In order to investigate the crustal structure features, the geometry of buried faults, the fault activities and the relationship of deep and shallow tectonics, a deep seismic reflection profile was completed in south region of Beijing plain. The results show that the crust beneath the survey area is divided into the upper and the lower crust by a strong reflection zone at about 6 similar to 7 s TWT. The thickness of the upper and the lower crust is about 18 similar to 19 km and 16 similar to 17 km, respectively. The depth of the Moho interface is about 34 similar to 35 km. There are considerable fluctuation and variation of crystalline basement in the study region. Both the discontinuity of the upper and the lower crust and the Moho discontinuity are transitional zones. There are rich reflection layers, many fault structures in the upper crust, and the structure patterns are clear. In the northwestern region of the Xiadian fault, the deep seismic reflection section shows 4 similar to 5 groups of strong reflection seismic phase, which exhibit the typical characteristics of a bulge. In the southeastern region of the Xiadian fault, there is a set of dense southeastward-dipping reflection strata with relatively strong energy in the upper portion of the section, and there is a set of strong reflection strata which exhibit apparent differences and complicated structures in the lower portion. These reflection strata have the typical characteristics of a sedimentary basin and the maximum depth of the basin is about 11 km. The crustal deep fault revealed by the deep seismic reflection profile has a steep angle. It penetrates upwards through the interface of the upper and the lower crust, and extends upwards into the bottom of sedimentary basin in the upper crust. It penetrates downwards through the crust-mantle transitional zone and constitutes a special combination relationship with the upper faults and the sedimentary basin.
