流-固耦合作用对计算同震静态库仑应力变化的影响

摘要

In previous studies,the co-seismic static Coulomb stress changes were calculated by the analytical solution of Okada,which barely considered the impact of fluid on the mechanical behavior of the solid skeleton.However,fluid plays a very important role in solid deformation.Based on the fully-coupled poro-elastic theory,we build three typical fault-slip models (strike-slip fault,thrust fault and normal fault) to study the fluid-solid coupling Coulomb stress changes and compare them with the computation of traditional methods.The results show that three different faults exhibit completely different pore-pressure change distributions in 3D space.The transient pore-pressure change after a strike-slip faulting event is axially symmetrical with increase and decrease areas evenly distributed in a juxtaposed pattern.And the static Coulomb stress decreases significantly in the near field due to the fluid-solid coupling effect.The spatial distributions of the pore pressure generated by the thrust shock and the normal shock have similar patterns,but opposite positive and negative areas.The pore pressure rises in the epicenter of the thrust model but drops in the normal fault model.Compared with the traditional methods of Coulomb stress change calculation,the pore pressure changes of the thrust fault earthquakes can reduce the stress shadow,which may trigger more aftershocks.Conversely,the normal fault event increases the region of stress shadow near the epicenter and lowers the probability of aftershocks.Therefore,the fluid-sold coupling effect should be taken into consideration in studies of Coulomb stress triggering to make model prediction more realistic.%传统方法计算同震静态库仑应力变化一般都是基于Okada的解析解,模型中不考虑流体对固体骨架力学行为的影响.但实际上,流体对固体变形有着非常重要的作用.为此,本文基于孔隙弹性理论,考虑流固之间的完全耦合效应,针对三种典型的断层错动模型(走滑型、逆冲型以及正断型),分别计算静态库仑应力变化与传统算法之间的差别.计算结果表明:三种地震模型得到的介质孔隙压变化在空间的分布格局完全不同,走滑型地震产生的孔隙压变化图案在空间中呈正负相间的四象限分布,近场的静态库仑应力明显下降,流-固耦合作用对静态库仑应力变化影响较大.逆冲型和正断层型地震产生的介质孔隙压变化在空间的分布图案类似,但正负区域正好相反,孔隙压在逆冲型地震的震源区域上升,而在正断型地震的震源区域下降.同传统方法计算的库仑应力相比,逆冲型地震产生的介质孔隙压变化使得震源区的应力影区面积减小,这会触发更多的余震;而正断型地震产生的孔隙压变化则正好相反,增大了震源附近的应力影区范围,这样会降低该区域余震发生的概率.可见,介质的流-固耦合作用对计算库仑应力变化的影响不可忽视.因此,在利用库仑应力变化研究地震触发时,应考虑流-固耦合作用,使得库仑模型的预测结果更符合实际.