Abstract: The Ganzi-Yushu Fault, the boundary of Bayan Har active tectonic block, Qiantang active tectonic block and Sichuan-Yunan active tectonic block, is a sinistral strike-slip fault zone with intensive Holocene activity. Thus, the study of activity characteristics and rupture behavior of paleoearthquakes in the late Quaternary on the Ganzi-Yushu Fault is of fundamental importance for understanding the future seismic risk of this fault. The southeast section of Ganzi-Yushu Fault is made up of three segments of Ganzi, Manigange and Dengke, where a M_S7.3 earthquake in 1866, a M_S7.7 earthquake in 1854 and a M_S7.3 in 1896 occurred, respectively. There is still lack of in-depth study on the active features and the cascading rupture possibility of these segments, which hindered the evaluation of seismic risk for the southeast section of Ganzi-Yushu Fault. By the means of field geological survey and micro topography measurement, this paper studied the geological and geomorphological features of the southeast section of the Ganzi-Yushu Fault. The results show that the Ganzi and Dengke segments show obvious extension movement, in addition to the left-lateral movement. For Manigange segment, the characteristics of the movement are mainly left-lateral strike-slip and thrusting, and the maximum vertical displacement of the Holocene strata is greater than 2m. In part areas, the movement is normal faulting, which perhaps relates to the left stepping zone in the local stress environment. Therefore, combining the research results such as the fracture distribution in different motion characteristics, rupture behavior of paleoearthquakes, and the distribution of historical earthquake surface ruptures, we divide the southeast section of Ganzi Yushu Fault into Ganzi, Manigange and Dengke segment, and consider the Yakou and the Dengke Basin as the stepovers and the segments' boundaries. As the small scale of impermanent barriers including Dengke Basin and the ridge near Yakou, of which the width is about 1~2km, they may be broken through in great earthquake rupture in future. A trench was excavated in Zhuqing township to investigate the paleoearthquakes on the Manigange segment, radiocarbon dating was employed and 3 paleoseismic events were revealed in the Zhuqing trench, which are the seismic events occurring respectively at 3875~3455BC, after 775BC, and the latest one that ruptured the surface. Compared with the previous results of paleoseismology in the southeast section of Ganzi-Yushu Fault, it is found that the paleoseismic events in the Manigange segment are obviously different with that in Ganzi segment and Dengke segment. Due to the lack of sufficient data on the southeast section of the Ganzi-Yushu Fault, it still needs further discussion whether the cascade-rupturing between these segments exists.

Key words: Ganzi-Yushu Fault, characteristics of late Quaternary activity, paleoseismology, rupture behavior

摘要： 甘孜-玉树断裂是1条全新世强烈活动的左旋走滑断裂带，与鲜水河断裂共同构成了巴颜喀拉地块和羌塘地块、川滇菱形地块的边界断裂，研究其晚第四纪活动特征及大地震复发行为是认识甘孜-玉树断裂未来7级以上大地震危险性的重要基础工作。文中通过野外地质调查、微地貌测量等方法揭示出甘孜段和邓柯段除左旋运动性质外，还具有明显的拉张作用，而马尼干戈段运动性质则以左旋走滑兼逆冲性质为主，全新世地层最大垂直位移量>2m。结合断裂的几何展布特征、古地震破裂行为和历史地震地表破裂分布范围等方面的研究结果，可以将垭口和邓柯盆地2个阶区作为甘孜-玉树断裂东南段的分段边界。由于垭口附近的山脊及邓科盆地的障碍体规模均较小，阶区宽度仅1~2km左右，为非持久性障碍体，在未来大地震的破裂中可能会被突破。文中在马尼干戈段上的竹庆乡进行了探槽开挖，通过样品年代学测试共识别出3次古地震事件，分别为3875~3455BC之间、晚于775BC和最新1次断错地表事件。通过与前人在甘孜段和邓柯段的古地震研究结果进行对比，揭示出马尼干戈段与甘孜段、邓柯段的古地震破裂事件具有明显的不同，但限于甘孜-玉树断裂东南段的古地震研究仍缺乏足够的数据支撑，各几何分段之间是否存在级联破裂仍需要进一步的探讨。

关键词: 甘孜-玉树断裂, 晚第四纪活动特征, 古地震, 破裂行为