Abstract: The Xiaojiang fault zone is located in the southeastern margin of the Tibetan plateau, the boundary faults of Sichuan-Yunnan block and South China block. The largest historical earthquake in Yunnan Province, with magnitude 8 occurred on the western branch of the Xiaojiang Fault in Songming County, 1833. Research on the Late Quaternary surface deformation and strong earthquake rupture behavior on the Xiaojiang Fault is crucial to understand the future seismic risk of the fault zone and the Sichuan-Yunnan region, even crucial for the study of tectonic evolution of the southeastern margin of Tibetan plateau. We have some new understanding through several large trenches excavated on the western branch of the Xiaojiang fault zone. We excavated a large trench at Caohaizi and identified six paleoseismic events, named U through Z from the oldest to the youngest. Ages of these six events are constrained at 40000-36300BC, 35400-24800BC, 9500BC-500AD, 390-720AD, 1120-1620AD and 1750AD-present. The Ganhaizi trench revealed three paleoearthquakes, named GHZ-E1 to GHZ-E3 from the oldest to the youngest. Ages of the three events are constrained at 3300BC-400AD, 770-1120AD, 1460AD-present. The Dafendi trench revealed three paleoearthquakes, named E1 to E3 from the oldest to the youngest, and their ages are constrained at 22300-19600BC, 18820-18400BC, and 18250-present. Caohaizi and Ganhaizi trenches are excavated on the western branch of the Xiaojiang Fault, the distance between them is 400m. We constrained four late Holocene paleoearthquakes with progressive constraining method, which are respectively at 500-720AD, 770-1120AD, AD 1460-1620 and 1833AD, with an average recurrence interval of 370~440a. Large earthquake recurrence in the late Holocene is less than the recurrence interval of~900a as proposed in the previous studies. Thus, the seismic hazard on the Xiaojiang Fault should be reevaluated. We excavated a large trench at Dafendi, about 30km away south of Caohaizi trench. Combining with previous paleoseismological research, it is found that the western branch of Xiaojiang Fault was likely to be dominated by segmented rupturing in the period from late of Late Pleistocene to early and middle Holocene, while it was characterized by large earthquakes clustering and whole segment rupturing since late Holocene.

Key words: the western branch of the Xiaojiang fault zone, large scale trench, paleoearthquake, recurrence interval, rupture characteristics of the fault

摘要： 小江断裂带地处青藏高原东南缘，系川滇菱形地块与华南地块的边界断裂带。1833年小江断裂带西支曾发生过云南历史上最大的嵩明8级特大地震。研究小江断裂带晚第四纪地表变形和强震破裂行为对认识断裂带乃至川滇地区未来地震危险性以及青藏高原东南缘构造演化都至关重要。文中通过在小江断裂带西支开挖的多个大型探槽取得了一些新的认识：草海子探槽共揭露出6次古地震事件，从老到新依次为事件U、V、W、X、Y和Z，分别发生于40000—36300BC、35400—24800BC、9500BC—500AD、390—720AD、1120—1620AD和1750AD至今。干海子探槽共揭露出3次古地震事件，从老到新依次为事件GHZ-E1、GHZ-E2和GHZ-E3，分别发生于3300BC—400AD、770—1120AD和1460AD至今。大坟地探槽古地震事件分别为：事件E1，发生于22300—19600BC；事件E2，发生于18820—18400BC；事件E3，发生于18250BC至今。干海子探槽与草海子探槽同在小江断裂带西支上，且两探槽相距仅约400m，结合历史地震记录并使用古地震研究的逐次限定法可限定该段全新世晚期记录较完整的4次古地震事件发生时间分别是500—720AD、770—1120AD、1460—1620AD和AD1833，平均复发间隔约370～440a，这一结果小于前人所得的约900a的平均复发间隔。因此，应重新评估小江断裂带未来的地震危险性。结合前人研究结果发现，小江断裂带西支在晚更新世晚期至全新世早、中期很可能以分段破裂为主，而在全新世中晚期以来具有丛集及整个西支断裂全段破裂的特征。

关键词: 小江断裂带西支, 大型探槽, 古地震, 复发间隔, 断裂破裂特征