S-wave Velocity Structure Of Crust And Upper Mantle Beneath North China

North China is one of the regions with high level of strong earthquake activities in China. It is of great significance to study the deep structure beneath North China for understanding the seismogenic environment and activity mechanism. North China craton is considered to be the typical example of the destruction of the ancient craton. Studying the structure of crust and mantle in North China is of great help for better understanding the genetic mechanism for the devastating destruction of the stable continental craton. Seismic tomography is one of the effective methods to detect the Earth's interior structure. With the development of the permanent seismic network and the dense broadband seismic array, seismic tomography has made a grate of important advances in the research of the deep structure in mainland, ocean, subduction zones, mantle plumes, orogenic belts, volcanic activity areas, earthquake faults and deep structure of strong earthquakes and so on.In this paper, we used the high-quality digital teleseismic waveform data recorded by dense temporal broadband seismic array and permanent seismic network, obtained S-wave relative residuals by cross-correlation method, and finally determined a fine S-wave velocity structure in North China by applying local and teleseismic tomography method.The main results are as follows:1,Based on the observe data recorded by dense temporal seismic array, our tomography results show that shallow S-wave velocity structure correlated with the thickness of sedimentary cover very well. The uplift and depression areas are imaged as high and low velocity anomalies, respectively. High velocity anomalies are shown in Yanshan-Taihangshan Mountains, while low velocity anomalies are shown in Yanqing-Huailai basin, Datong Basin. Distinct low S-wave velocity anomalies are shown in Tangshan region and the surrounding areas in the middle crust, which may be caused by fluids, and low velocity anomalies are shown in Shanxi rift basin in the middle and lower crust, which may be caused by hot materials, thus easily to form the seimogenic environment.2,The thickness of lithosphere maybe beyond 120km in Yanshan orogenic belt and about 100km in Taihangshan Mountain. Integrated with the previous studies, we consider that it may be 70-80km in North China basin and between west of Taihangshan Mountain and east of Ordos block.3,Based on the teleseisimc data recorded by dense temporal seismic array and permanent seismic network, our results show that Yanshan-Taihangshan Mountains uplift, Ordos block, southwestern fold belt, Dabie orogenic belt and Liaodong block are imaged as distinct high velocity anomalies, reflecting their stable characteristics. On the other hand, areas around the Ordos block, in the Bohai Bay Basin and along the Tan-Lu faults are imaged as obvious low velocity anomalies. These low velocity anomalies are closely related with the locations of the faults, suggesting that the existence of the upwelling materials from mantle beneath the fault zones, thus caused large-scale tectonic extending activities.4,The low velocity anomalies are probably reached to 400km beneath Shanxi rift basin, and the pattern of the low velocity anomalies expands to the northeast direction from deep to shallow. We infer that due to the plate movement heading to the northeast caused by India-Eurasia collision, the deep low-velocity and high-temperature materials are driven to migrate to northeast with the upwelling process.5,Dabie orogenic belt is imaged as obvious high velocity anomalies in the mantle transition zone, which may be related to the subduction remnants of the ancient Yangtze. Low velocity anomalies are shown in and near the edge of the subducting Pacific plate lingered in the transition zone beneath East Asia. As the mantle transition zone and the top of lower mantle are the main regions for the plate dehydration, we speculate that low velocity anomalies may be caused by the plate dehydration or the erosion by hot lower mantle materials beneath the Pacific plate remnants in the transition zone.