动态触发对断层亚失稳过程影响的实验研究

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

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.

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/σ_(n),shear stress/normal stress)of the fault.Two kinds of dynamically triggered earthquakes are identified.If the fault is far from instability(τ/σ_(n)<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/σ_(n)>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 inτ/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/σ_(n))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.