青海玉树地震(M_s=7.1)产生超剪切破裂过程的动力学机制研究

Dynamic mechanisms for supershear rupture processes of the Yushu earthquake(M_S=7.1)

由于2010年玉树地震(Ms=7.1)产生了超剪切地震破裂,所以地震灾害特别严重.国内外地球科学家对该地震产生超剪切破裂过程的物理机制一直非常关注,但至今没有给出满意的解答.为此,文中根据玉树地震发震断层的实际几何构建有限单元数值模型,模型中的断层由2个断层段构成,它们之间有约10°的夹角,形成断层拐折.模拟结果表明,玉树地震的破裂由2个子事件组成;当破裂在震源所在的断层上成核后,先在第一个断层段上传播,其速度为亚剪切波速度;当破裂一旦越过断层拐折,在第二个断层段上传播时,破裂速度就立即转变为超剪切波速度.计算结果显示,当断层发生超剪切破裂时,断层上的位错幅度、破裂产生的地震波速度及加速度都会显著增大,从而造成地震灾害大大增加,这很可能是玉树地震的震害特别严重的重要原因.从模拟实验中还看到,若是模型中的断层没有发生拐折,在模型的其他参数都保持不变的情况下,破裂速度不会发生变化.但是,若初始应力场的方位与断层之间的夹角发生变化,这时断裂系统中尽管存在断层拐折,也不是一定能产生超剪切破裂.只有当初始应力方位与断层之间的夹角以及断层走向变化的偏角二者之间的关系恰到好处时,断层拐折才有可能促使断层破裂由亚剪切转化为超剪切破裂.所以,玉树地震之所以能产生超剪切地震破裂,恰恰是发震断层几何与初始应力场方位之间的关系达到某种"最佳状态"的结果.这也可能是天然地震中超剪切破裂事件稀少的原因之一.因此,研究超剪切地震破裂过程的动力学机制,对于深入研究地震震源过程、地震灾害评估等有着非常重要的科学意义.

Owing to supershear ruptures, seismic hazards in the 2010 Yushu earthquake is particularly severe. Earth scientists, both at home and abroad, have been paying more and more attention to the physical mechanisms of the supershear ruptures produced in the Yushu event. But, so far, no satisfactory explanation is given. To this end, we construct a finite element model based on the actual geometry of the seismogenic fault in the Yushu earthquake. In the model, the fault consists of two segments, and there is an intersection angle of 10° between them, forming a fault bent. The simulation result shows that the main rupture of the Yushu event is composed of two sub-events. Once the rupture was nucleated at the hypocenter, the rupture first propagated along the first fault segment at a speed of subshear wave velocity. When the rupture goes over the fault bent, the rupture speed turned into supershear wave velocity immediately along the second fault segment. Moreover, the calculation results suggest that dislocations at fault surface, seismic wave speed and strong ground motion acceleration are largely amplified by supershear ruptures, giving rise to terrible damage. This may be an important reason why severe seismic damage was caused in the Yushu earthquake. In particular, we can see from numerical experiment that rupture speed will not change if the strike of the fault in the model does not bend when all other model parameters keep unchanged. However, if the intersection angle between the orientation of initial stress and the fault varies, rupture will not necessarily propagate at supershear wave speed, even though there is a fault turning in the fault system. Only if the relationship between the orientation of initial stress and the strike of the fault is just perfect, the fault bent possibly promotes supershear ruptures. Therefore, supershear rupture occurred in the Yushu event may be the result of optimum state in which perfect relationship between orientations of initial stress and the strike of seismogenic fault is formed. It is presumably one of the reasons why natural earthquakes with supershere ruptures are rarely seen. Thus, studying supershear rupture will be helpful to deeply understand seismic source process and seismic hazard assessment.