Experimental Study on Stress-induced Pre-failure Damage in Rocks and Its Applications to Earthquake Source-process Research Based on AE

This paper reviews recent studies concerning the evolution of the pre-failure damage in several common lithologies, subjected to differential compression, by using detailed acoustic emission (AE) data. The experimental results exhibit a damaging process characterized by three typical phases of microcracking activity: the primary, secondary, and nucleation phases, respectively. The primary phase reflects the initial opening or rupture of pre-existing microcracks, and it is characterized by an increase, with increasing stress, both in the event rate and b value. The secondary phase involves sub-critical growth of the microcrack population in the test sample, revealed by an increase in event rate and a decrease of the b value with increasing stress. The nucleation phase corresponds to the initiation and accelerated growth of the ultimate fracture along one or more incipient fracture plane. During the nucleation phase, the b value decreases rapidly to the global minimum value around 0.5. The temporal variation of b value in every phase correlated clearly with the major grain size of the test sample, indicating that a comparatively larger grain size results into a lower b value. Based on the constitutive laws of sub-critical growth of crack populations, a damage model was improved and tested, in order to investigate the fracture mechanism of every phase. At the same time, we have also made simultaneous timeto-failure measurements of the energy release, seismic b value, fractal dimension and spatial correlation length (SCL) accompanying pre-failure damage. Our results indicate that each parameter is a function of the time-to-failure and thus can potentially be used as an indicator of the critical point. It is clear that the pre-failure damage evolution is generally characterized by: 1) accelerated moment release, 2) a decrease in the fractal dimension and SCL with a subsequent precursory increase, and 3) a decrease in b value from ~1.5 to ~0.5 for hard rocks and from ~1.1 to ~0.8 for soft rocks such as S-C cataclasite.