动态触发对断层亚失稳过程影响的实验研究

为探究动态应力扰动对断层亚失稳过程的影响，本文基于实验室地震研究方法，对准静态加载下的断层施加动态应力扰动，模拟不同应力状态下断层的地震动态触发过程，利用高速摄影和数字图像相关方法（DIC）观测动态触发地震的全过程.研究结果表明，当动态扰动恒定时，对于在亚失稳之前触发的事件，其破裂模式取决于初始的应力水平（τ/σ_n，剪应力/正应力）：当应力水平较低时（τ/σ_n < 0.42），破裂发生之后无法持续传播，形成自停止破裂，破裂速度（V）相对较低；而当应力水平较高时（τ/σ_n>0.42），断层更加接近临界状态，易产生完全破裂，破裂速度较高.整体而言，破裂长度随着初始应力水平的提高而增长，直至发生完全破裂.而当断层进入亚失稳阶段后，可以看到明显的成核阶段，在此期间施加的动态扰动均能触发整个断层的破裂.失稳时的应力降及破裂速度均高于亚失稳阶段之前的动态触发事件，甚至能触发超剪切破裂.此外，应力降（Δτ）随着初始应力水平的提高呈增加趋势.以上结果表明，动态触发地震的破裂特征与断层所处的应力状态存在对应关系.本研究系统性地揭示了动态触发地震的破裂特征，既丰富了地震亚失稳理论研究，也为利用动态触发地震的震源特征推测断层应力状态提供实验依据.

Experimental study of the effect of dynamic triggering on the meta-instable stage of faults before earthquakes

To explore the effect of dynamic triggering on the meta-instable stage, a series of highly controlled experiments are conducted to simulate the dynamic triggering of earthquakes under different stress states based on the servo-controlled biaxial apparatus and digital image correlation (DIC)method combined with high-speed photography technology in the laboratory. The results show that the triggered rupture is controlled by the stress state (τ/σn, shear stress/normal stress) of the fault. Two kinds of dynamically triggered earthquakes are identified. If the fault is far from instability (τ/σn < 0.42), the triggered rupture always assumes low rupture velocity and quenches before sweeping the entire fault (i.e., self-arrested rupture). On the contrary, if the fault is sufficiently stressed (τ/σn>0.42), the triggered rupture would run away at a velocity approaching Rayleigh wave speed or even exceeding the shear wave speed. Overall, the increase in τ/σn facilitates the rupture of the entire fault. Specifically, when the fault enters the meta-instable stage, the dynamic disturbance persistently triggers the rupture of the entire fault. In addition, the stress drop and rupture velocity of the triggered events in the meta-instable stage are significantly higher than those before the meta-instable stage. Moreover, the relative stress drop (Δτ/σn) increases with the initial stress state. The results indicate that when the dynamic disturbance is fixed, there is a clear correlation between the triggered ruptures and the static stress state of the fault. This study systematically reveals the rupture characteristics of dynamically triggered earthquakes, which enriches the study of the meta-instable stage of faults. Meanwhile, our results provide an experimental basis for measuring the stress state of the fault by dynamically triggered earthquakes.