| The Sichuan-Yunnan region is located on the southeastern edge of the Qinghai-Tibet Plateau,where the largest secondary block,the Sichuan-Yunnan rhomboid block,has experienced extensive differential movement since the Miocene,developing a number of active fault zones,such as the Xianshuihe-Xiaojiang fault zone,the Red River fault zone,etc.,which undergo tectonic rotation at different rates,giving rise to a number of typical moderate-strong earthquakes.The complex geological structure environment and the widespread presence of low-velocity layers in depth act as a deep decoupling layer that regulates fault and block movement,resulting in complex and variable source characteristics.Source parameter studies,such as focal mechanism solutions,source depths,earthquake rupture directions,rupture scales,etc.,provide earthquake generation mechanisms and energy release processes,which are important for understanding earthquake activity patterns and conducting earthquake prediction.Especially for moderate-strong earthquakes,source parameters provide precursory triggering factors for large earthquakes and provide possibilities for detecting crustal stress states in a wider area.This paper first calculates the Green’s functions based on the regional broadband seismic records in the Sichuan-Yunnan region using the Sichuan-Yunnan public velocity model CVM2.0,and uses the CAP method to solve the focal mechanism solutions of four typical moderate-strong earthquake sequences that occurred in the Sichuan-Yunnan region in the past two years,and obtains the P and T axis polar coordinate rose histograms of the earthquake sequences.The inversion results show that the seismogenic fault planes of Yangbi earthquake,Luding earthquake and Maerkang earthquake are respectively:strike/dip/rake:140°/68°/-168°,165°/76°/-6°,326°/80°/-6°,all of which are typical strike-slip earthquakes near fault zones,and Yangbi earthquake and Luding earthquake sequences also develop normal faulting aftershocks,indicating a strong and complex regional stress environment.The Lushan earthquake is a typical thrust-type earthquake that occurred in the middle-south section of Longmenshan fault zone,with a seismogenic fault plane strike of 17°,dip of 46°and slip angle of 62°.It is arranged in NE direction with Lushan earthquake in 2013 and Wenchuan earthquake in 2008.It is speculated that it is controlled by deep seismogenic environment of Longmenshan.The P and T axis orientations show that Yangbi earthquake is mainly controlled by near NS-oriented principal compressive stress,Luding earthquake and Maerkang earthquake are mainly controlled by NWW-oriented principal compressive stress,and stress action causes faults to undergo different types of strike-slip movement.Secondly,based on the finite fault model(FMT)inversion method,this paper solves the rupture process of the main shocks of the Luding earthquake and the Yangbi earthquake,and obtains the optimal fault slip distribution map and seismic moment release map.The optimal slip distribution map shows that the maximum slip of the Yangbi earthquake is about 0.4m,and the area with the maximum slip is limited to a small elliptical area on the right side of the epicenter.The seismic energy is relatively concentrated and basically released within the first 10s.The rupture direction of the Luding earthquake is NW,with a maximum slip of about 1.8m,located between depths of 6～12km,and there are two secondary rupture areas on the northwest and southeast sides of the epicenter.The rupture energy extends to the surface,and there may be a surface rupture zone of about 15km,which is consistent with the field investigation results.Based on the optimal slip distribution of the Luding earthquake obtained in this paper,the PGV distribution of the Luding earthquake is obtained by superimposing ETOPO1 and velocity model.The PGV distribution is consistent with the long axis direction of seismic intensity,which is NW.The villages and towns that suffered severe damage in the seismic intensity zone are all located in the extreme seismic zone of this paper.Based on the comprehensive focal mechanism solution results,it is believed that the seismogenic structure of the Yangbi earthquake and Luding earthquake may be related to the rupture of a complex fault system.Finally,based on the 2008 Panzhihua M_S6.0 earthquake sequence,this paper obtains the source depths of four aftershocks using the s PL near-source depth phase method,and the inversion error can be controlled within 1～2km range.The results reflect that the overall depth is within 20km;using the short-period waveform inversion method,using small earthquakes with known focal mechanism solutions as empirical Green’s functions,based on grid search method,this paper obtains the rupture direction and rupture scale of the 2008 Panzhihua Mw5.4 earthquake,and obtains that the rupture direction propagates along N185°direction,with a fault scale of 3.6km,which is basically consistent with the fault strike direction,aftershock distribution direction,etc. |
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