Variation characteristic of S-wave splitting on the 2019 Changning M_S6.0 earthquake sequence，Sichuan

In this paper, the S-wave splitting parameters of the waveform data recorded at 10 stations in the source area of the 2019 Changning M_S6.0 earthquake, Sichuan, from April 25, 2013 to July 31, 2019 were measured by the particle motion discriminant method combined with the polarization analysis method. More than four effective S-wave splitting parameters are obtained at nine stations. The results show that the S-wave splitting parameters at the stations in the studied region are characterized by partition in space and variation over time. The characteristics of fast wave polarization direction in space are as follows: the predominant polarization direction of fast S-wave at three stations in the southeastern source area of the Changning earthquake is in the direction of NE, which is consistent with direction of regional principal compressive stress in the southeastern source area. In the northwestern source area, the predominant polarization directions of fast S-wave, nearly EW, is consistent with the directions of regional principal compressive stress in the northwestern source area. Due to the combining effect of crustal stress and complex fault structure, the three stations CJW, GXA and LQS all have two predominant polarization directions of fast S-wave. The polarization directions of fast S-wave change with time as follows: After the main shock, the polarization directions of fast S-wave at each station gradually tended to be convergence after the dispersion increases; the polarization directions of fast S-waves at the station CJW changed three months before the main shock, indicating that with the accumulation of the stress during the seismogenic process, the anisotropic characteristics at the station CJW are controlled mainly by stress instead of structure. As for the temporal distribution, the average normalized delay time of slow S-wave at each station decreased as the distance increasing from the main shock and aftershock dense area to stations, reflecting the stronger accumulation and releasing of stress during seismogenic process in the aftershock dense area. In addition, the normalized delay time of slow S-wave at the station CNI decreased significantly about six months before the main shock, and increased rapidly after the main shock, suggesting the accumulation of stress before the earthquake and the abrupt release of stress after the earthquake lead to the change in geometry of the micro-cracks in upper crust.