关键词: 断层接触状态, 地震活动性, 声发射, 应力降, 正应力

Abstract:

Fault-contact heterogeneity is a common property of natural fault zones. Studying the relationship between seismic activity and fault-contact heterogeneity may provide new perspectives on the spatial relationship between small seismic events and mainshocks. In this study, granite samples were prepared by polishing, and experimental faults with heterogeneous contact characteristics were constructed by prefabricating the geometric morphology of the fault surfaces. The experimental faults were designed with a periodic, alternating structure of strong-contact and weak-contact zones of equal area, thereby achieving controllable fault-contact heterogeneity. On this basis, shear tests were performed under different average normal stresses to investigate fault-slip instability, generating stick-slip motion and simulating natural earthquakes at the laboratory scale. During the experiments, acoustic emission(AE)signals and local fault strain were synchronously monitored, and the magnitudes of AE events were calculated using the empirical Green’s function method. This method provides a unified scale for results under different experimental conditions, facilitating not only horizontal comparisons between groups but also scaling analogies between laboratory earthquakes and natural earthquakes, thus improving the extrapolation applicability of experimental results. Meanwhile, the μDA method was adopted to supplement the calculation of the mainshock magnitude. Key parameters, including the spatial-temporal distribution, magnitude characteristics, event frequency, and stress drop of foreshocks and mainshocks within the fault stick-slip cycle, were systematically analysed to reveal the coupled effects of fault contact heterogeneity and average normal stress on seismic activity. The experimental results showed that: 1) The fault contact heterogeneity has a significant influence on the distribution of small AE events and the initial locations of mainshocks. Namely, the small AE events are concentrated in areas of high normal stress or in areas with high gradients of normal stress. At the same time, the initial locations of the mainshocks are usually relative to the areas of high ratio of shear stress to normal stress, suggesting that the mechanisms of small AE events and mainshocks may be different. Additionally, the frequency of small AE events increases significantly as the mainshocks approach, while spatially, the small AE events activity is not in the same contact zone as the mainshocks. 2) The moment magnitudes of the AE events, which were obtained based on an empirical Green’s function method, are mostly between -8 and -7 for the small AE events but are between -4.6 and -4.1 for the mainshocks. Both the displacement results are used to calculate the main shock magnitude, which is between -3.4 and -3.0. The results of different methods can provide important constraints for the laboratory seismic scale system. 3) The average normal stress has significantly different effects on the magnitude and stress drop between the small AE events and main shocks. The magnitudes and stress drops of small AE events are not sensitive to the average normal stress. On the other hand, both the magnitudes and stress drops of the main shocks increase significantly as average normal stress increases. In either small AE events or the mainshocks, the logarithm of the stress drop increases with magnitude, and the relationship between the two is approximately linear, consistent with results from some other laboratory experiments. In summary, fault-contact heterogeneity significantly affects the spatiotemporal distribution of acoustic emission events. In conclusion, the heterogeneity of fault contact significantly influences the spatiotemporal distribution of acoustic emission events. Notably, the spatial inconsistency between major earthquakes and minor seismic activity highlights potential uncertainties in strong earthquake prediction using minor seismic data. The non-uniform distribution of fault-contact stress serves as a key factor controlling the spatial distribution of minor and major earthquakes, providing crucial laboratory evidence for understanding the relationship between background seismic activity and strong earthquakes in natural fault systems.

Key words: Fault contact state, Seismic activity, Acoustic emission, Stress drop, Normal stress