断层自发破裂动力学过程的有限单元法模拟及其在地震研究中的应用

发布时间：2018-06-24 00:16

本文选题：自发破裂 + 双材料断层　；参考：《中国地震局地球物理研究所》2017年博士论文

【摘要】：地震是断层的自发破裂动力学过程。数值模拟断层的自发破裂动力学过程对于认识地震的力学本质、减轻地震灾害等有着重要的科学意义及应用价值。本文首先对经典的滑移弱化摩擦关系进行了改进,然后对断层的破裂过程进行动态数值模拟。模拟结果表明,利用改进后的摩擦关系能够产生脉冲型(pulse-like)破裂模式。断层自发破裂过程受初始应力场及摩擦关系影响,若初始应力场中的剪应力水平较低或滑移弱化摩擦本构关系中的动摩擦系数较大,则容易产生脉冲型破裂;反之,则容易产生裂纹型(crack-like)破裂。另外,为了研究双材料(bimaterial)断层破裂对强地面运动的影响,我们采用正则化的速率-状态相关摩擦本构关系计算了破裂沿着双材料断层传播的二维有限元模型。模拟结果表明,双材料机制对地震破裂过程以及断层周边区域的强地面运动有显著影响。由断层破裂辐射出的地震波导致的强地面运动在整个空间上的分布是不对称的,其不对称性会随着断层两侧材料差异程度的增加而增加。断层破裂能否跨越断层阶区(stepover)继续传播,从而引发更大震级的地震,地震时断层是否发生超剪切破裂导致地震灾害加剧,都是震源动力学研究的重要内容。本文利用有限单元方法模拟断层阶区对地震破裂传播的控制作用以及对产生超剪切地震破裂的促进作用。研究结果表明:断层面上的摩擦系数减小、断层周边区域内初始剪应力增大以及较小的阶区间距等,都将增加断层破裂跳跃阶区传播的可能性;此外,这些物理因素都会对破裂的传播速度产生影响。在一定条件下,破裂传播速度会由在初始断层上的亚剪切波速度,转为在次级断层上的超剪切波速度。结合以上在概念模型中对断层自发破裂过程的模拟研究结果,我们根据汶川地震和玉树地震发震断层的实际几何分别构建有限单元数值模型,研究了汶川地震单侧破裂过程的动力学机制以及玉树地震产生超剪切破裂过程的动力学机制。2008年汶川大地震的破裂过程极其发杂,向东北方向的破裂距离长达300km,而向西南方向的破裂长度很小,呈现出单侧破裂的主要特征。文中模拟并分析了汶川地震的破裂过程,结果表明:龙门山断裂带两侧的物性差异是造成汶川大地震单侧传播的决定性因素。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。文中在模拟并分析玉树地震的破裂过程后认为:玉树地震发震断层走向与初始主应力方向之间的关系断层破裂由亚剪切转化为超剪切破裂的可能原因。
[Abstract]:Earthquake is a dynamic process of spontaneous rupture of faults. Numerical simulation of the dynamic process of spontaneous rupture of faults has important scientific significance and application value in understanding the nature of earthquake mechanics and mitigating earthquake disasters. In this paper, the classical slip weakening friction relationship is improved firstly, and then the dynamic numerical simulation of the fracture process of the fault is carried out. The simulation results show that the pulse type (pulse-like) fracture mode can be generated by using the improved friction relationship. The spontaneous fracture process of a fault is affected by the initial stress field and the friction relationship. If the shear stress level in the initial stress field is lower or the dynamic friction coefficient in the sliding weakening friction constitutive relationship is large, the pulse rupture is easy to occur. The crack mode (crack-like) rupture is easy to occur. In addition, in order to study the effect of bimaterial (bimaterial) fault fracture on strong ground motion, we use regularized rate-state dependent friction constitutive relation to calculate the two-dimensional finite element model of fracture propagation along the bimaterial fault. The simulation results show that the bimaterial mechanism has a significant influence on the earthquake rupture process and the strong ground motion around the fault. The distribution of strong ground motion caused by seismic waves radiated by fault rupture is asymmetrical in the whole space, and the asymmetry will increase with the increase of material difference on both sides of the fault. Whether the fault rupture can continue to propagate across the fault order region, thus causing an earthquake with a larger magnitude, or whether the earthquake disaster is aggravated by the fault supershear rupture during the earthquake, is an important content of the focal dynamics research. In this paper, the finite element method is used to simulate the controlling effect of the fault order on the earthquake rupture propagation and the promoting effect on the supershear seismic rupture. The results show that the decrease of friction coefficient in fault plane, the increase of initial shear stress in the peripheral region of the fault, and the smaller interval between the steps will increase the probability of propagation of the fracture jump stage of the fault. These physical factors have an effect on the propagation speed of the rupture. Under certain conditions, the velocity of fracture propagation will change from the velocity of sub-shear wave on the initial fault to the velocity of the supershear wave on the secondary fault. Based on the simulation results of the spontaneous rupture process of the fault in the conceptual model above, a finite element numerical model is constructed according to the actual geometry of the Wenchuan earthquake and the Yushu earthquake. The dynamic mechanism of the unilateral rupture process of Wenchuan earthquake and the dynamic mechanism of the supershear rupture process of the Yushu earthquake are studied. The rupture process of the 2008 Wenchuan earthquake is extremely mixed. The rupture distance to northeast is 300km, while the length to southwest is very small, showing the main characteristics of unilateral rupture. The rupture process of Wenchuan earthquake is simulated and analyzed in this paper. The results show that the difference of physical properties on both sides of Longmenshan fault zone is the decisive factor for the unilateral propagation of Wenchuan earthquake. The earthquake disaster is especially serious because of the super shear earthquake rupture caused by the Yushu earthquake (Msl7. 1) in 2010. After simulating and analyzing the rupture process of the Yushu earthquake, it is concluded that the relationship between the strike of the earthquake generating fault and the direction of the initial principal stress of the Yushu earthquake may cause the transformation from sub-shear to super-shear fracture.
【学位授予单位】：中国地震局地球物理研究所
【学位级别】：博士
【学位授予年份】：2017
【分类号】：P315

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