| The characteristics of crust-mantle deformation reflected by seismic anisotropy are of great significance for understanding continental dynamics background of strong earthquakes.Shear wave splitting is the most obvious form of seismic wave propagation in anisotropic media of the earth,and it is one of the main means to study seismic anisotropy.The fast polarization direction and the delay times are the two most important parameters of shear wave splitting,which reflect the direction and intensity of medium deformation respectively.The anisotropy parameters of the upper crust can be measured by the shear wave splitting analysis of the direct S-waves of local earthquakes.Based on the two shear wave splitting parameters,we can analyze the characteristics of the upper crustal anisotropy and deformation,and the reasons of upper crustal anisotropy related to the influence of local stress field and tectonic characteristics.In general,the fast polarization direction is closely related to the direction of maximum principal compressive stress and/or the strike of fault structure,while the delay times reflect the strength of anisotropy and the characteristics of local stress field.Especially,it is helpful to understand the dynamic changes of the tectonic stress field in the focal region based on the shear wave splitting analysis of the microseism sequences before and after the strong earthquake,and the dynamic implication of strong earthquake formation and occurrence.The source region of the 2017 Jiuzhaigou M_S7.0 earthquake is located in the northeastern margin of the collision between the Bayan Har Block and the Sichuan Basin,where the faults crisscross and the geological structure is very complex.There are three major faults in the study area.They are the Minjiang fault,the Huya fault and the Tazang fault,respectively,which jointly bear the eastward sinistral strike-slip component of the Eastern Kunlun Fault.The Jiuzhaigou M_S7.0 earthquake was caused by the extension and release of the left-lateral strike-slip component accumulated by the Huya fault in the NW direction.The measurement of GPS deformation field further indicates that there is obvious crustal shortening deformation near E-W direction on the Minjiang fault and the Huya fault after the Jiuzhaigou earthquake.Based on the shear wave splitting analysis in the focal region of the Jiuzhaigou earthquake,we analyzed the characteristics of the upper crustal deformation and the spatio-temporal variation of shear wave splitting.It is helpful to better understand the seismogenic mechanism of the Jiuzhaigou earthquake and the tectonic mechanism of the Minshan uplift.A total of 2800 high quality splitting measurements were obtained from six temporary seismic stations(L6202,L6201,L5113,L5112,L5111,and L5110)and one permanent station(JZG)which were deployed in the vicinity of the epicentral region.We obtained the shear wave splitting parameters at each station by analyzing the direct s-waves of the local earthquakes.By analyzing shear wave particle motion of local events in the shear wave window,the fast polarization direction and the delay times between fast and slow shear waves were derived from seismic recordings at seven stations in the source area of the Jiuzhaigou earthquake.In the study region,the fast polarization directions show partition characteristics.And the systematic changes of the delay times between two split shear waves were also observed.As for spatial distribution,there are three stations L5111,L5112 and L6202 in the aftershock area of the northern source area of the Jiuzhaigou earthquake,stations L5112 and L5111located on the east side of the seismogenic fault of the Jiuzhaigou earthquake have only one predominant fast direction(NNE),while station L6202 located on the west side of the seismogenic fault has two predominant fast direction(EW and NNE),which shows a synthetical action of the crustal stress and complex faults structure in the aftershock area;With reference to the three stations JZG,L5110 and L5113 outside of the aftershock area,the NW fast polarization direction of station JZG located in the eastern source area and near the Tazang fault(east)is consistent with the strike of the Tazang fault(east),while the EW fast polarization directions of stations L5110 and L5113 in the southern source area are consistent with the direction of regional principal compressive stress;The average delay times of the stations in the aftershock area are greater than those of the external stations in the aftershock area,which shows that the stress accumulation in the aftershock area is stronger than that of the surrounding area during the seismogenic process of the Jiuzhaigou earthquake.In the time domain,the dispersion of the predominant fast direction is larger in the initial period after the main shock,but it tends to decrease with time,while the delay times is larger after the main shock,but it also gradually decreases and stabilizes with time.The characteristic that the shear wave splitting parameters tend to decrease and stable with time reflects that with the occurrence of the main shock and aftershocks,the stress accumulated during the seismogenic process of the Jiuzhaigou earthquake release and adjust gradually,while the magnitude and the amplitude of the adjustment gradually decreased and stabilizes gradually in the later period. |
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