大青山山前断裂土右旗西段莲花山探槽揭露的古地震事件及年龄

Paleoseismic events and ages revealed by the Lianhuashan trench in the western section of the Tuyouqi of the Daqingshan piedmont fault

大青山山前断裂位于河套盆地北缘,晚第四纪以来活动强烈,前人对该段古地震特征做过较多研究,但是因测年技术手段限制,以及在同一断层剖面拥有较多能有效约束古地震事件的测年数据较少、且能揭露出较多古地震事件的剖面偏少。本文通过对大青山山前断裂土右旗西段莲花山东侧古地震探槽的开挖,利用光释光年代学数据对古地震事件进行有效约束,并结合前人在该段探槽的年代数据,运用断层逐次限定法综合分析,得出土右旗西段断裂距今约11 500年发生了5次古地震事件,平均复发间隔2 260年。该段断裂5次古地震事件由新到老分别为:1.28~1.42 ka、3.41~3.60 ka、4.08~5.01 ka、8.26~9.32 ka和10.70~11.30 ka。最新一次活动事件应该为公元849年地震。同时,探槽揭露该段断层呈正断层性质,倾角在∠58°~∠77°之间。根据经验公式,包头段、土右旗西段和土左旗西段的潜在发震能力分别在M_W6.8~7.1、M_W6.8~7.1和M_W7.1~7.2左右。本文经验公式得出的震级偏小,可能存在级联破裂,后期需进一步结合其他手段对大青山山前断裂西段的危险性进行综合评价。

Daqingshan piedmont fault is located in the northern margin of Hetao basin and has been active strongly since the Late Quaternary.Previous researchers had conducted extensive research on the characteristics of paleoseismic in this area,but limited by the dating techniques and the fact that there were fewer data available on the same fault profile that can effectively constrain paleoseismic events,and fewer profiles that can reveal more paleoseismic events.This paper explores the excavation of an paleoseismic trench on the east side of Lianhuashan in the western section of Tuyouqi of the Daqingshan piedmont fault,using Optically Stimulated Luminescence data,and combineing with the age data of previous exploration trenches.By using the fault successive limit method,it is found that the western section of the Tuyouqi fault has experienced five paleoseismic events approximately 11500 years ago,with an average recurrence interval of 2260 years.The five paleoseismic events in this section of the fault,from new to old,are 1.28 to 1.42 ka,3.41 to 3.60 ka,4.08 to 5.01 ka,8.26 to 9.32 ka,and 10.70 to 11.30 ka,respectively.The latest event of activity should be the earthquake of 849 AD.Meanwhile,the trench exploration revealed that the fault in this section exhibits a normal fault nature,with an inclination angle between 58 and 77 degrees.According to empirical formula,the potential seismic capacity of the Baotou section,the western section of Tuyouqi,and the western section of Tuzuoqi are around MW6.8~7.1,MW6.8~7.1 and MW7.1~7.2,respectively.The magnitude obtained by the empirical formula in this paper is relatively small,and there may be cascade rupture.In the later stage,it is necessary to conduct a comprehensive evaluation of the risk of the western section of Daqingshan piedmont fault combined with other means.