Geometry and tectonic deformation of seismogenic structures in the Rongxian-Weiyuan-Zizhong region, Sichuan Basin : insights from focal mechanism solutions

The Rongxian-Weiyuan-Zizhong (RWZ) region in central-western Sichuan Basin has been recognized to be a place with low historical seismicity, however, several destructive earthquakes occurred in 2019. Based on the seismic data from Sichuan Regional Seismic Network, our present study calculated the focal mechanism solutions, centroid depths and moment magnitudes of 26 M-s >= 3. 0 earthquakes since 2016 in the RWZ region by the CAP (Cut and Paste) waveform inversion method. The characteristics of the geometry and tectonic deformation of the seismogenic structures are then analyzed preliminarily. The results revealed that the centroid depths of 26 M-s >= 3. 0 earthquakes range from 1. 5 to 5 km, and the mean depth is about 3. 4 km. This finding indicates all of the M-s >= 3. 0 earthquakes occurred in the sedimentary layers of the shallow part of the upper crust. The focal mechanism solutions are all of thrust faulting mode, and with similar strike of NNE-NE direction and low dip angle toward SE. Based upon the statistics of strike and dip of two nodal planes, together with the vertical cross-section of hypocenters, we suggest that the seismogenic structures are a series of blind thrust faults along the south flank of the Weiyuan anticline. Dominant unique orientations of the P, T and B axes for the 26 earthquakes demonstrate a relatively simple tectonic stress settting. Our stress tensor inversion from focal mechanism solutions indicates that the maximum principal stress axis (sigma(1)) in the RWZ region is horizontal and orientated in NW-SE direction, which coincides with the regional horizontal compressional stress field. But this maximum principal stress axis is different from that in NE-SW direction for the 17 June 2019 M(s)6.0 Changning aftershock area in the southern Sichuan Basin. This observation indicates that tectonic activity in the RWZ region is controlled by the regional stress field. The tectonic deformation of the seismogenic structures here is generally featured by NW-SE pure compression from the strain rosette of the entire 26 M-s >= 3. 0 earthquakes, and it is contrast to the NE-SW compression with a small NW-SE extensional component in the 2019 M(s)6.0 Changning aftershock area. All of these above findings reveal that both of the tectonic stress field and tectonic deformation mode in Sichuan Basin are of an obvious domain feature.