基于唐山7.8级地震探讨动态应力作用

Discussion of dynamical stress effect on Tangshan M_s7.8 earthquake

运用三维非线性动态有限元方法(Non-linear Dynamic FEM)仿真模拟研究了菱形模型的动态应力作用,分析了1976年唐山7.8级地震.结果表明:(1)唐山地震前增强的中强地震产生的冲击力形成的应力波反射后发生半波损失,成为动态的拉伸应力,通过减小内摩擦而引起岩石产生裂纹、局部破裂并发生串通,一方面触发地震,另一方面为孕育唐山地震积累能量,表现为正反馈过程.(2)唐山地震产生的冲击力加卸载存在时间差,造成加卸载冲击波相位不同,结果产生的动应力和围压叠加致使围压波动和下降,导致抗剪切强度降低,使初始破裂分别向北东、西南方向传播.(3)冲击力的卸载,相对于产生拉伸性的应力波,反射后发生半波损失,成为动态的压缩应力,致使围压增大并导致破裂停止;而动态应力在传播过程中衰减.因此,余震的发生是一个负反馈过程.

We use the three-dimensional non-linear dynamic finite element method to perform a modeling study on dynamical stress effects with a rhombic model and analyze the Tangshan M7.8 earthquake in 1976. The result shows that stress wave is generated by shock force which is produced by strengthened moderately strong earthquakes before the Tangshan earthquake, when this wave encounters a fault, the reflected stress wave, becomes tensile due to half-wavelength loss, which causes cracks, local rupture and crack On the one hand, other earthquakes are triggered. earthquake is accumulated. This is a process of linkage in the rock by reducing rock friction. On the other hand, energy for the Tangshan positive feedback. The shock force from the Tangshan earthquake during processes of loading and unloading has a time difference, this forms the different phase of shock waves. The dynamical stress produced by stress wave, superposed on the confining pressure, makes confining pressure reduce and change at low values. These effects can reduce shear strength and make the initial fracture of the Tangshan earthquake propagate toward northeast and southwest respectively. The unloading of shock force generated by the Tangshan earthquake is equal to producing the effect of tensile stress wave, the reflected tensile stress waves, having lost half-wavelength, become compressive ones which cause confining pressure increase and rupture stop. Meanwhile during the process of dynamical stress propagation, its amplitude decays. So the aftershock occurrence of the Tangshan earthquake is a negative feedback process.