关键词: 浅层地震勘探, 钻孔联合地质剖面探测, 全新世活动断裂, 大兴凸起东缘断裂

Abstract:

The eastern marginal fault of Daxing Uplift is located in the southeast of the Beijing Plain, which is a boundary fault that controls the Daxing Uplift and the Langgu Sag. It intersects obliquely with the NNE-trending Xiadian Fault in the north where a magnitude 8 earthquake occurred in 1679. The overall strike of the fault is northeast, dipping southeast. Previous studies have suggested that the youngest stratum of the fault is the Mid Pleistocene of the Quaternary and it is not an active fault since the Late Quaternary. Based on high-precision shallow seismic exploration data, this study carried out high-density composite drilling geological section surveys and obtained evidence of obvious activity of the fault since the Late Quaternary. The fault is shown as an active normal fault in the composite drilling geological section. The top of the footwall of the fault is the 7m-thick silty clay marker layer buried at the depth of 74m and the top of the hanging wall is 102m deep, the amount of dislocation is about 28.0m. Fault slip surfaces were found in the cores of two of the boreholes, with depths of 54.2m and 39.4m, respectively. The buried depths of the top surface of the marker layer in the two boreholes with a horizontal distance of 2m are 8m and 10m, respectively, the dislocation amount is 2m. Combined with the observation of core deformation characteristics of the two boreholes, it is believed that the buried depth of the upper breakpoint of the fault may be shallower. This research has changed the understanding that the fault zone on the eastern margin of the Daxing Uplift is not active. This new discovery not only has great application value for understanding the risk of large earthquakes of this fault zone and the risk of earthquake disasters in Beijing, but also has scientific significance for the study of fault development and evolution and the deep-shallow coupling process in North China since the late Cenozoic. The main knowledge gained is as follows: 1)Through high-precision shallow seismic exploration, it is found that the Neogene and above strata in the study area generally show an inclined morphology which is deep in the south and shallow in the north. The strata below the Neogene are in angular unconformity contact with the bottom interface of the Neogene, and the depth of the shallowest upper breakpoint is about 38~43m. 2)The combined drilling geological section exploration reveals rich dislocation information of stratigraphic markers and further confirms the existence of active faults by borehole stratigraphic correlation. In the drill cores, fault slip surfaces were observed in the late Pleistocene strata at the depth of 39.4m, 51.5m and 54.2m, respectively. The stratigraphic comparison of the boreholes 5^# and 9^# with a hole spacing of 2m further reveals a fault throw of about 2m in the stratum at the buried depth of 8~10m, thus, it is inferred that the depth of the upper breakpoint on the fault may be 8m or shallower. According to the stratigraphic age data of adjacent boreholes in this area, it is considered that the fault is a Holocene active fault. The specific age of the latest activity and its activity parameters will be further studied through the subsequent borehole chronological tests and large-scale trench excavation.

Key words: the eastern marginal fault of Daxing Uplift, shallow seismic exploration, composite drilling geological section exploration, Holocene active fault