The 2013 and 2017 Ms 5 Seismic Swarms in Jilin, NE China: Fluid-Triggered Earthquakes?

摘要

Understanding the mechanism of moderate earthquake swarm is important for the time-dependent hazard assessment. Five M_s ≥ 5.0 earthquakes occurred between October and November 2013 in Qianguo, Jilin, a region with low seismicity in northeast China. After 4 years, two more M_s > 5.0 earthquakes struck Ningjiang, Jilin. To investigate the swarm mechanism and the relationship between these two swarm activities, we determine the full moment tensor solutions for 12 M_s > 3.5 earthquakes, including the seven M_s ≥ 5.0 events. The focal mechanisms show that the 2013 M_s ≥ 5.0 events mostly exhibit thrust compression with significant volume closure components, whereas the 2017 M_s ≥ 5.0 earthquakes illustrate a dominated strike-slip type. In order to explain such differences, we conduct double-difference tomography in the study region. Our tomographic results reveal that both swarms are located in the low-velocity zones and the fault materials exhibit structural variations along the strike, possibly indicating that the structural contrast could be critical for the variations in the focal mechanism and hypocenter migration. The faults provide pathways for the upward intrusion of fluids that control the distribution and migration of the swarm sequences. A long-term fluid-rock interaction could lead to the fault materials intensely fractured in the 2013 swarm area but relatively less damaged in the 2017 swarm area. We can conclude that fluids play an important role in triggering the two seismic swarms.