RELOCATION OF THE AFTERSHOCK SEQUENCE OF THE M_S6.5 LUDIAN EARTHQUAKE AND ITS SEISMOGENIC STRUCTURE

We integrated two-month phase data recorded by Yunnan Seismic Network,Zhaotong Seismic Network,Qiaojia Seismic Array and temporal stations deployed around the Ludian earthquake source region and relocated the aftershock sequence of the Ludian earthquake. The locations of 1 750 aftershocks were determined using double-difference location algorithm. The relocation result shows that the aftershock distribution has two predominant directions,to the southeast and southwest,and shows itself as an asymmetric conjugate shape. The lengths of the two aftershock strips are about16 km. The angle between the two strips is about 100°. Aftershock distribution shows that the seismogenic fault of the Ludian earthquake is a high-angle strike-slip fault. The mainshock is located at the middle at southwest of the two aftershock strips. Early aftershocks are distributed mainly along the NW- SE direction,perpendicular to the Zhaotong-Ludian Fault. The aftershocks located to the southwest of the mainshock may be triggered by the mainshock. According to the aftershock distribution and its relations with neighboring faults,focal mechanism of the mainshock,the long axis orientation of seismic intensity map,and distribution of landslides,we speculate that the seismogenic fault is the Baogunao-Xiaohe Fault. There are significant differences not only in seismic activity,deep velocity structure,but also the block movement direction and rate on both sides of the BaogunaoXiaohe Fault. The northward expansion of aftershock activity may be blocked by the high-velocity anomaly zone located on the north side of the Baogunao-Xiaohe Fault.