Relocation, Focal Mechanisms and Stress Field Characteristics of Earthquake Swarms in the Songyuan Area of Jilin Province, China

Two earthquake swarms, the Qianguo swarm and Ningjiang swarm, have been observed since 2013 in the Songyuan area, Jilin Province, China. Using the seismic data of broadband permanent and temporal stations that recorded a larger number of earthquakes in the two swarms, we analyze the distribution of aftershocks and the focal mechanisms of these earthquakes and investigate the stress field and faulting properties of the Songyuan area. The relocated hypocenters of the Qianguo earthquakes are distributed along a NNW direction at focal depths of 4-13 km, while the dominant distribution of relocated Ningjiang earthquakes is along a NE direction at depths of 8-16 km. The focal mechanism solutions of 36 earthquakes with M-L >= 3.6 demonstrate a combined strike-slip and thrust faulting mechanism with centroid depth concentrated at 3-7 km for the Qianguo swarm and a strike-slip mechanism with centroid depth at 5-7 km for the Ningjiang swarm. The full moment tensors of seven moderate-strong earthquakes show that these earthquakes contain a high percentage of non-double-couple components. The regional stress field is inverted using the focal mechanism parameters of the 36 earthquakes. In the Qianguo area, the horizontal maximum principal compressive stress (sigma(1)) is in the E-W direction with a dip angle of 14.1 degrees. In the Ningjiang area, the sigma(1) is in the ENE-WSW direction with a dip angle of 9.2 degrees. The sigma(1) directions of the two areas are slightly different, but both approximately along an E-W direction. The comprehensive results of this study show that the Qianguo swarm is likely caused by the movement of a buried fault in the NNW direction oblique to the Changling-Dashanzi fault, whereas the Fuyu-Zhaodong fault in the NE direction is responsible for the Ningjiang swarm. Both the Qianguo and Ningjiang swarms are probably closely related to the thrusting effect of the subduction of the Pacific Plate into the Northeast Asian continental margin and the complex geological conditions in the Songyuan area.