Grain-scale 4D visualisation of strain partitioning during brittle creep in sandstone

The partition of strain between seismic and aseismic processes, notably brittle creep, is highly variable in both tectonic and induced seismicity settings. The two processes have a complicated relationship, with brittle creep generally being associated with more distributed deformation and dynamic rupture with strain localisation. While the overall macroscopic strain behaviour during this process is reasonably well established, the mechanisms by which localised damage regions develop, interact, and ultimately coalesce to form localised fault zones remain under active investigation. The recent development of in-situ X-ray tomography during rock deformation experiments enables direct, time‑resolved, three‑dimensional interrogation of these processes at sub-grain scale.

 

Here, brittle creep was induced in a water-saturated sample of heavily cemented Clashach sandstone under triaxial conditions (σ₃ = 20 MPa, P_pore = 5 MPa) using the University of Edinburgh’s “Stór Mjölnir” deformation rig (Cartwright-Taylor et al., 2022). This triaxial rig is equipped with piezoelectric transducers to monitor acoustic emissions and seismic velocity change, and was mounted on synchrotron beamline I12 at Diamond Light Source, UK. In-situ X-ray microtomography was conducted throughout the creep process with a voxel edge length of 7.91 μm, comfortably smaller than the average grain diameter of ≅ 300 μm. These coupled datasets allow for simultaneous monitoring of changes in seismic velocity, acoustic emissions, macroscopic and grain-scale strains as the sample creeps (Cartwright-Taylor et al., 2022, Mangriotis et al., 2025).

 

Main (2000) proposed a damage mechanics model that explains the three stages of decelerating, steady-state and accelerating creep through a combination of two mechanisms: initial deceleration due to local hardening processes, with later acceleration driven by interactions between cracks. These three stages were observed in the macroscopic axial strain data and seismic velocity variation, which fit the model closely. Digital Volume Correlation was used to observe the strains within the sample throughout creep. During primary creep, these strains are predominantly dilation, with a steep positive correlation between volumetric and shear strains. These dilational strains are strongly localised around where the eventual failure-plane nucleates. As the sample transitions into secondary creep at ε_z ≅ 1.85%,  v_p reduces to around 85% of its initial value. More mixed compaction and dilation strains are observed, again localised around the eventual failure plane. A sharp burst of more widely distributed shear strain is observed at ε_z ≅ 1.9% as the strain transitions into tertiary creep, and v_p falls to around 80% of its initial value. These strains correspond approximately to the onset of acoustic emissions. The DVC strains then revert to a largely dilational mode prior to dynamic failure. This localised combination of dilation and shear strain development, and evolution of their relative importance over time, independently validates the combination of localised hardening and crack interaction proposed by Main (2000).

 

Main (2000); https://doi.org/10.1046/j.1365-246x.2000.00136.x

Cartwright-Taylor et al. (2022); https://doi.org/10.1038/s41467-022-33855-z

Mangriotis et al. (2025); https://doi.org/10.1038/s41598-025-03105-5