鲁甸MS6.5级地震发生后,余震分布呈现两个优势方向,其发震断层是NE向的昭通—鲁甸断裂,还是NW向的次级断裂,引起了广泛的讨论.详细的野外调查发现,从龙头山镇南东方向的谢家营盘光明村王家坡一线N22°W～N55°W走向断续展布长约8 km地表破裂带,呈左行右阶排列,伴有30～35 cm左右的走滑分量,局部走滑分量达40～60 cm,表明此次地震的发震构造为北西走向的包谷垴小河断裂.探槽工程揭露出发震构造光明村小垭口段4次古地震事件;8个炭屑样品的14C测年分析表明,事件E1可能发生在9190-8870BC,E2为1000BC至900AD,E3为910-1240AD,最新一次事件E4为2014年鲁句Ms6.5级地震.该研究表明,云南地区6.5级强震可以产生地表破裂并可能在地质记录中留下遗迹.结合本次地表破裂特征,影像几何不连续以及探槽剖面揭露出古地震断面信息容易隐形等特征表明,包谷垴—小河断裂在历史活动中强度不大,属中强地震发震构造.%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 Zhaotong-Ludian fault or a NW oriented secondary fault.Through detailed field investigation,we found that there were estimated 8 km long surface ruptures oriented N22°W-N55°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 30-35 cm overall and 40-60 cm locally,demonstrating that the Baogunao-Xiaohe 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 El,E2 and E3 at 9190-8870 BC,1000 BC-900 AD,and910-1240 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 Baogunao-Xiaohe fault was not very strong historically,and this fault represents a seismogenic structure of a moderately strong earthquake.
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