Abstract: Double-difference location algorithm and the CAP method (Cut and Paste) were used for the precision mapping and inversion of the focal mechanism solution for the Songyuan earthquake swarm above M5 in 2013. The double-difference location results show that the epicenter distribution sequence of the Songyuan M5 earthquake swarm is NW—SE, with a length of about 10 km, width of about 5 km, and a major axis of about 317°. It is almost perpendicular to the Changling-Dashanzi fault, and the focal depth is mainly concentrated at 2~14 km. The results of the focal mechanism solution of 19 M_S≥3.5 earthquakes indicate that the Songyuan M5 earthquake swarm is dominated by thrust, thrust and strike-slip earthquakes, and the dominant orientation of the focal plane solution I is NW—SE orientation. The depth of 19 earthquakes are focused on 4~6 km. The orientation of the epicentral distribution after the fine orientation is consistent with the dominant orientation of the focal mechanism solution plane I. The seismogenic structure of the Songyuan M5 swarm is an invisible fault that is almost perpendicular to the Changling—Dashanzi fault. The background stress field in the source area is affected not only by the push of the Pacific plate to the North East Asia plate, but also by the internal secondary fault zone of the Songliao basin, under the influence of deep seismic activity in the underground subduction zone of the Northeast China. The deep seismic activity has strengthened the disturbance of mantle material and further promoted the accumulation of stress in shallow medium. The junction of the secondary fault zone in Songliao basin is relatively weak, and when the stress accumulates to a certain extent, a moderate-strong earthquake will occur.

Key words: Earthquake relocation, Focal mechanism solution, Songliao basin, The Songyuan earthquake swarm in 2013