Shear Wave Velocity Structures In Crust And Mantle Of North China And Study On The Relationship With Geological Evolution Since Late Cenozoic

The North China block has experienced long and complicated geological histories, and formed different tectonic units such as relatively stable block, fault basins and orogenic (tectonic) belts. Many faults crossed each other vertically and horizontally, with different active state. While, strong earthquakes occurred frequently in the region, so it is of theoretical and practical significance to study whatever the geological tectonic evolution itself or the preparation background of strong earthquakes. In this Ph.D thesis, using methods of pure S wave waveform inversion and T function, and analogue annealing method to search the best fitted model, fine velocity structures were obtained in the crust and mantle in a depth of 100km beneath 140 stations and mantle velocity structures in 800km deep beneath 50 stations; as well, 2-dimension shear wave velocity profile were also constructed. Moreover, P wave velocity structures were computed by independent P wave waveform inversion, to constrain the physical properties of sub-surface media in view of a few parameters. The general characteristics of shallow velocity structures show that, there exists regional similarity for different tectonic units in North China, but present great differences from each other. They are as follows: (1) The Ordos block is the most steady one, with thick and stable layers showed in the mid and lower crust, and its average velocity ratio is less than 1.732. At its surrounding fault basin belts, the velocity structures are complicated. The stronger the tectonic activity is, the more complex the velocity structure is. The features of velocity structures reflect that, the fault basins were developed from west to east, as well from the mountains on the north and south sides to Ordos block. (2) The crustal thickness differs largely in 10km on the two sides of the Taihang Mountains belt, which is consistent to the distribution of regional gravitational field. The velocity structures are also complicated in fault basins on the east side of the Taihang Mountains belt, and only a few stations in Cangxian upwelling area maintain features of the ancient stable block.