Comparison of the microfracture localization in granite between fracturation and slip of a pre-existing macroscopic healed joint by acoustic emission measurements

Experiments of fracturation and slip of a preexisting macroscopic healed joint have been performed under triaxial deformation on granite from Mayet de Montagne (France). This granite shows high grain-scale inhomogeneity. Acoustic emissions have been recorded and hypocenters have been determined during the entire experiments. For both rupture experiment and slip experiment, precursory localization of microfractures in the final rupture plane has been observed in the early stage of deformation, well before the dilatancy. It is likely that not only initial closure of favorably oriented cracks but also breaking of partially cemented grains or slipping between grains may occur in the pseudoelastic phase and are already localized on the final rupture plane where the shear stress seems to be concentrate. This behavior is observed in both cases where stress heterogeneity and rupture nucleation are controlled by (1) medium-scale heterogeneity at the grain scale (HS sample) or (2) macroscopic heterogeneity in the form of a preexisting healed joint (JS sample). The sample with the healed joint exhibited ~ 1.6 times more acoustic emission events than the intact sample. The presence of the healed joint significantly weakened the sample

Detailed analysis of acoustic emission activity during catastrophic fracture of faults in rock

The detailed time-space distribution of acoustic emission (AE) events during the catastrophic fracture of rock samples containing a pre-existing joint or potential fracture plane is obtained under triaxial compression using a high-speed 32-channe waveform recording system, and the results are discussed with respect to the prediction and characterization of catastrophic fault failure. AE activity is modeled quantitatively in terms of the seismic b-value of the magnitude–frequency relation, the self-excitation strength of the AE time series, and the fractal dimension of AE hypocenters. Consistent with previous studies on rock samples containing a fracture plane with several asperities, the present analyses reveal three long-term phases of AE activity associated with damage creation on heterogeneous faults, each clearly identifiable based on the above parameters. Along-term decreasing trend and short-term fluctuation of the b-value in the phase immediately preceding dynamic fracture are identified as characteristic features of the failure of heterogeneous faults. Based on the experimental results it is suggested that precursory anomalies related to earthquakes and other events associated with rock failure are strongly dependent on the heterogeneity of the fault or rock mass. A homogeneous fault or rock mass appears to fracture unpredictably without a consistent trend in precursory statistics, while inhomogeneous faults fracture with clear precursors related to the nature of the heterogeneity