2017年九寨沟Ms7.0地震揭示青藏高原东缘岷山地区一条新的左旋走滑断裂

A New Sinistral Strike-slip Fault in the Minshan Area of Northeastern Tibetan Plateau Revealed by the 2017 Jiuzhaigou Ms7.0 Earthquake

2017年8月8日,青藏高原东缘川西九寨沟地区发生Ms7.0地震。基于震源机制解和余震分布特征,结合历史地震和区域新构造分析,揭示了一条新的左旋走滑断裂:九寨沟-虎牙断裂,该断裂斜切岷山隆起,呈弧形展布,北接塔藏罗叉断裂,南连虎牙断裂,是东昆仑左旋走滑断裂的最新东延。新构造研究结果表明,第四纪以来,青藏高原东部巴颜喀拉地块向东挤出,在其东缘岷山地区边界运动学性质发生了明显的转换:早-中更新世之交的昆黄运动时期(1.0～0.6Ma),向东挤出运动主要转换为地壳缩短变形和块体隆升,沿岷山隆起东西两侧发生逆冲断裂作用;而自晚更新世晚期以来(ca 120ka),地块向东挤出伴随着块体顺时针旋转,构造变形以左旋走滑活动为主,主要集中在塔藏-九寨沟-虎牙断裂带。新构造运动性质的转换使得一些早期强烈活动的断裂,如岷江断裂、漳扎断裂等,其现今活动性明显减弱,而新的左旋走滑断裂正在孕育诞生。这一活动断裂演化图像的新认识为本地区未来强震评估和预测提供了新的视角。

On 8 August 2017,an earthquake of the magnitude of Ms7.0 struck the Minshan area of the northeast Tibetan Plateau.Focal mechanisms of the main shock and the epicenter distribution of aftershocks,together with historic earthquakes and neotectonic analysis,clearly reveal a newly developing sinistral strike-slip fault,namely the Jiuzaigou-Huya fault.The fault obliquely cut the Minshan uplift zone,forming an arc-shaped zone.It is the easternmost extension of the East Kunlun sinister strike-slip faulting zone,linking to the Tazangcha fault in the north and the Huya fault in the north.The neotectonic studies show that the Bayan terrane in the eastern Qinghai-Tibet plateau extruded eastward and kinematic nature of the faults in the Minshan uplift zone has changed remarkably during the Quaternary.It was predominated by reverse faulting during the Early-Middle Pleistocene Kun-Huang Movement stage(ca.1~0.6Ma),which accommodated eastward extrusion of the Bayan Har Block.Since the late Late Pleistocene(ca 120ka),the terrane extruded eastward accompanied by clockwise rotation and tectonic deformation was dominated by sinistral strike-slip motion along the newly developed Jiuzaigou-Huya fault zone.Transition of the fault mechanism made some early active faults(such as the Minjiang fault,the Zhangzhai fault,etc.)less active and even dead at present,while new active faults are on the way.This evolutionary image of Quaternary faults may give a new insight into seismic assessment along the eastern margin of the Tibetan Plateau.