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.
Worldwide, slip on earthquake faults causes numerous disasters, resulting in large losses in human life and built structures. To minimize future losses associated with earthquakes along such faults, it is important to precisely locate the faults relative to the built environment and to determine the subsurface geometry of the faults. In Beijing, China, we used shallow-depth geophysical methods to evaluate the location and subsurface geometry of the Huangzhuang-Gaoliying fault (HGF), one of the principal tectonic faults of Beijing area. We used seismic reflection and refraction tomography, multi-channel analysis of surface waves (MASW), and paleoseismic trenching to characterize the north section of HGF near the Gaoliying section of Beijing. Our seismic images indicated that there are at least two strands of the HGF that are distributed over an approximately 200-m-wide zone. We identified a principal fault strand (F1) that is observed in all the seismic images, as well as in a paleoseismic trench. The F1 strikes approximately N49°E and dips southeastward at 70° to 75°. Over the past few years, surface ruptures have occurred along the HGF in several locations, but it is unclear if the surface ruptures were the result of tectonic slip on the HGF or were related to land subsidence along the fault.
The paper involves the application of the surface geological method for regional site classification in Beijing. Geological maps are collected through GIS to create a relationship of classification between the geological factors of the geological maps and the site classification index of the US NEHRP code. First, the site classification of the US NEHRP is obtained, and based on the relationship of conversion, given by Chinese researchers, between the site classification of the US NEHRP and the China’s Code for Seismic Design of Buildings (GB50011-2010), the results of the site classification of Beijing are obtained based on China’s Code (GB50011-2010). Finally, based on the collected borehole data in Beijing, the results of the site classification map are verified and analyzed.
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