龙门山前缘的芦山地震与逆冲-滑脱褶皱作用

摘要

龙门山式构造以推覆作用和滑脱作用为主要特征,近5年来地震活动性显著,在2008年和2013年分别发生了汶川(Ms 8.0)地震和芦山(Ms 7.0)地震.根据已公开发布的芦山地震的震源机制解、破裂过程、地震烈度、地表变形特征、余震分布规律等方面的研究成果,结合本次对龙门山南段活动断裂和芦山地震构造变形与地表响应的野外实地调查等方面的科研成果,作者将龙门山南段和前缘地区划分为龙门山冲断带和前缘扩展变形带2个构造变形带,对比了它们在构造变形样式、活动断裂、历史地震的差异性,提出了以推覆-滑脱岩片为特点的龙门山冲断带地震构造模式和以逆冲断层-滑脱褶皱为特点的龙门山前缘扩展变形带地震构造模式.在此基础上,分析了龙门山前缘扩展变形带的逆冲-滑脱作用与芦山地震的发震断层模式与成因机制,认为芦山地震形成于龙门山前缘扩展变形带的逆断层-滑脱褶皱带,其发震断裂为大邑断层,该断层倾向北西,呈铲状向下并汇交于滑脱面.该滑脱面就是芦山地震的震源层.%The tectonic pattern of the Longmenshan mainly is detachment and napping.With apparent seismic activities in recent five years,the Wenchuan (Ms 8.0) earthquake and the Lushan (Ms 7.0) earthquake occurred in the Longmenshan Mountains and its front in 2008 and 2013,respectively.Based on the focal mechanism solution,rupture process,seismic intensity,surface deformation,aftershocks of the Lushan earthquake and active faults on the southern segment of the Longmenshan,the authors divide the southern segment of the Longmenshan and its front into two tectonic deformation belts including the Longmenshan thrusting belt and the frontal propagation belt in this paper.By comparing the differences of tectonic deformation style,active faults and history earthquake between the two belts,the authors put forward two kinds of seismotectonic models.One is the thrusting belt characterized by the napping and detachment and another is the frontal propagation belt characterized by thrusting and detachment folding.By analyzing the seismogenic mechanism of thrusting and detachment folding in the frontal propagation belt during the Lushan earthquake,the authors consider that the Lushan earthquake formed by thrusting and detachment folding in thefrontal propagation belt of the Longmenshan.The seismogenic fault of the Lushan earthquake is the Dayi fault,which dips to NW with listric surface and conver over the detachment surface.The detachment surface is just the seismic source layer of the Lushan earthquake.