Abstract: After the Ludian MS 6.5 earthquake occurred, the distribution of aftershocks presented two possible directions, causing intensive debate over whether the seismogenic structure was the NE oriented ZhaotongLudian fault or a NW oriented secondary fault. Through detailed field investigation, we found that there were estimated 8 km long surface ruptures oriented N22°WN55°W along the line connecting Xiejiayingpan and Guangming villages and Wangjiapo. These ruptures are rightstepwise and leftlateral and with strikeslip components of 3035 cm overall and 4060 cm locally, demonstrating that the BaogunaoXiaohe fault is the seismogenic structure of the Ludian MS 6.5 earthquake. A large trench along the Xiaoyakou fault trough of the seismogenic structure opened, from which we identified four paleoearthquakes including the 2014 Ludian MS 6.5 earthquake and named them E1 through E4 from the oldest to the youngest. Eight charcoal samples were dated to constrain the ages of E1, E2 and E3 at 91908870 BC, 1000 BC900 AD, and 9101240 AD, respectively, and the youngest event E4 was associated with the 2014 Ludian MS 6.5 earthquake. This study illustrates that a 6.5 magnitude earthquake in the Yunan Region could produce surface rupture and leave its trace in the geologic record. Based on the surface rupture characteristics, image geometric discontinuity and the characteristic adelomorphic footprint of the paleoseismics found on the trench walls, it may conclude that the activity of the BaogunaoXiaohe fault was not very strong historically, and this fault represents a seismogenic structure of a moderately strong earthquake.

Key words:  the Ludian earthquake, surface rupture, seismogenic structure, rupture characteristics, paleoearthquake