Complexity of initiation and evolution of the 2013 Yunlong earthquake swarm

3D fault geometry has recently been shown to play a key role in controlling the kinematics of earthquake swarms. With the aid of a temporary dense seismic array, machine-learning-based detection, and accurate relocation, we outline a complex 3D fault zone accommodating a small earthquake swarm near Yunlong city, western Yunnan, China from February to May 2013. Our results show that the swarm initiated from a compressive stepover zone and subsequently activated a complex fault zone including six planar fault segments. The migration front of the swarm can be well-modeled by fluid diffusion, indicating the swarm was primarily driven by pressurized fluid. A nearby GPS site exhibits a slight displacement that started immediately before the swarm, then gradually decayed in the following months, implying the stress fluctuations caused by a possible aseismic slip likely initiated the swarm and, along with fluid diffusion, drove the migration of the swarm. Within the stepover zone, complex and dense fractures act as conduits connecting the reservoir and fault zone, facilitating fluid flow. Meanwhile, the stress in the stepover zone tends to increase in response to the compression of the two boundary faults, which not only makes the stepover zone more susceptible to be triggered by those transient stresses but also forms a fluid pumping mechanism that drives fluids from the stepover zone into the complex fault zone. We suggest the complexity of the initiation and evolution of the 2013 Yunlong swarm is strongly controlled by 3D fault geometry.(c) 2023 Elsevier B.V. All rights reserved.