Abstract: The Xianshuihe Fault, the boundary of Bayan Har active tectonic block and Sichuan-Yunnan active tectonic block, is one of the most active fault zones in the world. In the past nearly 300 years, 9 historical earthquakes of magnitude ≥ 7 have been recorded. Since 2008, several catastrophic earthquakes, such as Wenchuan M_S8 earthquake, Yushu M_S7.1 earthquake and Lushan M_S7 earthquake, have occurred on the other Bayan Har block boundary fault zones. However, only the Kangding M_S6.3 earthquake in 2014 was documented on the Xianshuihe Fault. Thus, the study of surface deformation and rupture behavior of large earthquakes in the late Quaternary on the Xianshuihe Fault is of fundamental importance for understanding the future seismic risk of this fault, and even the entire western Sichuan region. On the basis of the former work, combined with our detailed geomorphic and geological survey, we excavated a combined trench on the Qianning segment of Xianshuihe fault zone which has a long elapse time. Charcoal and woods in the trench are abundant. 30 samples were dated to constrain the ages of the paleoseismic events. Five events were identified in the past 9  000 years, whose ages are:8070-6395 BC, 5445-5125 BC, 4355-4180 BC, 625-1240 AD and the Qianning earthquake in 1893. The large earthquake recurrence behavior on this segment does not follow the characteristic earthquake recurrence model. The recurrence interval is 1000~2000 years in early period and in turn there is a quiet period of about 5 000 years after 4355-4180 BC event. Then it enters the active period again. Two earthquakes with surface rupture occurred in the past 1000 years and the latest two earthquakes may have lower magnitude. The left-lateral coseismic displacement of the 1893 Qianning earthquake is about 2.9m.

Key words: Xianshuihe fault zone, Qianning segment, paleoearthquake, earthquake recurrence behavior, Bayan Har block

摘要： 鲜水河断裂带是巴颜喀拉和川滇2个强烈活动次级块体的边界带，为全球近代最活跃的断裂带之一，近300a就记录有9次7级以上大地震。2008年以来，巴颜喀拉块体东边界龙门山断裂带上相继发生了2008年汶川M_S8和2013年芦山M_S7大地震，南边界带西段玉树-甘孜断裂带上发生了2010年玉树M_S7.1地震，东段鲜水河断裂带上发生了2014年康定M_S6.3地震。研究其晚第四纪地表变形和大震复发行为是认识鲜水河断裂带乃至川西地区未来7级以上大地震危险性的重要基础工作。在前人工作基础上，文中通过地质地貌调查，在离逝时间相对较长的乾宁段上选择龙灯地点进行了组合探槽开挖，获得了鲜水河断裂带乾宁段距今约9 000a以来的5次（古）地震事件序列：8070-6395BC、5445-5125BC、4355-4180BC、625-1240AD和1893年乾宁地震，其大震复发行为并不遵循准周期复发模式，早期复发间隔1 000~2 000a，至4355-4180BC事件后，有过5 000a左右时长的平静期，之后又进入活跃期，1 000a BP左右以来发生了2次地表破裂型大地震。1893年乾宁地震在探槽地点一带的同震左旋位错量约2.9m。

关键词: 鲜水河断裂带, 乾宁段, 古地震, 地震复发行为, 巴颜喀拉块体