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Fracture network localization preceding catastrophic failure
  • Jessica McBeck,
  • Yehuda Ben-Zion,
  • Francois Renard
Jessica McBeck
University of Oslo

Corresponding Author:j.a.mcbeck@geo.uio.no

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Yehuda Ben-Zion
University of Southern California
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Francois Renard
University Joseph Fourier Grenoble
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We quantify the evolving spatial distribution of fracture networks throughout six in situ X-ray tomography triaxial compression experiments on monzonite and granite at confining stresses of 5-35 MPa. We first assess whether one dominant fracture continually grows at the expense of others by tracking the proportion of the maximum fracture volume to the total fracture volume. This metric does not increase monotonically. We next examine if the set of the largest fractures continually dominates deformation by tracking the proportion of the cumulative volume of fractures with volumes >90th percentile to the total fracture volume. This metric indicates that the fracture networks tend to increase in localization toward the largest set of fractures for up to 80% of the experimental time (differential stress), consistent with observations from southern California of localizing and delocalizing seismicity. Experiments with higher confining stress tend to have greater localization. To further assess the fracture networks localization, we compare the geometry of the set of the largest fractures to a plane. We find the best fit plane through the fractures with volumes >90th percentile immediately preceding failure, and calculate the distance between these fractures and the plane, and the r2 score of the fractures and the plane throughout each experiment. The r2 scores and the distance indicate greater localization in the monzonite experiments than in the granite experiments. The smaller mean grain size of the minerals in the granite may produce more sites of fracture nucleation and termination, leading to more delocalized fracture networks that deviate further from a plane. The higher applied confining stress in the monzonite experiments (25-35 MPa) relative to the granite experiments (5-10 MPa) may also contribute to the more localized fracture networks in the monzonite experiments. The evolution of the clustering the fractures toward the plane and the Gini coefficient, which measures the deviation of a population from uniformity, closely match each other. Tracking these metrics of localization also reveals that macroscopic yielding appears to occur when the rate of fracture network localization increases.