A Seismic Tomography Study On Crust And Upper-mantle Structure Beneath The North-South Zone In China

There is an important zone of seismicity in the middle of China, also it is a transitional zone of tectonic boundaries between Tibet and eastern China. This region is stretched from the Ningxia province in the north to the Yunnan province in the south, almost traverses all over the China. The earthquake activity in this region has a high frequency high intensity and short periodic features for the influence of collision between India and Eurasia plates. To study the velocity structure of this region is a necessary work for understanding the focal mechanism, seismogenic environment and geodynamic background. In the recent years .state and local government invested lots of fund in the construction of seismic stations for the seismic monitoring and prediction, and so there accumulated abundant seismic data recorded by these stations which established foundation to study earthquakes, especially in high resolution seismic tomography.The study area is between 22°N~40°N and 96°E~110°E. Data were collected at 439 seismic stations where a total of about 9000 regional earthquakes occurred between 1983 and 2006 and 125 teleseismic events occurred between 2005 and 2006, and just the P phase is used in tomography. The inversion method used in tomography is suggested by professor Zhao DP. Finally, the resolution of the results achieved 1° x l°. There are 11 crosscut slices to depicted the velocity images of the North-South seismogenic zone in China.These slices showed that the north-south zone not only comprised many primary geotectonic units which are segmented by lots of giant deep faults in geology, but also the velocity structures beneath it are with significant heterogeneity The deeper with the depth, the smoother in the heterogeneity . These velocity patterns suggest that in the upper crust, the velocity heterogeneity seems more likely to be related to surface structures of the region, in which the compressive uplift mountains and the plateau are characterized by high-velocity anomalies, whereas low velocities...