The influence of confining stress and preexisting damage on strain
localization in fluid-saturated crystalline rocks in the upper crust
Abstract
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 stresses (5-20 MPa) and fluid pressures (zero to 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 stress 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.