22 :55 on June 17, 2019, a magnitude 6. 0 earthquake (28. 34 degrees N, 104. 90 degrees E) occurred in Changning County, Yibin City, Sichuan Province, China. The deep seismogenic environment and potential earthquake risk in the inner and marginal areas of Sichuan Basin have once again attracted the close attention of seismologists and scholars at home and abroad. In order to reveal the characteristics of the deep medium structure and the seismogenic environment of the Changning M(s)6. 0 earthquake area, and to interpret the tectonic background and genesis of the earthquake activities comprehensively, In this paper, we fully collected a total of 35 sets of observation seismic equipment which deployed in the Changning earthquake area in Southeast Sichuan and its surrounding areas, including Sichuan digital seismological network, Yibin local seismological network and new mobile seismological stations in Yibin after 2016, P -wave arrival data of 17305 earthquakes recorded from January 2013 to July 2019. Double-difference seismic tomography method was used to invert the three-dimensional P-wave velocity structure characteristics of the upper crust around the Changning earthquake area, combined with the three-dimensional magnetotelluric array sounding and gravity intensive measurements obtained in the earthquake region by the postearthquake scientific research, we comprehensively analyzed and discussed the scientific issues such as the relationship between the velocity structure characteristics and seismic activity in the Changning earthquake area, the deep tectonic environment and the seismic risk. The results show that the P -wave velocity structure of the upper crust around the Changning earthquake zone shows obvious lateral inhomogeneity, the physical properties of the sedimentary layer in the earthquake zone are obviously different, and there is a high-velocity anomaly distribution in the sedimentary layer where the Changning anticline is located, There is a clear difference between the velocity structure of the periphery around Shuanghe anticline and its east. The spatial distribution characteristics of the relocated Changning M(s)6. 0 earthquake sequence are closely related to the velocity structure of the upper crust. The sequence generally spread in the NW-SE direction along the boundary of high-low velocity anomaly, and terminate near the east section of the Baixiangyan-Shizitan anticline, The inhomogeneous variation of the velocity structure of the Changning earthquake area and its surrounding medium is the deep structural factor controlling the spatial distribution of the mainshock and its sequence. The three-dimensional P-wave velocity structure also shows that there are different deep-shallow structural backgrounds near the Shuanghe folds in Changning M(s)6. 0 earthquake. The different activities of local structures in the earthquake region not only cause the variety of fold structure axial and associated fault strike, but also cause the obvious differences between coupling relationship of deep and shallow structures and physical characteristics of sedimentary cover in the earthquake area. The epicenter of the Changning M(s)6. 0 earthquake was located near the boundary zone where the velocity structure changes. This boundary zone of physical property change may be a favorable location for earthquake preparation and occurrence of moderate-strong earthquakes. The Changning M(s)6. 0 earthquake and most of its sequence occurred above the basement detachment zone, due to the joint action of the regional NE-SW principal compressive stress and the NW-SE present stress field transmitted through the Huayingshan structural belt, the following earthquakes with magnitude 5. 1 in Gongxian, magnitude 5. 3 in Changning, magnitude 5. 4 and magnitude 5. 6 in Gongxian, as well as a large number of small and medium-sized earthquake events, were triggered by the Changning M(s)6. 0 earthquake. The P-wave velocity structure also reveals that there are different shallow velocity structures on both sides of Shuanghe fold, Dadiwan fault and Dafuya fault in NW direction. Combined with the ground geological survey results, such as the relatively developed NW fault structure associated with the axis of Changning-Shuanghe anticline, and the strike of associated faults of the east fold structure also shows diversity and complexity. The results show that in addition to the influence of regional tectonic movement, the differential activities of local structures in the Changning earthquake area are also prominent. The axial structures and associated faults in the Changning-Shuanghe anticline structural area have certain earthquake generating capacity and deep seismogenic background, this may also be the main reason for the large aftershock intensity and long duration of Changning earthquake. The seismicity trend and potential seismic risk in southeast Sichuan still deserve further attention.
