Fig. 1: Bulk mechanical behaviour of Clashach sandstone.

a Evolution of differential stress, σ (blue line), and acoustic emission (AE) event rate, \(\dot{N}\) (green line), with time. Orange circles denote the times of the X-ray tomographic volumes. The plotted AE event rate was calculated from all recorded events, binned into 10 s time intervals. Consistent with previous studies^75,76, we identified five stages of deformation: (i) initial compaction and then quasi-elastic behaviour up to the yield point, (ii) strain hardening approaching peak stress, σ_P, (iii) damage zone localisation and strain softening beyond σ_P, (iv) sample weakening due to shear zone development through the sample and (v) shear sliding along a contiguous sub-planar fault. The transition from constant strain rate loading (10⁻⁵ s⁻¹) to constant AE event rate loading (1 ± 1 AE s⁻¹) occurred early in stage (ii) shortly after the sample yield point, which was defined by the point at which the stress-strain curve deviated from linearity and the AE event rate accelerated beyond the steady but low rate observed during the elastic region (linear portion of the stress-strain curve). b Photograph of the failed sample showing the localised shear damage zone. c Differential stress plotted against axial strain. Number labels in a and c refer to the tomogram slice and strain increment labels in Figs. 2 and 3, with tomogram 16 acquired at the yield point (transition from stage i–ii), tomogram 19 acquired at peak stress (transition from stage ii–iii), tomogram 22 acquired as microcracks localised along the critically oriented shear zone (transition from stage iii–iv), tomograms 25 and 28 acquired during shear zone development, tomogram 31 acquired at the onset of coherent sliding (transition from stage iv–v) and tomogram 34 acquired during coherent sliding. Young’s modulus, E = 19.369 ± 0.028 GPa, was calculated over the range shown. AE activity began at 40% of peak stress, σ_P, with initial strain localisation evident in the strain increments from 0.7σ_P onwards, and sample yield following at 0.85σ_P. The AE feedback control (1 AE s⁻¹) modulated the strain rate from 0.93σ_P.