The Influence of Confining Pressure and Preexisting Damage on Strain Localization in Fluid‐Saturated Crystalline Rocks in the Upper Crust.

The spatial organization of deformation may provide key information about the timing of catastrophic failure in the brittle regime. In an ideal homogenous system, deformation may continually localize toward macroscopic failure, and so increasing localization unambiguously signals approaching failure. However, recent analyses demonstrate that deformation, including low‐magnitude seismicity, and fractures and strain in triaxial compression experiments, experience temporary phases of delocalization superposed on an overall trend of localization toward large failure events. To constrain the conditions that promote delocalization, we perform a series of X‐ray tomography experiments at varying confining pressures (5–20 MPa) and fluid pressures (0–10 MPa) on Westerly granite cores with varying amounts of preexisting damage. We track the spatial distribution of the strain events with the highest magnitudes of the population within a given time step. The results show that larger confining pressure promotes more dilation, and promotes greater localization of the high strain events approaching macroscopic failure. In contrast, greater amounts of preexisting damage promote delocalization. Importantly, the dilative strain experiences more systematic localization than the shear strain, and so may provide more reliable information about the timing of catastrophic failure than the shear strain. Plain Language Summary: The ability of deformation, such as fractures and strain, to spatially cluster or localize produces a wide range of geologic features on Earth, such as crustal fault networks and plate tectonics. Previous work demonstrates that deformation can evolve toward more localized distributions. However, recent analyses show that deformation can temporarily decrease in localization. These decreases in localization complicate efforts to use the spatial organization of seismicity, e.g., as a precursor of approaching large earthquakes. The factors that promote phases of delocalization remain unconstrained. Here, we perform a series of experiments to identify the factors that control the delocalization of local strain events within low porosity, Westerly granite rock cores. We find that both the confining pressure, indicative of depth within the crust, and the amount of preexisting damage of the rock cores control the amount of localization that the strain events experience, and the phases of delocalization. Increasing confining pressure produces more localization of the high strain events. More preexisting damage produces more delocalization. Key Points: Digital volume correlation of X‐ray tomograms reveals strain localization in triaxial compression experimentsLarger confining pressure promotes localization approaching macroscopic failureMore preexisting damage promotes episodes of delocalization [ABSTRACT FROM AUTHOR]