In recent years, the southern Liaoning Province is the main area of seismic activity in Liaoning Province, and the main geological structure units in this area include the Liaohe rift and Liaodong uplift in the east. As an important manifestation of modern tectonic activity, earthquakes are less distributed in Liaohe rift. Most of the seismic activities are concentrated in eastern Liaoning uplift area on the east side of Liaohe rift. The structure in this area is relatively complex. The revival of old faults during Quaternary is obvious, and there are more than 10 Quaternary faults. Among them, Haichenghe Fault and Jinzhou Fault are the faults with most earthquakes. The 1975 Haicheng MS7.3 earthquake occurred in the Haichenghe Fault and the 1999 Xiuyan MS5.4 earthquake occurred in the east of the fault. In this paper, the seismic phase bulletins are used for earthquakes from August 1975 to December 2017 recorded by 67 regional seismic stations of Liaoning Province. These stations were transformed during the Tenth Five-year Plan period. Using the double-difference tomography and tomoDD program, we relocated the earthquakes and inversed the velocity structures of the southern Liaoning area. In the study, grid method is used for model parameterization of seismic tomography, ART-PB is used for forward calculation, damped least square method is used in inversion, and checkerboard test is used for the solution evaluation. The theoretical travel time is forward calculated by taking the checkerboard velocity model of imaging meshing and plus or minus 5% of anomaly as the theoretical model. The checkerboard test results show that the checkerboard P-wave velocity model at the depths of 4km, 13km, 24km and 35km in the study area can be restored completely, and most areas at the depth of 33km can also be restored completely. We calculated and got the relocations of almost all of the earthquakes in southern Liaoning area and obtained a better distribution of P wave velocities at the depth of 4km, 13km, 24km and 33km. The results show that earthquakes mainly concentrated in two areas: the Haicheng aftershock area and the Gaizhou earthquake swarm activity area. The distribution of seismicity in this area is obvious in NW direction. The result of P-wave tomography in 4km depth indicates the consistent characteristics of shallow velocity structure with the surface geological structure in southern Liaoning Province area. The two sides of the Tanlu fault zone are characterized by different velocity structures. The high and low velocity discontinuities are located in the Tan Lu fault zone, which is in good agreement with the geological structure of the region. In Haichenghe Fault in the Haicheng aftershock area, there are high-velocity zone in the shallow layer and low-velocity zone in the depth of 4~12km, and the low-velocity zone intrudes and deepens eastward. The Xiuyan earthquake with MS5.4 in 1999 occurred on the boundary section of high and low velocity zones. At the same time, there is a gap between Xiuyan and Haicheng sequences, which is located at the junction of high and low velocities, and there is a significant low-velocity zone underground in the region. From the perspective of mechanism of the seismogenic model, this velocity structure model may generate large earthquakes. There are high-velocity zones at the ends of different segments of Jinzhou Fault, and the Gaizhou earthquake swarm occurred in the high-velocity area at the end of the fault. It is speculated that the activity of the Gaizhou earthquake swarm may be caused by the rise of water saturation in rocks due to the intrusion of liquid under the condition of stress accumulation. © 2022, Editorial Office of Seismology and Geology. All right reserved.
