Neelima Kandula

Faults in carbonate rocks show both seismic and aseismic deformation processes, leading to a wide range of slip velocities. We deformed two centimeter‐scale cores of Carrara marble at 25°C and imaged the nucleation and growth of faults using dynamic synchrotron X‐ray microtomography. The first sample experienced a constant confinement of 30 MPa and no pore fluid. The second sample experienced confinement in the range 35–23 MPa and water as a pore fluid at 10 MPa pore pressure. We increased the axial stress by steps until creep deformation occurred and imaged deformation in 4‐D. The samples deformed with a quasi‐constant or increasing strain rate when the differential stress was constant, a process called creep. However, for both samples, we also observed transient events that include the acceleration of creep, that is, creep bursts, phenomena similar to slow slip events that occur in continental active faults. During these transient creep events, strain rates increase and correlate in time with strain localization and the slow development of system‐spanning fault networks. In both samples, the acceleration of opening and shearing of microfractures accommodated creep bursts. High‐resolution time‐lapse X‐ray microtomography imaging and digital image correlation during triaxial deformation quantify creep in laboratory faults at subgrain spatial resolution. This work demonstrates that transient creep events, that is, creep bursts or slow slip events, correlate with the nucleation and slow growth of faults and not only with slip on preexisting faults.

Faults in carbonate rocks show both seismic and aseismic deformation processes, leading to a wide range of slip velocities. We deformed two centimeter-scale cores of Carrara marble at 25°C and ...

Faults in carbonate rocks show both seismic and aseismic deformation processes, leading to a wide range of slip velocities. We deformed two centimeter-scale cores of Carrara marble at 25°C, und...

Faults in carbonate rocks show both seismic and aseismic deformation processes, leading to a wide range of slip velocities. We deformed two centimeter-scale cores of Carrara marble at 25°C, under in-situ conditions of stress of 2-3 km depth, and imaged the nucleation and growth of creeping faults using dynamic synchrotron X-ray microtomography with micrometer spatial resolution. The first sample was under a constant confinement of 30 MPa and no pore fluid. The second sample was under a confinement in the range 35-23 MPa, with 10 MPa pore fluid pressure. We increased the axial stress by steps until creep deformation occurred and imaged deformation in 4D during creep. The samples deformed with a steady-state strain rate when the differential stress was constant, a process called creep. However, for both samples, we also observed transient events that include the acceleration of creep, i.e., creep bursts, phenomena similar to slow slip events that occur in continental active faults. During these transient creep events, strain rates increase and correlate in time with strain localization and the development of system-spanning fault networks. In both samples, the acceleration of opening and shearing of microfractures accommodated creep bursts. Using high-resolution time-lapse X-ray micro-tomography imaging, and digital image correlation, during triaxial deformation allowed quantifying creep in laboratory faults at sub-grain spatial resolution, and demonstrates that transient creep events (creep bursts) correlate with the nucleation and growth of faults.

Centuries of work have highlighted the importance of several characteristics on fracture propagation. However, the relative importance of each characteristic on the likelihood of propagation remains elusive. We rank this importance by performing dynamic X‐ray microtomography experiments that provide unique access to characteristics of evolving fracture networks as rocks are triaxially compressed toward failure. We employed a machine learning technique based on logistic regression analysis to predict whether or not a fracture grows from 14 fracture geometry and network characteristics identified throughout four experiments on crystalline rocks in which thousands of fractures propagated. The characteristics that best predict fracture growth are the length, thickness, volume, and orientation of fractures with respect to the external stress field and the distance to the closest neighboring fracture. Growing fractures tend to be more clustered, shorter, thinner, volumetrically smaller, and dipping closer to 30–60° from the maximum compression direction than closing fractures.

Microscale heterogeneities influence failure mechanisms in the crust. To track the microstructural changes in rock samples when loaded until failure, we employed a novel experimental technique that couples dynamic X‐ray microtomography imaging with a triaxial deformation apparatus. We studied the brittle failure of Carrara marble under triaxial compression. Dynamic tomographic data revealed the spatial organization of microfractures and damage increments characterizing the precursory activity toward catastrophic failure. We quantified the emergence of scaling relationships between microstructural parameters, including total damage volume, incremental damage volume, the largest connected microfracture, and the applied differential stress. The total volume of microfractures accumulated from the beginning of the experiment as well as the incremental damage showed power law increase. The growth of the largest connected microfracture was related to differential stress as a power law with divergence at failure. The microfracture volume increments were distributed according to a power law with an upper cutoff that itself spanned the entire volume toward failure. These characteristic features of brittle failure in Carrara marble under compression are in agreement with theoretical models that consider failure as a critical phase transition. We also observed that, very close to failure, several power law relationships broke down, which we interpret to be related to the coalescence of the largest microfractures in a finite size volume. Scaling laws and associated exponents computed from our data are compared with predictions made from theoretical and numerical models. Our results show that precursors of macroscopic brittle failure in Carrara marble follow predictable trends.