The aftershock sequence and spatial stress field of the 2014 <italic>M</italic>_W6.1 Ludian, Yunnan, Earthquake

The 2014 MW6.1 Ludian earthquake was a complex seismic event involving sequential rupturing along two conjugate faults: the Baogunao-Xiaohe fault and an orthogonal EW-trending fault. To better understand the tectonic characteristics and seismogenic environment, this study utilized 43 broadband seismometers deployed around the Ludian area to detect, locate, and determine the focal mechanisms of aftershocks, as well as to invert the stress field. The results indicate that aftershocks were distributed along two dominant directions, consistent with the mainshock rupture faults. Depth profiles from earthquake locations reveal the spatial relationships between the Zhaotong-Ludian fault and the two seismogenic faults. The aftershock sequence expanded westward over time, triggering the activity of a concealed fault. Based on the located aftershocks, 753 focal mechanism solutions were obtained, predominantly of the strike-slip type. A small number of normal-faulting earthquakes were concentrated along the near EW-trending fault, with a significant angle to the main fault. Focal mechanism solutions indicated a strike-slip with an extensional component along the near EW-trending fault. Thrust faulting earthquakes were primarily concentrated along the Baogunao-Xiaohe fault. The stress field inversion results showed that the stress field rotated across different regions, with the maximum compressive stress axis rotating approximately 10° clockwise from west to east in the study area. The Baogunao-Xiaohe fault exhibited a steeply dipping extensional stress axis, with an R value of approximately 0.87, indicating significant horizontal and vertical shear stresses, characterized by strike-slip with compression. The complexity of fault geometry, due to the strike-slip extension and extensive shear conjugate rupturing along the near EW-trending fault, likely impeded the late-stage propagation of the mainshock rupture and prevented larger-scale ruptures along this fault.