Abstract: Qikou depression is an important tectonic unit in the eastern part of the Huanghua depression,which constitutes the Cenozoic depocenter,with the Cenozoic maximum deposition thickness reaching to 8000～10,000m. Several profiles of acoustic survey have been conducted across the buried faults in Tianjin Qikou depression. The surveying results show that the high-angle near EW trending normal faults are well developed. The latest upper break points of these faults are 35m from the seabed,being very shallow,and the dislocated stratums are very new.
Combined with regional stratigraphic analysis,the buried faults should be late Pleistocene active faults. But the deep seismic profiles reveal that these faults in shallow strata appear as negative flower structures,which form a compound graben structure,and gradually integrate into the Qibei Fault and Haihe Fault in the deep part,as branch faults of Qibei Fault and Haihe Fault. So,these faults don't have great tectonic significance,as the normal faults associated with the coseismic slip at the shallow strata when earthquakes occurred at the deep part of Haihe Fault. The earthquake risk assessment of these faults should be different from other active faults. We can't think they have a high seismic risk,just because they are late Pleistocene active faults. But,there is a high seismic risk for the Haihe Fault.
The deep seismic profiles show that Haihe Fault and Haiyi Fault are boundary faults of Qikou depression,Qibei Fault and Qizhong Fault are the secondary faults within Qikou depression; Haihe Fault is shown as a listric normal fault on the seismic profile, and in its shallow part,multiple shallow branch faults are developed,which control the Cenozoic sedimentary development of Qikou depression. The Qibei Fault is the main branch of Haihe Fault,and as shown on the acoustic shallow stratigraphic profiles,it is a late Pleistocene active fault,though it is smaller in size relative to Haihe Fault,we can't ignore the seismic risk of Qibei Fault.

Key words: marine buried fault, acoustic surveying, Qikou depression, Haihe Fault, Qibei Fault

摘要： 针对天津歧口凹陷内的隐伏断裂开展了浅地层声学探测。结果显示，歧口凹陷内近EW走向的高角度正断层十分发育。这些断层上断点埋藏较浅，距海底约35m，断错地层较新，结合区域地层年代学分析，应为晚更新世活动断裂。但是中深层地震剖面显示，这些断层在浅部地层中表现为“负花状构造”，形成复式地堑结构，在深部都逐渐归并到海河断裂，是海河断裂的派生断裂。因此，对这些断裂的地震危险性评价应有所区别，不能因为其是晚更新世活动断裂，就认为它们具有很高的地震危险性，而真正具有强震危险性的断裂是海河断裂。

关键词: 海域隐伏断裂, 声学地层探测, 歧口凹陷, 海河断裂