甘孜-玉树断裂西段晚第四纪活动性研究

The Ganzi-Yushu fault together with the Margaichaka and Xianshuihe fault, at the south boundary of the Bayan Har block, accommodates the relative motion between Bayan Har and Qiangtang-Chuandian block. Divided by the step area at Yushu, the west segment extends to Fenghuoshan and the east segment extends to Ganzi. The east segment is linear in satellite images and systematically offset fluvial terraces and alluvial fans, along which large earthquakes have occurred frequently along this fault, such as the 1854 ~M7.1 earthquake at Ganzi, the (1320±65) ~M8 earthquake at Manigange, the 1896 ~M7.5 earthquake at Dengke, the 2010 ~M7 earthquake at Yushu and the 1738 ~7.5 earthquake northwest of Yushu. The west segment，of which the geometry is more complex and composed by several branches, is a seismic gap at the south boundary of Bayan Har block. Displaced late Quaternary geomorphic units can be seen on satellite images, however, due to the high altitude and sparse population in this area, little is known about the late Quaternary activity of this segment. Based on the integrated methods of interpretation of remote sensing images, mapping of micro tectonic geomorphology, Quaternary dating of geomorphic units and fault related sediments, we will try to study the late Quaternary activity of the west segment of the Ganzi-Yushu fault. This research is beneficial to the long-term seismic hazards assessment of the Ganzi-Yushu fault and can provide basic data to the late Quaternary kinematic analysis of the south boundary of Bayan Har block.

甘孜-玉树断裂位于巴颜喀拉地块的南界，与玛尔盖查卡断裂、鲜水河断裂共同协调巴颜喀拉地块与羌塘-川滇地块之间的相对运动。以玉树阶区为界，西段延至风火山，东段延至甘孜。东段线性影像清晰，沿线阶地、洪积扇发生明显断错，大震频发，发生过1854年甘孜~7.1级地震、公元(1320±65)年马尼干戈~8级地震、1896年邓柯~7.5级地震、2010年玉树7级地震，1738年玉树西北~7.5级地震。西段断裂结构相对复杂，由多条分支断裂组成，属于巴颜喀拉地块南缘的地震空区。遥感影像上具有断错地貌迹象，由于地处高原人烟稀少，西段晚第四纪活动性研究至今非常欠缺。本文拟通过遥感影像解译、构造微地貌填图、地貌面以及断层相关沉积物年代测定，对甘孜-玉树断裂西段晚第四纪活动特征进行研究，该研究有利于甘孜-玉树断裂的长期地震危险性分析，也为探讨巴颜喀拉地块南界的晚第四纪运动学特征提供了基础资料。

甘孜-玉树断裂带是青藏高原中部的一条大型左旋走滑断裂带，甘孜-玉树断裂位于巴颜喀拉地块的南界，与鲜水河断裂共同协调巴颜喀拉地块与羌塘-川滇地块之间的相对运动。当江断裂位于甘孜-玉树断裂带西北段，其晚第四纪活动性相对于甘孜-玉树断裂带其它断裂段研究程度较低，不利于断裂带整体活动性的研究和地震危险性分析，阻碍了对青藏高原大型走滑断裂带运动学模型的检验。通过遥感影像解译、数字高程地形模型（DEM）数据分析、详细的野外构造地貌填图以及OSL和14C年代限定，获得该断裂晚第四纪滑动速率为6.0-8.7mm/yr。以小纹沟、阶地作为标志体，获得该断裂历史地震的同震位移分布特征，认为1738年历史地震发生在该断裂上，地表破裂长度约为85km，最大同震位错为5m，根据破裂长度、位移量与震级的经验关系获得潜在震级为7.5。该研究有利于甘孜-玉树断裂的长期地震危险性分析，也为探讨巴颜喀拉地块南界的晚第四纪运动学特征提供了基础资料。