九寨沟地震发震区周边构造特征及发震断裂

Study on the structural characteristics and seismogenic faults around the earthquake-stricken area of the Jiuzhaigou earthquake,China

探讨九寨沟地震发震构造背景与构造机制,为防灾减灾及灾后重建提供科学依据。根据2017年8月8日发生的九寨沟地震的主震及余震的精确定位,投影到九寨沟地震震中周边构造纲要图及1∶20万地质图上,发现九寨沟地震主震及余震呈NW-SE向线性展布,位于塔藏断裂东段尾端呈马尾状散开的复杂断裂分布区;主震及余震都没有出现在已知的地表断裂上,而是分布在2条地表断裂(塔藏断裂和岷江断裂)之间石炭纪海相浅灰色薄-厚层致密灰岩夹页岩地层中。通过野外地质调查以及对断裂、褶皱、断块构造特征的研究,确定巴颜喀拉断块东边界由岷江断裂北段-雪山梁子断裂-虎牙断裂-龙门山后山断裂(茂汶断裂)构成;岷山断块是新近纪以来强烈挤压隆升形成的近南北向的新生断块,地貌上表现为高耸的岷山山脉。本次九寨沟地震的发震断块为摩天岭断块,发震断裂是摩天岭断块内隐伏的基底走滑断裂——夏莫断裂;发震动力主要是印度板块向北碰撞挤压,导致相对薄弱的基底夏莫断裂发生左行走滑活动。

The tectonic background and structural mechanism of the Jiuzhaigou earthquake are discussed in order to provide scientific basis for disaster prevention and mitigation as well as post-disaster reconstruction.According to the precise location of the main shock and aftershocks,the Jiuzhaigou earthquake occurred on the 8 of August,2017 is projected on the structural outline map of the fault block around the Jiuzhaigou earthquake epicenter and on the 1∶200,000 sheet geological map,respectively.It shows that the main shock and aftershocks of the Jiuzhaigou earthquake are linearly distributed along NW-SE direction,located in the complex fault area at the end of the east part of the Tazang fault.The main shock and aftershocks occur in the Carboniferous marine limestone intercalated with shale,which clamped by two surface faults(Tazang fault and The Minjiang fault).The field investigation and study over the characteristics of faults,folds and fault blocks indicate that the eastern boundary of the Bayan Har Block is composed of the northern segment of the Minjiang fault-Xueshanliangzi fault-Huya fault-Maowen fault.The Minshan fault block is a newly formed north-south trending block resulted from the strong uplift since Cenozoic,topographically characteristic of high towering Minshan Mountains.It is revealed that the earthquake-induced fault block is the Motianling fault block and the seismogenic fault is the Xiamo fault,a concealed basal strike-slip fault in the Motianling fault block.It is considered that the seismogenic force is the northward compression and collision of the Indian plate which result in the left lateral strike slip of Xiamo fault.