| Seismic tomography is a technique of imaging three-dimensional seismic velocity structure from seismic observations that provide direct coverage of the region of interest with multiple crossing propagation paths. Being the latest and most powerful tool to explore the earth's interior, the appearance and widely application of the technique of Seismic tomography has already and will have far-reaching and profound impacts on earth sciences.The Capital Circle Area (CCA)is Located between Yanshan Mountain and Taihang-shan Mountain. Despite its important role in China's political, economic and cultural development, due to the complex tectonics interaction, it's also one of the regions most severely affected by kinds of geologic hazard and risk, e.g. doubles of earthquakes with magnitude greater than 7 occurred here in the latest 300 years. In Oct. 2001, the modern Capital Circle Digital Network was set up and operated routinely, and abundant seismic data with high quality have been accumulated gradually. The development of observatory network provide a good way to peer into the structure and dynamic process under the earth, and also a useful tool to detect the seismic activity, especially the moderate and small earthquakes happened in this region.Focused on the the investigation of detailed velocity structure under the CCA, an experiment of six man-made explosions was made in the region of CCA during Apr. 2002. The travel times of Pn, which travels directly in the uppermost layer of mantle are used to invert the lateral variation and anisotropy of Pn velocity of the uppermost mantle; and also the reflect wave PmP collected from the six explosions are investigated to map the 3D velocity variation image at different depths in the upper crust.A general review of the developments and findings of seismic tomography and in particular the critical issues in travel-time tomography was given, and the theory and method of refraction tomography at uppermost mantle was made, with detailed studies of the uniqueness, regularization, preconditioned, and smoothing process, as well as analyses of the construction of the forward model and errors introduced. Different kinds of seismic data, like the original seismic wave records and earthquake phases report were collected and dealed with, and more than 3200 first arrival Pn rays between 1.8-9.0?were inverted to map the velocity lateral variation. After a discussion of the trade-off in structure betweenthe velocity image and the anisotropy image, the major patterns of the velocity anisotropy was found. Synthetic checkerboard tests with different grid sizes were investigated and by using the statistical bootstrap method, parameter errors were estimated as the standard error of the parameters.Pn velocity is one of the fundamental parameters that are used to infer the thermal and tectonic history of the lithosphere. Lateral variations in Pn velocity have been associated with variations in upper mantle temperature, compositional differences, and pressure effects, while generally, temperature differences have the dominant effect. Compared with the thermal structure of the lithosphere of North China and Pn velocity structure in CCA, the relationship between the distribution of heat flow and Pn velocity variation was discussed, leading to a deep understanding to the high heat flow and spatial distribution of heat flow in the region of CCA.An interesting inverse method called " inversion without blocks", which is based on the Bayesian Theory was applied to get the 3D velocity disturbance image in the upper crust just beneath the 90 x 140km2 array and the resolution kernal was also calculated, in which totally 321 PmP travel time data of the six explosions recorded by the regional 3D array were involved. Finally considering the geologic tectonic in this region, the inhomogeneous distribution of P velocity and the active tectonics was discussed and also the link between spatial distribution of historic great earthquakes and many modern events with moderate and small sizes and P...
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