Relocation of the 2014 Ludian M_S6.5 earthquake and its large aftershock sequence

We use catalog data recorded by Yunnan seismic network to relocate the 2014 Ludian M_S6.5 earthquake and its aftershock sequence with double-difference location algorithm, and obtain focal parameters of 3 658 earthquakes. The results show that the relocated epicenter distribution of Ludian M_S6.5 earthquake sequence exhibits two predominant directions, approxi-mately EW and SES, and conjugated to each other. The length of the EW strip is about 30 km, while the length of SES strip is about 20 km. The Ludian earthquake sequence mainly distri-buted between 4−20 km in depth. It is deeper around the mainshock and becomes shallower along the strips. There are fourM_S≥4.5 large aftershocks （the earthquakes that occurred on the same day counts as one） occurred in the Ludian M_S6.5 earthquake source region till February 28, 2017. The distributions of four strong aftershock sequences are different from each other after relocation. The distributions of aftershock sequences of M_S4.7 occurred in September 10 and M_S4.6 occurred in October 27 are similar with the mainshock sequence, while the aftershock sequences located around 2016 M_S4.7 earthquake and 2017 M_S4.9 earthquake are different from the mainshock sequence. Synthesizing the distribution of relocated Ludian M_S6.5 earthquake sequence, geologic data in studied area and previous researches, we draw the following conclusion: the distribution of four strong aftershock sequences are resulted from combined action of regional stress field and source stress field caused by the mainshock, the strong aftershock sequences occurred in September 10 and October 27 in 2014 are mainly influenced by source stress field induced by the mainshock, while the strong aftershock sequences occurred in 2016 and 2017 are mainly influenced by regional stress field.