Recent Evidence of Slip Deformation of the Nankou-Sunhe Fault Based on Paleoseismic Results
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摘要: 南口-孙河断裂是北京地区重要的全新世活动断裂之一，也是张家口-渤海地震带的重要组成部分。该断裂总体走向310°，全长约80 km。前人通过大量的活动构造研究揭示，其全新世以来存在明显活动，发生了3次古地震事件，单次地震事件的垂直位错约为1.3 m，断裂的垂直滑动速率约为0.3 mm/a。但由于直接观察证据和测年精度限制，前人对南口-孙河断裂最近一次强震事件离逝时间的约束相对薄弱。同时，针对南口-孙河断裂可能存在的最新走滑变形特征，缺乏相对可靠的地表变形证据支撑。为此，本研究在南口-孙河断裂全新世活动段东南端进行了古地震研究，系统分析了强震事件造成的沉积地层和构造变形，采集了探槽内主断层面构造定向样品，并开展了扫描电镜分析。通过古地震探槽研究揭示，南口-孙河断裂的活动造成了沉积地层显著构造变形，表现为标志地层的错断、地层厚度的变化以及崩积楔的发育等。对探槽内沉积地层的对比分析表明，南口-孙河断裂的活动错断了探槽内1.3 m以下的沉积地层。14C测年结果揭示，南口-孙河断裂最新一次地震事件的离逝时间为2820～3700 a BP。依据古地震探槽内崩积楔、错断层位分布、地层厚度变化以及地层的倾斜变形等古地震事件识别标志分析，南口-孙河断裂在本探槽内共揭示了4次古地震事件，10020 a BP 以来发生了3次古地震事件。依据显微构造分析揭示，断裂主断层面存在平行于断层走向的黏土矿物的定向排列，揭示断层可能存在左旋水平运动，这也被探槽内揭示的主断裂约75°的陡倾角和显著的花状构造所验证。因此，南口-孙河断裂全新世以来表现为显著的走滑运动特征，启示北京地区发震构造分析需要考虑华北地区的走滑构造变形。Abstract: The Nankou-Sunhe Fault is a relatively significant Holocene active fault in Beijing and an important part of the Zhangjiakou-Bohai seismic zone. The strike of the rupture is 310°, and the total length is ～80 km. The elapse time for the most recent event is relatively weak. At the same time, there is lack of relatively reliable surface deformation evidence to support the possible latest slip deformation of the Nankou-Sunhe fault. In this study, a paleoseismic trench was excavated at the southeast end of the Holocene active section of the Nankou-Sunhe Fault, and the sedimentary stratigraphy and tectonic deformation features within the trench were analyzed, and the mechanically oriented samples of the main fault surface were collected and analyzed by scanning electron microscopy. The paleoseismic trenching revealed that the activity of the Nankou-Sunhe Fault caused significant tectonic deformation of the sedimentary stratigraphy, which is manifested by the misalignment of the layers, the change of stratigraphic thickness and the development of the colluvial wedge. According to the comprehensive analysis of the sedimentary stratigraphy in the trench, the Nankou-Sunhe Fault has misplaced the sedimentary stratigraphy below 1.3m in the trench. According to the 14C dating results, the latest seismic event of the Nankou-Sunhe Fault is 2820～3700 a BP, and the analysis of the identification marks of paleoseismic events, such as the distribution of colluvial wedges, fault location, stratigraphic thickness change and tilting deformation of strata in the trench, reveals that the Nankou-Sunhe Fault has revealed three seismic events during the past 10020 a in this trench. The presence of directional alignment of clay minerals parallel to the fault strike on the main fault surface reveals that the fault may have had left-rotation horizontal motion, which is also verified by the steep dip of ～75° of the main fault revealed in the trench. Thus, all these results evidenced the strike-slip deformation and the highly potential seismic hazard for Nankou-Sunhe fault.
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Key words:
- Nankou-Sunhe fault /
- Trenching /
- Paleoseismic /
- Scanning electron microscopy
1)1 2 胡平，徐锡伟，孟勇琦，2006. 北京市活动断层探测与地震危险性评价工程技术报告.2)2 3 赵帅，2021. 北京市昌平区地震灾害风险普查技术报告. -
图 2 探槽和南口-孙河断裂空间相对位置示意图
Figure 2. Schematic diagram of the spatial relative position of trenching and the Nankou-Sunhe fault
图 3 探槽南壁沉积地层拼接照片及解释结果
Figure 3. Sedimentary stratigraphic splice photo and interpretation result for the south wall of the trenching
图 4 探槽北壁沉积地层拼接照片及解释结果
Figure 4. Sedimentary stratigraphic splice photo and interpretation result for the north wall of the trenching
图 5 识别的4次古地震事件及其层位分布
Figure 5. Recognized paleoseismic events and its occurrence horizon
图 6 本研究与前人古地震事件对比
Figure 6. Paleoseismic events comparison between this study and previous research
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