Relocation and seismogenic structure of the 2021 Guyuan earthquake swarm

In this paper, the 2021 Guyuan earthquake swarm was relocated by using the methods of first arrival P phase location, multi-stage location and microearthquake matching location. Meanwhile, the focal mechanism solutions of the M_L≥2.8 earthquakes from the swarm sequence were calculated based on Snoke method, HASH method and gCAP method, and the central solutions of various focal mechanisms for the same earthquake were determined. Then the fault plane was fitted according to the relocation results and the seismogenic structure of Guyuan earthquake swarm was preliminarily analyzed. The results show that 893 microearthquake events above M_L0.0 were detected, which is about 2.6 times of the original catalog of the regional seismic network. In the epicenters area, the dominant distribution is in the NWW direction, and is about 8km long and 3km wide. The focal depth is 8~17km, and the average depth is 13km. M_L≥2.8 earthquakes mainly occured in the relatively dense earthquakes area of 12~14km. The focal mechanism results of M_L≥2.8 earthquakes shows that all of them are strike slip type, and most of them have a little thrust component, and the dominant azimuth of the P-axis is nearly EW direction, which is basically consistent with the direction of the main compressive stress axis of the local tectonic stress in the Liupanshan area. Combined with relocation, focal mechanism and fault plane fitting, it is suggested that the main seismogenesis structure of Guyuan earthquake swarm may be controlled by two hidden NWW strike faults with opposite dipping.The NE-dip and SW-dip faults form upward-branching flower structure, and the larger M_L≥2.8 earthquakes are located at the intersection of faults with different dip, and all may have left-lateral strike-slip movement. In the whole swarm earthquake sequence, the NE-dip fault activity is dominant and the SW-dip fault activity is second. According to previous studies, it is found that the earthquake sequence is located in the low-velocity anomalous lens about 10km thick at the bottom of the upper crust in Liupanshan area. The preliminary analysis suggests that the seismogenic environment of Guyuan earthquake swarm may be related to the action of deep fluid.