Seismogenic structure and crustal stress field in Jiashi earthquake zone

The Xinjiang Jiashi earthquake area is located in the boundary region of the Tarim Block and the southern Tianshan block, the north of which is the Kepingtag thrust nappe structure belt and the south is the Tarim Block. Due to the interaction of the two blocks, seven $ {M}_\mathrm{S} $≥6.0 earthquakes occurred in the southern region between 1997 and 2003, while $ {M}_\mathrm{S} $6.4 occurred in the northern region on 19 January 2020, with the epicenter located in the Kepingtag thrust nappe structure on the northwest margin of the Tarim Basin. The characteristics of earthquake swarm activity associated with nappe structure and the interior of the Tarim Block, the structural geometry revealed by the earthquake, and the correlation with the local tectonic stress field are the basic problems of earthquake dynamics in this area. In this paper, the seismic precise locating events of the two seismic regions in the north and south of Jiashi are collected, and the specific parameters and locations of the faults are obtained by using the fault fitting method. In the northern earthquake area, f1 fault strike: 276.5°, dip: 15.8°, f2 fault strike: 172.1°, dip: 87.1°; In the southern earthquake area, the strike of f3 fault is 237.9°and dip angle is 89.2°, the strike of f4 fault is 300.1°and dip angle is 88.5°, the strike of f5 fault is 38.5°and dip angle is 88.3°, and the strike of f6 fault is 230.9°and dip angle is 83.8°. Then the focal mechanism data of the two seismic regions were collected and the regional tectonic stress fields were estimated. The results show that the stress field type of the northern earthquake region is thrust type, with azimuth of 141.68°and plunge angle of 20.84°of the principal compressive stress axis, and azimuth of 301.39°and plunge angle of 67.91°of the extensional axis. The type of stress field in the southern earthquake area is strike-slip type, with azimuth of 186.79°and plunge angle of 18.89°of the compressive stress axis, and azimuth of 91.25°and plunge angle of 15.76°of the extensional axis. Finally, by projecting the obtained stress fields onto their respective fault planes, we found the f1 fault in the northern part exhibits low-dipping thrust characteristics, consistent with folded structures identified in geological surveys, and f3,f5 and f6 in south part show left-lateral strike-slip property and f4 shows right-lateral strike-slip property. The relative shear stress and relative normal stress of fault f2 in the southern stress field are both larger than those in the northern stress field, especially the relative normal stress increases significantly. Which indicates that f2 is a rupture at the edge of the Tarim Block, and the sudden southward thrust caused by the 2020 earthquake triggered the deeper, more vulnerable f2 fault, leading to its rupture.