华北大震序列的断裂系模式及破裂过程的联合反演

本文从断裂系扩展的角度,研究了华北邢台、渤海、海城和唐山地震序列的发生过程.通过对前震-主震-余震地震活动图象的剖析,我们发现,这些地震序列是由不同长度、不同几何分布的相关断裂系相互作用和扩展的结果.根据地震活动图象和断裂扩展理论,对每个地震序列提出了相应的断裂系模式,并用断裂有限单元法将震源机制、地震活动图象的分期变化和大地形变等观测资料相结合,进行联合反演,再次对断裂系模式的几何和力学参数进行了修正,从而得到各个地震序列破裂过程的最佳结果.同时我们还发现,一个走滑大地震序列临近结束时,地震活动图象的总体轮廓呈现Z形.这是由于断裂系内的拐折扩展分支相互作用和连通成为一体的结果,这种总体Z形的出现,可作为地震序列结束的标志. 

FRACTURE SYSTEM MODELS OF EARTHQUAKE SEQUENCES OF Ms7.0 IN NORTH CHINA AND SYNTHETIC INVERSION OF FRACTURE PROCESSES

Four large earthquake sequences of Ms7.0 occurred in North China during 1966-1976. They are the Xingtai earthquake of 1966,Bohai earthquake of 1969,Haicheng earthquake of 1975 and Tangshan earthquake of 1976. The occurrence processes of these earthquakes are studied from the view point of fracture system propagation in this paper. In terms of the analyses of their activity patterns of foreshock-mainshock-aftershock,it is found that the occurrence of each earthquake sequence is the results from the propagation of the associated fracture system in which the length and geometric distribution of fractures may be different. Based on the theory of fracture propagation,four mechanical models of fracture systems suited to each earthquake sequence respectively are presented. The geometric and mechanical parameters of these models are corrected once more by means of the synthetic inversion for the data of focal mechanism solutions,variations of aftershock activity patterns during different periods and geodetic deformation using the fracture finite element method. Then,the best solutions modelling the fracture processes of the four earthquake sequence are selected. It is also found that the pattern of aftershock distribution of a large strike-slip earthquake sequence always show a Z-shape as a whole due to the propagation and combination of fracture branches when an earthquake sequence is at its final stage. It can be a mark to recognize the termination time of an earthquake sequence.