中国大陆古地震研究的关键技术与案例解析(2)——汶川地震地表变形特征与褶皱逆断层古地震识别

摘要/Abstract

摘要：

汶川M_S8.0地震是近代少有的大陆褶皱逆断层型巨大地震,其地表破裂带是研究和解剖褶皱逆断层地表同震变形样式,并以此探讨古地震遗迹的不可多得的现实案例。在整理和分析汶川地震地表破裂带地质地貌调查资料的基础上,选择可能仅记录1次事件的平通、邓家(北川-映秀断裂)和九龙(江油-灌县断裂)等地为例,分析同震变形的特点和类型,并结合映秀、桂溪等地的古地震研究成果,讨论褶皱逆断层型古地震识别的技术要点。结果显示: 地表变形主要包括逆断层直接位错、折曲位错变形和弯曲褶皱变形等类型; 崩积楔、断层与地层切盖关系是分析断错地表型古地震事件的可行依据,而折曲位错变形型和弯曲褶皱变形型古地震识别则强调在上盘是否存在侵蚀不整合面,下盘是否存在生长地层,以及标志地层在断层两盘位差的突然增减; 断层陡坎高度的倍数关系在一定程度上与古地震次数相关,但不能简单地用同震位移量除以陡坎高度的方法确定古地震期次; 对于低角度逆断层的古地震识别,薄长状崩积楔、断层与堆积地层的切错关系和不同标志地层在断层两侧的累积位差的突变是重要的标志。识别古地震应因地制宜、思考多种因素的影响、用多种证据相互印证。

关键词: 汶川地震, 同震变形样式, 褶皱逆断层, 古地震识别, 技术要点

Abstract:

The M_S8.0 Wenchuan earthquake is a rare earthquake of fold-reverse fault type in mainland China. The rupture zone of Wenchuan earthquake is an indispensable case which can be used to study the surface co-seismic deformation of reverse fault and discuss the paleoseismic records. Based on the geologic and geomorphic features along the rupture zone of Wenchuan earthquake,we choose the Pingtong,Dengjia and Liulong three sites which were deformed only in the Wenchuan earthquake to analyze the characteristics of co-seismic deformation. And we combine with paleoseismic studies in the Yingxiu and the Guixi areas to discuss some key techniques in the paleoseismic study of the fold-reverse fault type. The conclusion shows surface deformation types include fault dislocation,bending dislocation and fold deformation. The cut-cover relationship of colluvial wedge,fault and strata is the feasible evidence of the fault dislocation type. But the indicator for bending dislocation and fold deformation types emphasizes the unconformity on the hanging wall,growth strata on the footwall and sudden change of maker strata position between the hanging wall and footwall. The multiple relationship of fault scarp height is related to paleoseismic times in some degree,but it is not wise to decide the paleoseismic times simply by a direct division of the height of fault scarp by the co-seismic displacements. In addition,there are two important paleoseismic indicators for identifying paleoseismic event on low-angle thrust,that is,the sudden change of displacements in maker strata and the cut-cover relationship of thin-long colluvial wedge,fault and strata. A good paleoseismic study needs to consider many factors and use more evidences with consideration of local conditions to support or supplement the analysis.

Key words: Wenchuan earthquake, co-seismic deformation, fold-reverse fault, paleoseismic indicators, key technique

中图分类号:

P315.2

冉勇康, 陈立春, 陈文山, 王虎, 李安. 中国大陆古地震研究的关键技术与案例解析(2)——汶川地震地表变形特征与褶皱逆断层古地震识别[J]. 地震地质, 2012, (3): 385-400.

RAN Yong-kang, CHEN Li-chun, CHEN Wen-shan, WANG Hu, LI An. KEY TECHNIQUES AND SEVERAL CASES ANALYSIS IN PALEOSEISMIC STUDIES IN MAINLAND CHINA(2): SURFACE DEFORMATION CHARACTERISTICS OF WENCHUAN EARTHQUAKE AND PALEOSEISMIC INDICATORS ON FOLD-REVERSE FAULT[J]. SEISMOLOGY AND GEOLOGY, 2012, (3): 385-400.

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