地震块体模型的共轭剪切破裂带数值模拟

对地震块体模型的应变率场的数值模拟结果表明 ,地震块体模型 ,尤其是当界面法向和切向刚度较大时 ,能产生明显的共轭剪切破裂带。地震块体模型存在老断层 ,呈现各向异性 ,因而 ,促进了某一方向上的主条带充分发展 ,与主条带共轭的次条带被抑制。存在复杂地质构造可能是地震带的间隔距离并不严格相等的原因。地震发生在剪切破裂带交叉部位的原因是这一位置剪切应变率较高。考察应变率场能揭示出地震块体模型的应变局部化特征。在构造运动作用下 ,新生构造与老构造并不重叠是可能的 ,因而 ,新生构造的交汇点就不一定出现在老断层上。地震块体模型的中心块体的尺寸越大 ,中心块体越稳定

Numerical Simulation of Conjugate Shear Fracture Bands for Seismic Block Model

FLAC (Fast Lagrangian Analysis of Continua) can allow localized bands to develop and evolve partly because the code models the dynamic equations of motion and has been adopted to investigate some kinds of strain localization phenomena, though the numerical results concerning strain localization and strain softening are mesh-dependent. To obtain a further understanding of localized failure of rock under plane strain compression, some numerical investigations, such as the effect of specimen height and width, strain rate, shear dilatancy, pore pressure, and end constraint on shear bands patterns, were carried out by Wang et al. In addition, shear bands patterns of two and five seismic blocks models as well as thick-walled cylinder were also modeled by Wang et al. Shear strain rate field for seismic block model is numerically modeled by FLAC. The adopted failure criterion is a composite Mohr-Coulomb criterion with tension cut-off and post-peak constitutive relation of rock is linear strain-softening. Numerical simulations of seismic block model are carried out in plane strain and small deformation mode. In the seismic block model, old fault lies between arbitrary two adjacent blocks; the numbers of old faults and blocks are 4 and 5, respectively. In addition, four rectangular surrounding blocks envelop a center square block. The seismic block model is loaded at constant velocity of 5×10~(-6) m/s at top and base of the model. In addition, at left and right sides of the model the confining pressure remains a constant. Numerical results show that distributions of shear strain rate are highly non-uniform and conjugate shear fracture bands can be formed in seismic block model, especially for the higher normal stiffness and tangential stiffness of old fault. Anisotropy of the model leads to main shear fracture bands developing fully along a certain direction and at the same time the development of secondary shear fracture bands conjugate with the main bands are inhibited. Probably, complex geological structures are the reason for different spacings of shear fracture bands. The reason for earthquake occurring in the region where one shear fracture band intersects with another is that shear strain rate of the region is higher. Characteristics of strain localization can be revealed through investigation of shear strain rate fields. However, localization phenomenon cannot be observed from stress field due to the fact that in strain-softening stage stress will decrease, as is not similar to change in field of plastic shear strain that is increased sequentially though peak strength is reached. In the action of driving forces of plate tectonics, new shear fracture bands and old fault can unoverlap. Therefore, it is possible that the regions where arbitrary two shear fracture bands intersect are not in the old fault. The larger size of the center block results in the stability of the center block, which means that earthquake does not occur easily in the center block, as is in agreement with many observations in field. However, earthquake probably takes place in the surrounding blocks or at the boundary between the center block and the surrounding blocks.