Relocation and seismogenic structure of the 2021 Guyuan earthquake swarm

According to the Ningxia seismic network, Guyuan earthquake swarm activity occurred from June 11 to July 15, 2021. The 2021 Guyuan earthquake swarm had the most earthquake frequency ever recorded in historical swarm events of southern Ningxia, and hence attracted more attentions of relevant departments. Therefore, studying on the earthquake swarm provides fundamental data for the further comprehension of the seismic generation, evolution process of the earthquake swarm in this region and the investigation of the earthquake mechanism, thereby offering relevant supports for the earthquake tracking mission.

In this research, M_L≥3.0 earthquakes of 2021 Guyuan earthquake swarm were determined using the methods of first arrival P phase location, and the earthquake swarm was also relocated by using multi-stage location and microearthquake matching location. Meanwhile, the focal mechanism solutions of the M_L≥2.8 earthquakes from the swarm were calculated based on Snoke method and HASH method. Focal mechanism and centroid depth of the M_L3.6 Guyuan earthquake on June 22, 2021, the largest in the swarm, was also determined by gCAP inversion method, and the central solutions of various focal mechanisms for the M_L≥2.8 earthquakes 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 901 microseismic events above M_L0.0 were detected, which is about 2.6 times as much as those in the original catalog of the regional seismic network. The epicenters area is about 8 km long and 3 km wide, spreading in WNW direction predominately, with focal depths ranging from 8 km to 17 km, and the average depth of 13 km. M_L≥2.8 earthquakes are mainly concentrated in the earthquake dense area with focal depth of 12−14 km. The focal mechanism of M_L≥2.8 earthquakes show that all of them are strike-slip type, and most of them have a little thrust component, and the dominant azimuth of the P-axes is in nearly EW direction, which is basically consistent with the direction of the principal compressive stress of the local tectonic stress field in Liupanshan area. Combined with relocation, focal mechanism and fault plane fitting, it is suggested that the main seismogenic structure of Guyuan earthquake swarm may be controlled by two WNW-striking hidden faults with opposite dipping.The NE-dipping and SW-dipping faults form upward-branching flower structure, and the M_L≥2.8 earthquakes are located at the intersection of faults with different dip, and all may have left-lateral strike-slip movement. As for the whole earthquake sequence, the NE-dipping fault activity is dominant, and the SW-dipping fault activity is secondary. According to previous studies, it is found that the Guyuan earthquake sequence is located in the low-velocity anomalous lens with about 11 km thickness at the bottom of the upper crust in Liupanshan area. The crust-mantle interaction deep fluid action and high Poisson’s ratio zone due to the upwelling of the upper mantle in the Liupanshan area are the possible seismogenic reasons for the formation of the low-velocity lens where Guyuan earthquake swarm is located. The preliminary analysis suggests that the seismogenic environment of Guyuan earthquake swarm may be principally related to the action of deep fluid. This research is of significance for the deep insight into the relationships between earthquake relocation, focal mechanism and seismogenic structure.