Abstract: More attention has been paid to the late Quaternary activity of the boundary fault of the Sichuan-Yunnan block in eastern Tibet. The Lijiang-Xiaojinhe Fault (LXF) locates along the boundary of the northwest Sichuan and central Yunnan sub-blocks in the Sichuan-Yunnan block. Clear displaced landforms show that the fault has undergone strong late-Quaternary activity. However there is no surface-rupturing earthquake occurring on the LXF in the historical record. The LXF crosses the city of Lijiang, one of the most important tourist cities in Southwest China. The rupture behavior on this fault remains unclear and it is hard to assess its seismic hazard in the future. In this study, on the base of the interpretation of high-resolution satellite imagery, we chose the middle segment of the LXF and dug three trenches at Muzhuda, Hongxing, and Gantangzi sites to constrain the ages of paleoearthquakes combined with radiocarbon dating and OxCal modeling. The Muzhuda trench shows that at least three events occurred on the middle segment of the LXF at 7 940~6 540a BP, 4 740~4 050a BP and 1 830~420a BP, respectively. The Hongxing trench indicates that the LXF underwent two events at 5 120~3 200a BP and 2 100~1 220a BP. The Gantangzi trench reveals at least three paleoearthquakes at 44 980~17 660a BP, 7 210~3 810a BP and 2 540~1 540a BP, respectively. The events in the Gantangzi trench might be incomplete because of stratigraphic gap. These three trenches indicate that three events occurred on the middle segment of the LXF in the Holocene at 7 940~7 210a BP, 4 740~4 050a BP and 1 830~1 540a BP, respectively. Large earthquakes on the middle segment of the LXF appear to fit the quasi-periodic model with the mean recurrence interval of~3 000a and the estimated magnitude 7.5. Given the strong late-Quaternary activity of the middle segment of the LXF and a long elapsed time, we propose that the middle segment of the LXF might have a high seismic hazard potential in the near future.

Key words: Sichuan-Yunnan block, Lijiang-Xiaojinhe Fault, paleoearthquake, trench, recurrence interval

摘要： 丽江-小金河断裂为川滇菱形块体内部重要的次级边界断裂，沿断裂断错地貌清晰，晚第四纪活动强烈，但历史上无地表破裂型地震的记录，其大震复发行为仍不清楚，难以评价其地震危险性。文中选取断错地貌最为显著的丽江-小金河断裂中段，在高分辨率卫星影像解译的基础上，在母猪达、红星和干塘子开挖3个探槽，并利用放射性碳测年和古地震事件的OxCal建模对古地震事件年龄进行了限定。研究结果表明，母猪达探槽揭示了3次古地震事件，分别发生在7 940～6 540a BP、4 740～4 050a BP和1 830～420a BP；红星探槽揭示了2次古地震事件，分别发生在5 120～3 200a BP和2 100～1 220a BP；干塘子探槽揭示了至少3次古地震事件，分别发生在44 980～17 660a BP、7 210～3 810a BP和2 540～1 540a BP，因为地层存在缺失，该探槽中揭示的事件可能不完整。综合3个探槽结果，全新世以来丽江-小金河断裂中段有3次古地震，分别发生在7 940～7 210a BP、4 740～4 050a BP和1 830～1 540a BP，其大震复发大致符合准周期模式，平均复发间隔约3 000a，震级达M7.5。考虑到丽江-小金河断裂中段较强的晚第四纪活动和较长的大震离逝时间，其未来地震危险性值得关注。

关键词: 川滇菱形块体, 丽江-小金河断裂, 古地震, 探槽, 复发间隔