华北平原沧县隆起边界断裂的第四纪活动性及深部构造特征

Quaternary activity and deep structural characteristics of the boundary faults of the Cangxian uplift in the North China Plain

摘要:

沧东断裂是渤海湾裂谷盆地西侧规模最大的正断层,也是沧县隆起与黄骅坳陷的分界断裂,其活动和发展直接控制了沧县隆起和黄骅坳陷的发育与演化。采用近三十年来横跨沧东断裂不同段落获取的5条浅层地震勘探成果资料,并结合测线附近的钻孔资料,深入研究沧东断裂的第四纪活动性及深部构造特征。浅层地震勘探结果发现,沧东断裂上断点埋深从南至北由深变浅,但集中在埋深118~300 m范围,错断的最新地层为中更新统,推断其最新活动时代为中更新世,而且断裂的活动时代未发现具有分段性特征。进一步综合深地震反射、大地电磁测深、沧州地区小地震分布与中强地震震源机制等资料,对沧东断裂的深部构造特征分析后发现,沧东断裂具有典型的铲状正断层特点,在深度10 km左右转为了近水平延伸,沧东断裂并非区域上主要控震断裂,或者只是低风险活动断裂,未来发生中强地震的风险可能相对较低。

Abstract:

Cangdong fault, the largest normal fault in the west of the Bohai Bay basin, is also the boundary fault between Cangxian uplift and Huanghua depression. In this paper, the Quaternary activity and deep structural features of the Cangdong fault were studied in depth by using the seismic exploration data from five shallow survey lines obtained in the past 30 years and the borehole data near the lines. Based on the shallow seismic exploration results, it was found that the buried depth of Cangdong fault up-breakpoint changes from deep to shallow from south to north, but it is concentrated in the range of 118-300 m. It is inferred that the latest activities occurred at the middle Pleistocene, and the active age of the fault is not characterized by segmentation. Based on the data of deep seismic reflection, magnetotelluric sounding, distribution of small earthquakes, and focal mechanisms of moderate-strong earthquakes in the Cangzhou area, the deep structural features of the fault were further analyzed. It was found that the Cangdong fault is characterized by a typical listric normal fault, which turns into nearly horizontal extension at a depth of about 10 km. Cangdong fault is not the main earthquake-controlling fault in the study region, or it is only a low-risk active fault, so its risk of medium-strong earthquakes in the future may be relatively low.