断层自发破裂动力过程的有限单元法模拟

断层自发破裂动力过程的研究对于认识地震过程及减轻地震灾害有着重要的科学意义.为合理地模拟断层的自发破裂过程，本文首先对经典的滑移弱化摩擦关系进行了改进，然后利用有限单元方法对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，而经典的滑移弱化摩擦关系不能产生这种破裂形态.模拟结果还显示，断层自发破裂过程受初始应力场及摩擦关系影响，当初始应力场中剪应力水平较低时，容易产生脉冲型破裂；但当初始剪应力较高时，会产生裂纹型（crack-like）破裂.这个现象与在实验室里进行的岩石破裂实验结果是一致的.在相同的初始应力情况下，若滑移弱化摩擦本构关系中的动摩擦系数较大，断层将易于产生脉冲型破裂；若动摩擦系数较小，将倾向于产生裂纹型破裂.此外，本文也采用速率弱化摩擦关系对断层自发破裂过程进行了模拟，结果发现，在初始场及其他条件不变时，如果摩擦关系中的b-a值较小，容易产生脉冲型破裂；如果b-a值较大，会产生裂纹型破裂.

FEM simulation of the dynamic processes of fault spontaneous rupture

The study of the dynamic process of fault spontaneous rupture is important for understanding of earthquake processes and earthquake disaster reduction. In order to reasonably simulate the rupture processes, this paper first improved the classical slip-weakening friction law, and then used the finite element method to carry out simulation. The computed results demonstrate that pulse-like rupture is possible by making a modification of the classical slip-weakening friction law, while impossible with the classical slip-weakening friction law. It also shows that the behavior of the fault spontaneous rupture process is affected by the initial stress field of the region and the friction law. When the initial shear stress is low, it is easier to generate pulse-like rupture. But it is a crack-like rupture in the high level of initial shear stress. This is consistent with the laboratory result of small-scale rock rupture. With the same initial stress, fault tends to produce pulse-like rupture with large dynamic friction coefficient and crack-like rupture with small dynamic friction coefficient. In addition, this paper also simulated the spontaneous rupture process of faults with rate-weakening friction law. It turns out that the rupture is generally pulse-like for small value of b-a but crack-like for large value.