Abstract:

The stress-triggering earthquake, particularly the mechanisms and conditions of stress-induced earthquakes, is of great theoretical and practical significance for understanding seismic mechanisms and earthquake prediction. Monitoring changes of seismic activities caused by stress perturbations makes it possible to predict regional seismic hazards, which has become one of the primary methods in earthquake forecasting. Acoustic Emission(AE)is the laboratory signal analogous to seismic events. Studying the evolution of AE activities under stress perturbations in laboratory experiments helps identify the sub-instability stage of fault slip and captures the precursors of fault stick-slip, providing crucial reference for the studies of earthquake forecasting.

To investigate the effects of stress perturbations on fault stick-slip and to identify and analyze precursors of fault stick-slip, this study conducted frictional experiments using a servo-controlled biaxial loading machine with a direct shear structure of three granite blocks. Small-amplitude sine or square waves with different amplitudes and periods were modulated to normal and shear loading to investigate the effects of stress perturbations on fault slip instability, acoustic emission(AE) activityand fault strain.

The findings reveal that normal stress perturbations can significantly reduce the failure stress at the fault stick-slip, and the difference in failure stress before and after stress perturbation increases as the stress perturbation grows. In contrast, shear stress perturbations lead to failure stresses that were sometimes higher and sometimes lower than those under unperturbed conditions, with the differences between the high and low shear stress at the fault stick-slip increasing with the perturbation stress. The perturbation period has little influence on fault slip, and the square-wave stress perturbations exhibit greater effects on fault slip than the sinusoidal ones. AE activity shows a rapid increase before fault stick-slip. Normal stress perturbations suppress this AE activity, increasing before stick-slip, whereas shear stress perturbations enhance this increase. Normal stress perturbations do not alter the macroscopic distribution of fault stress along the fault strike, but shear stress perturbations increase spatial heterogeneity in fault stress. The amplitude, period, and loading rate of stress perturbations, along with fault normal stress, influence only the failure stress of stick-slip and the number of AE events, not the AE evolution.

Based on the experimental results, it is concluded that normal stress perturbations can change the contact between asperities on the fault, thereby affecting the fault frictional strength, without altering the spatial distribution of fault stress. Conversely, shear stress perturbations modify the shear stress only at certain fault sections, increasing the heterogeneity of the fault stress. This indicates fundamentally different mechanisms between normal and shear stress perturbations on triggering fault stick-slip. Furthermore, normal stress perturbations weaken the increasing of AE activities before fault stick-slip, thereby increasing the difficulty of predicting fault stick-slip according to the AE activities. In contrast, shear stress perturbations enhance this phenomenon, thereby increasing the possibility of predicting fault stick-slip based on AE activities.

Key words: frictional experiment, stress perturbation, acoustic emission, shear strain, forecast fault stick-slip

摘要：

文中利用双轴伺服控制加载装置, 采用由3块花岗闪长岩标本组成的含有2个滑动面的直剪结构开展了断层摩擦实验。实验中, 在断层面法向及滑动方向上施加不同振幅和周期的应力扰动, 并讨论了应力扰动对断层滑动失稳行为、 声发射活动及应变时空演化的影响。结果表明, 作用在整个断层的正应力扰动改变了断层面凹凸体的接触状态, 影响断层的摩擦强度, 但未改变断层面应力的空间分布; 作用在断层滑动加载点的剪应力扰动仅改变断层的局部剪应力, 增加了应力分布的非均匀性, 但不影响断层的摩擦强度。声发射活动在断层黏滑失稳前显著增强。正应力扰动抑制了断层黏滑失稳前声发射活动增强的现象, 增大了根据声发射活动预测断层黏滑失稳的难度。相反, 剪应力扰动强化了声发射活动增强的特征, 提高了根据声发射活动预测断层黏滑失稳的可能性。应力扰动的振幅、 周期和加载速率, 以及断层的正应力仅影响断层的失稳强度和声发射数量, 不影响声发射的演化特征。

关键词: 摩擦实验, 应力扰动, 声发射, 剪应变, 预测断层黏滑失稳