Accelerating Moment Release of Acoustic Emission During Rock Deformation in the Laboratory

The Accelerating Moment Release (AMR) of seismicity before large earthquakes has been discussed by an increasing number of seismologists over recent years; however, most of their research is concentrated on theoretical descriptions based on statistical physics. In this paper, we investigate the laboratory AMR phenomenon of acoustic emission (AE), and attempt to understand the physical mechanism of AMR behavior from the point of view of rock deformation. AE data used in this paper are from a granitic porphyry (GP) sample with heterogeneous structure, including grains of different size and a naturally healed joint. Based on a stochastic AMR model, the microfracturing activity during rock deformation is analyzed. Three stages, Pre- AMR, AMR and nucleation, that cover the entire deformation period, are defined according to their different microfracturing features. The fractal structure of each stage is investigated. Our results indicate that the AE activity is highly sensitive to both the stress load and the rock structure. The AMR stage, in which the moderate AE events demonstrated typical AMR behaviors, features a process of stress concentration and stress transfer on the fault plane. The AMR stage had a constant stress load condition and was characterized by a much earlier increase of AE rate than the elevation of mean AE magnitude, both of which are consistent with the results derived from the damage rheology model (BEN-ZION and LYAKHOVSKY, 2002). The AMR stage was immediately followed by the nucleation stage, caused by quasi-static/dynamic fracture of the main fault. Therefore, regarding the GP sample, the AMR stage is a long-term preparatory process for dynamic fault fracture.