Relocations and focal mechanism solutions of the 3 August 2014 Ludian, Yunnan Ms6.5 earthquake sequence

Using the double-difference algorithm, we relocated the Ludian M(s)6.5 mainshock and its 647 aftershocks during 3 to 7 August 2014 and finally obtained 471 relocated earthquakes. Our results show that the focal depth of main shock is 13.3 km, which is close to the initial rupture depth of the main shock. The aftershock sequence presents an asymmetric conjugate shape with lengths of 17 km in the EW direction and of 22 km in the NW direction. Small earthquakes are predominately located above 10 km depth, and extend toward the shallow parts above 10 km depth along the southeast and eastwest directions of conjugate faults from the main shock. In addition, distribution of small earthquakes also shows that the seismogenic fault has a high dip and is the NW Baogunao-Xiaohe fault that is one branch of the Zhaotong-Ludian fault. Because the mainshock centroid depth can provide important evidence for understanding the serious disaster caused by the mainshock, we determined 5 focal mechanism solutions (M-s >=-4.0) including the main shock using the gCAP(generalized Cut And Paste) method. Our results show that the centroid depth of the main shock is only 5 km, which is consistent with the 2 similar to 8 km depth of larger slip in the obtained rupturing process. The conjugate rupture and shallower centroid depth of main shock could be the important causes for the serious disaster of this earthquake.