Size effect of accelerated seismic release from failures under compression in naturally fractured media

It is crucial to understand the scaling behaviors of acoustic emissions (AEs) preceding damage localization in order to predict failures of brittle solids under compression. Yet, the effect of length scale on the complex interplay between the initiation, propagation, and coalescence of pre-existing fracture networks and corresponding AE behaviors is poorly understood. In this study, we perform laboratory compressional experiments on naturally fractured rocks at four sample sizes from 50 to 300 mm whose strength generally exhibits a finite-size effect. We analyze the time history of AE energy distribution and accelerated seismic release (ASR) until catastrophic failures of the specimens. We find their time evolution towards failure resembles the observations from specimens containing a single fault and highly microstructurally disordered materials. We observe clear evidence that a size effect exists at small AE magnitude, where larger specimens tend to produce a higher proportion of smaller microcracks. However, the AE energy distribution is scale-independent at high energies. Near to failure, the power-law component of the AE energy population is almost stationary. The temporal evolution of the AE activity rate is independent of sample size; instead, there exist fast and slow periods of the AE activity rate that could be attributed to the stress heterogeneities around the fracture network. ASRs are exclusively observed in the AE activity rate and are more general in the lack of criticality. Our interpretation of observed foreshock activities at varying sample sizes in fractured media may have significance for understanding and predicting failures from natural hazards and engineering instabilities.