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

摘要：

实验研究断层接触非均匀性对地震活动的影响可为分析小地震活动与主震的关系提供重要参考。文中通过预制断层面来实现非均匀接触条件, 并进行了不同正应力条件的滑动失稳实验, 结果显示: 1)断层非均匀接触对小地震分布与主震发生位置具有显著的影响, 主震前出现明显的小地震频度增加的特征。其中, 小地震主要集中发生在高正应力或正应力变化显著的区域, 而主震的起始位置则与高应力比的区域有关。这预示着小地震与主震的力学机制可能不同。2)通过经验格林函数法获得了小地震的矩震级大多分布在-8~-7级, 与主震(-3.4~-3.0级)相差达5个震级单位。3)平均正应力对小地震和主震的震级和应力降的影响存在明显不同。小地震震级与应力降对平均正应力变化不敏感, 但主震应力降与震级均随平均正应力增加而变大。总之, 断层面接触的强弱分布对声发射事件时空分布具有明显影响, 尤其是主震与小地震活动的空间不一致性, 有助于阐明根据小地震活动预测强震存在不确定性的观点。

关键词: 断层接触状态, 地震活动性, 声发射, 应力降, 正应力