关键词: 北京平原、大兴断裂、晚第四纪活动性、联排钻孔探测、浅层地震勘探

Abstract: Beijing plain is a strong earthquake tectonic area in China. There was a Sanhe -Pinggu earthquake with M8 in1679. The seismogenic fault of this earthquake is called Xiadian fault. There are about 10km long earthquake surface fault, which strikes northeast. Deep seismic profile exploration reveals that this is a direct exposure of a deep fault cutting the whole crust on the surface. It is concealed in the quaternary layers to both ends. Previous studies have not yet revealed how the deep fault with M8 earthquake extended to the southwest and northeast. In the study of Xiadian fault, it is found that there is a fault with similar strike and opposite dip in the west of Xiadian fault. It is called Xiadian west fault in this paper.In this paper, six shallow seismic profiles are used to determine the location of the fault in Sanhe city, and the late Quaternary activity of the fault is studied by using the method of combined drilling, magnetic susceptibility logging and luminescence dating. The results of shallow seismic exploration show that the fault is zigzag and generally strike NE and incline NW. In vertical section, it is generally normal fault. There is one section showing the flower structural, which indicates that the fault may have a certain strike slip property. On two long seismic reflection profiles, it can be seen that the northwest side of the fault is a half graben structure. This half graben like depression, which has not been introduced by predecessors, is called Yanjiao fault depression in this paper. The maximum quaternary thickness of the graben is 300m. Xiadian west fault is the main controlling fault in the southern margin of the sag. The Xiadian fault, which is opposite to the Xiadian west fault, controls the Dachang depression, which is a large-scale depression with a Quaternary thickness of more than 600 m. The Xiadian west fault is opposite to the Xiadian fault, and there is a Horst between the Xiadian west fault and the Xiadian fault. The width of the Horst varies greatly, and the narrowest part is less than 1km. Xiadian west fault may form an echelon structure with Xiadian fault in plane, and they are closely related in depth. According to the core histogram and logging curves of ten boreholes and eight effective dating data, the buried depth of the upper breakpoint of the concealed fault is about 12m, which dislocates the late Pleistocene strata. The effective dating result of this set of strata is 36.52 ± 5.39ka. There is no evidence of Holocene activity of the fault, but it is certain that the fault is an active fault in the late Pleistocene in Sanhe City. The vertical slip rate is about 0.075mm/a since late Pleistocene, and about 0.03mm/a since the late period of late Pleistocene. These slip rates are less than those of the Xiadian fault in the same period. According to our study, the vertical slip rate of Xiadian fault since late Pleistocene is about 0.25 mm /a. Although the latest active age, the total movement amplitude since Quaternary and the sliding rate since late Pleistocene of Xiadian west fault are less than those of Xiadian fault. However, its movement characteristics are very similar to Xiadian fault, which is close to Xiadian fault in space, closely related in deep structure. It can be inferred that the fault is probably a part of the seismogenic structure of the 1679 Sanhe-Pinggu M8 earthquake. In a broad sense, the Xiadian fault zone is likely to extend to the southwest along the Xiadian west fault.

Key words: Beijing plain， Daxing fault， Late Quaternary activity， Row drilling exploration， Shallow seismic exploration