The fine crustal structure and tectonics of the Beijing region are explored by using a 100 km long, NW-trending deep seismic reflection profile. This profile passed through the Sanhe-Pinggu earthquake (M 8.0) area and main faults in the Beijing region. The results show that the crust beneath the investigated area is divided into upper and lower crust by a strong reflective zone at about 6∼7 s TWT. The thickness of the upper and lower crust is about 18∼21 km and 13∼15 km, respectively. There are rich reflective layers and clear structural patterns above 3∼4 s TWT as well as obviously different structural features along the profile. In the west of the Sanhe-Pinggu earthquake area, the stacked deep seismic reflection section shows 2∼3 groups of strong reflective layers and a series of basement faults. In the east of the Sanhe-Pinggu earthquake area, there is a set of dense, west-dipping, reflective strata with relatively strong energy, which have the typical characteristics of a sedimentary basin. The largest depth of the sedimentary basin is about 8∼9 km. The deep fault in crust revealed by the deep seismic reflection profile has a steep plane, and it cuts and disturbs the lower crust and crust-mantle transitional zone. This deep fault extends upwards into the upper crust, and joins the crustal deep structure to the shallow fault. The profile reveals that the deep-shallow fault system represents the major deep-shallow tectonic feature in the study region.

Geological and geophysical studies suggest that there are many buried active faults and buried Quaternary basins below the Beijing plain. In order to investigate shallow crustal structures, geometry of buried faults, fault activity and the relationship between shallow and deep structures in this area, a mid‐deep seismic reflection profile with 60‐fold and several shallow seismic reflection profiles across buried faults were completed in the northwestern region of Beijing plain in 2006. The results show that the depth of crystalline basement is about 3~6 km along the profile. The seismic reflection section shows a set of strong reflected phases with good lateral continuity above the crystalline basement, which should be the sedimentary rock systems of Cenozoic, Mesozoic, and Paleozoic era. Beneath the crystalline basement, there is a series of weak reflected events with poor continuity possibly representing the crystallized rock system with comparatively strong metamorphism or other non‐layered rock systems. The faults revealed by mid‐deep and shallow seismic reflection profiles present consistent upper‐lower relationship and have been significantly active since Quaternary, which play an important role in controlling the rift valley‐horst like basin and range tectonics as well as the thicknesses of Cenozoic strata in this region. The study results can further improve the understanding of the neotectonic activity in the Beijing area, and the used exploration method and technology can serve as a reference for the survey of deep‐shallow structures in other areas.

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