactory solution by elucidating the evolving microstructure while the rock is being deformed. We studied the effect of chemo-mechanical coupling on fracturing induced by hydration reaction in serpentinite. We found that under the same stress conditions, reaction-induced fracture propagation is considerably slower at higher pore fluid pressure. A quantitative characterization of evolving mechanical behavior and microstructure leads to a better understanding of the underlying deformation mechanisms that control fault instabilities.