Rupture process of the Minxian-Zhangxian, Gansu, China M(S)6. 6 earthquake on 22 July 2013

Abstract

On July 22,2013,a major earthquake(MS6.6)occurred on the boundary of Minxian and Zhangxian in Gansu province,with its epicenter near the Lintan-Dangchang fault.Based on the finite fault model constructed,the rupturing process of this earthquake was determined by the waveform method using 12 near source strong motion three-component records provided by China Earthquake Data Center.Strike,dip angle,and rake angle of seismogenic fault as well as focal depth are supposed to be known before the waveform inversion based on a finite fault model.The focal mechanism of the earthquake is obtained by a dislocation source model utilizing 30far-field P vertical waveform records with uniform azimuth coverage and 12near-field P-wave initial motions.Geometric parameters of the seismogenic fault plane and focal depth adopted in the inversion were determined by considering information such as results of different institutions and scholars,waveform fitting obtained by treating two nodal planes as fault planes,and strike of existing faults in this area.The strike is300.2°,dip angle is 66°,slip angle is 47.7°,focal depth is 7km.In the process of invertingrupture process of this earthquake using strong motion records,Green′s function was calculated by f-k integration and source parameters over the fault,including slip amount,slip rake,rise time and rupture velocity,which were inverted by simulated annealing method.In order to check the accuracy of source parameters deduced by strong motion records,we use the deduced source parameters to calculate the far-field synthetic waveforms and then compare them with observed ones.Results show that slip over the seismogenic fault is centralized nearby the focal source,maximum slip is around 80 cm,and no slip on the surface of ground.This earthquake is dominated by over-thrust with sinistral strike-slip component,consistent with other faults in this area.The rupturing ended at 8s.Seismic moment M0 is 1.60×1025dyn·cm,moment magnitude MWis 6.1.Considering the strike of the seismogenic fault and aftershocks′distribution matches with Lintan-Dangchang fault,rake and dip of seismogenic fault are also similar to those of eastern part of Lintan-Dangchang fault,we deduce this earthquake was related with the north forward extrusion of the east Kunlun fault and resulted from the further activity of the Minxian-Dangchang fault.