Investigation of the 2013 Eryuan, Yunnan, China MS 5.5 Earthquake Sequence: Aftershock Migration, Seismogenic Structure and Hazard Implication

The Chuan-Dian block is one of the most seismically active regions in China. However, its western boundary – Weixi-Qiaohou-Weishan fault is an exception, which had maintained nearly 65-year hiatus in major seismic activity until 2013. Recently, several moderate earthquakes (MS 5.0 – 6.4) have successively occurred near the Weixi-Qiaohou-Weishan fault, attracting attention from the public. In this study, we conduct a comprehensive analysis for the 2013 MS 5.5 Eryuan earthquake sequence that occurred ~20 km southwest of the Weixi-Qiaohou-Weishan fault. We integrate a machine-learning-based earthquake detection and location workflow and a template matching technique to derive 749 high-precision earthquakes directly from continuous streams of raw seismic data. These aftershocks started from NE of the mainshock, then bilaterally expanded towards SE and NW directions, and eventually activated a SE-trending complex fault structure with ~18 km length. We also use a waveform-based inversion algorithm to calculate the focal mechanisms of the mainshock and its relatively large aftershocks, resulting in four predominantly normal-faulting events and seven strike-slip events, which further illuminate the 2013 Eryuan sequence was accommodated by a strike-slip fault system separated by extensional stepover zones. In addition, we identify six repeating earthquake doublets in the aftershock sequence, implying that some aftershocks of the MS 5.5 Eryuan mainshock may be driven by afterslip.