Seismic Tomography Beneath The North China

Seismic tomography has become a powerful tool for studying the three-dimensional crust and mantle structure.It can determine the different scales and the different depths of the three-dimensional velocity image,which provides reliable information for people studying the fine structure of the Earth's interior.North China is our country's economic and political center,and the earthquake activity of this area is one of the strongest area in China.Therefore,in-depth studying the fine structure of this area has great significance.In order to in-depth study the fine structure of the North China,Institute of Geophysics China Earthquake Administration launched a large-scale movement of seismic array observations in North China in 2006.We determined a high-resolution (horizontal resolution is about 36km) 3D P-wave velocity model of the crust and upper mantle down to the depth of 700km under North China by jointly inverting local events and teleseismic events recorded by North China Seismic Array and Capital Seismic Network.We detailed study the lateral heterogeneity of the crust and upper mantle,the deep tectonic environment of strong earthquake's occurrence and the deep structure of important fault in North China.The main results are as follows:(1) Different structure blocks in North China have the existence of significant differences in velocity structure.Under North China basin,a pronounced low-velocity anomaly exists at depth of 60-120km,which may relate with shallow depth of asthenosphere,and a pronounced high-velocity anomaly exists at depth of 150-350km,suggesting that old lithosphere residues still exists in the upper mantle through large-scale lithosphere thinning.A large pronounced low-velocity anomaly exists in the mantle transition zone under Bohai sea basin and locates on the west side of the front subducting Pacific slab,which may result from phase transition and dehydration of the subducting Pacific slab.At the depth of 60-350km,a pronounced low-velocity anomaly exists under Shanxi rift basin,which may result form the upwelling of high temperature and low-velocity mantle material under the studying zone.At the depth of 60-150km,a pronounced high-velocity anomaly exists under Yanshan uplift,which may relate with the thick lithosphere. (2)Through studying the relationship between the 3D P-wave velocity structure and strong earthquake distribution,we found that most strong earthquake(M≧6.0) often occurred between high-velocity and low-velocity anomaly transition zone.Low P-wave velocity anomaly exists under the hypocenter.The occurrence of strong earthquakes has the nearly relationship with the existence of fluid in the crust, heterogeneity in the crust-mantle transition zone and the upwelling mantle material.(3)The Taihangshan front fault and the Zhangjiakou-Penglai fault become flimsy tectonic boundaries.The Taihangshan front fault becomes the boundary between high-velocity and low-velocity anomaly in the crust,but the boundary becomes blurred in the crust-mantle transitional zone.The velocity structure of Zhangjiakou-Penglai fault is very complex.At the depth of 60-90km,the Zhangjiakou-Penglai fault also becomes the boundary between high-velocity and low-velocity anomaly,and this fault locates on the edge of the Yanshan uplift which has thick lithosphere,suggesting that the deep structure of this fault may be controlled by listhosphere thickness of the different block.