RELOCATIONS AND FOCAL MECHANISM SOLOTIONS OF MS5.5 QIANGUO EARTHQUAKE SWARM IN JILIN PROVINCE IN 2013

We relocated a seismic swarm, which started in a mass from 31 October, 2013 in Qianguo County of Jilin Province, by using double difference location method, based on the phase data of regional digital seismic network and the crustal velocity model of Sunliao Basin. The characteristics of seismogenic fault have been investigated based on the spatial distribution image of the seismic swarm and the geophysical data near the epicenter area. The relocated epicenters of the swarm earthquakes have a precision of 0.9km in E-W, 0.7km in N-S and 1.2km in U-D direction, and show an apparent concentrated seismic belt trending N-W, with a length and width of 12km and 6km, respectively. The source depths of all events are shallow, with 80%in a range of 6~8km, and the events are apparently crowded together on the depth cross section. According to the relocated spatial distribution characteristics of the seismic swarm, the features that the medium size events happened successively, and the focal mechanism of the large size events in the swarm, we infer that the seismogenic tectonics of Qianguo seismic swarm is the thrust nappe structure inside the Keshan-Da'an fault zone. The fault plane inclines to the East direction, and is steep when close to the ground surface, which shows the typical characteristics of a listric thrust fault. The longitudinal length of the rupture plane is greater than the transverse length. According to the features of seismogenic tectonics, we infer that the three M_S ≥ 5.0 earthquakes occurred at the lower layer of the thrust rupture surface of the fault, while the aftershocks were triggered by the three events and occurred mainly at the upper layer of the rupture surface.