中国强震前兆地震活动图像机理的三维数值模拟研究

3D numerical simulation on the mechanism of precursory seismicity pattern before strong earthquake in China

建立了含有母体岩石、硬包体和随机分布的小裂纹的三维有限元模型,计算了包体和各层实体中的应力分布.利用最高应力破裂准则、释放破裂单元刚度生与死的方法,模拟强震前岩石的破裂和小震的空间分布特征.结果表明,文中三种模型都显示出强震前在孕震体即包体附近出现了高应力集中单元.它们是形成小震空区、条带和地震空间丛集图像的基础.随机裂纹的存在,有利于在孕震体(包体)外的裂纹端部应力集中,先发生小震,形成包围孕震包体的前兆地震活动图像,而包体中的应力逐渐增加,为发生强震提供了条件.包体的形状和几何位置是影响强震前兆地震活动图像形态的重要因素.引入材料的黏弹性,导致了其中应力随时间迅速减小和弹性层某些部位应力随时间的增加.但在本文设定的构造模型框架和介质参数下,下层黏弹性的存在对上层母体的应力随时间的增加影响不大.

In this paper, we build three 3D finite-element models including mother rock body, hard inclusions and randomly distributed cracks and computed the stress distribution within the mother body and hard inclusions. Using the failure criterion of maximum equivalent stress and the method of Element Birth and Death, the space distribution features of small earthquakes （micro cracks in rock） before a strong earthquake are simulated. The results show that high stress elements are distributed around the hard inclusion in all the three models, and they construct the earthquake gap, earthquake belt and cluster before the strong earthquake. Stress concentrations in the tips of randomly distributed cracks around the inclusion lead to the precursory seismicity pattern. Stress increase in the inclusion is a condition of strong earthquake occurrence. The shape and geometry of inclusion is an important factor which affects the precursory seismicity pattern. The visco-elasticity of material in the bottom of rock body led to decrease of stress with time in the bottom part and to increase of stress at some places of elastic layers. But the stress increase due to bottom material visco-elasticity is small in the model of this paper.