An algorithm of fault parameter determination using distribution of small earthquakes and parameters of regional stress field and its application to Tangshan earthquake sequence
利用小震分布和区域应力场确定大震断层面参数方法及其在唐山地震序列中的应用

It is known that clustered small earthquakes often occur in the fault plane vicinities of large earthquakes.Based on the simulated annealing and Gauss-Newtonian nonlinear inversion algorithms,we develop a robust algorithm to estimate fault plane parameters of strong earthquakes such as strike,dip,and location using data of densely populated small events.Rake angle of the fault plane is further inferred from regional tectonic stress parameters.This method can also be employed for determination fault plane parameters of an active fault using data of densely populated small earthquakes.We apply the algorithm to investigate the fault planes of the 1976 Tangshan,Luanxian,and Ninghe earthquakes and the Lulong fault,using precisely relocated earthquakes which occurred near the earthquake rupture zone between 1 April 2002 and 31 May 2006,and obtain the parameters of strike,dip,location,and rake angle of these fault planes.Our estimated fault planes are close to the results of previous studies,testing the validity of our method.In addition,we discover an aligned seismic belt east of the Luanxian seismic zone trending NE-SW,in the form a fracture zone.The faulting parameters of the fracture zone such as the strike,dip and rake are determined accurately.It is still an open question whether the Luanxian and Ninghe faults have ruptured during the Tangshan earthquake sequence,answers to the question await for further multi-disciplinary studies and bear great significance for the understanding of dynamic processes of the Tangshan earthquake sequence.