Finite-frequency Traveltime Tomography By Body Wave And Its Application In The North China

Since tomography is introduced to the area of the internal earth structure study, Seismic tomography, with a noticeable progress, has become one of the most effective methods to explore the deep structure of the earth. Based on different theories seismic tomography can be sorted as ray theory tomography and finite-frequency tomography (FFT). The purpose of this thesis is to calculate the finite-frequency sensitive kernel, write the FFT codes and explore the upper mantle velocity structure of North China Craton (NCC) by FFT.The major studies and conclusions are as follow:We improved Fast Marching Method (FMM) ray tracing method. FMM can get all the traveltime in the study media space, and the ray path is the steepest path. We improved the FMM method in algorithm to make it more fast and efficient.FFT is a newly developing method in the age of broadband digital seismic observation. This method overcomes the abuses of the infinite frequency approximation, considers the actual cases such as wave frequency attenuation, wave scattering and wave front healing. We used the improved FMM method to construct a traveltime table, and based on this table we got a 1-D velocity model sensitive kernel by table look-up method. The results show that, table look-up method can enhance the computation speed, which give powerful support to FFT.In this thesis we depicted two methods to pick arrive time, multi-channel cross correlation (VanDecar & Crosson,1990) and adaptive stacking (Rawlinson & Kennett, 2004). The author simplified the operational procedures of picking arrive time by graphical interface. Compare the two results from these two method, we rejected those among which the difference is much big, which increased the credibility of relative traveltime residual.The following part of the thesis is an application on real seismic data. There were 190 broadband stations from North China Seismic Array, and all of the teleseismic occurred during Oct.2006 and Mar.2009, with the epicenter 30°?90°. We chose the relative traveltime residual from these data to invert the P- and S-wave velocity structure, the results reveal that: (1) An obvious low-velocity anomaly, extends to?400km, is located at the west region of study area. This low-velocity anomaly reflects the deep thermal material. Because this low-velocity anomaly doesn't penetrate the mantle transition zone, we deduced this low-velocity anomaly is not come from lower mantle. We can't conclude how this low-velocity formed just from only one aspect (velocity anomaly structure). We should consider the other elements such as thermal, density, quality factor Q, components.(2) The results from regional tomography show that the western edge of Pacific slab reaches to 120(?) longitude (Huang & Zhao,2006). In this thesis we also found a high-velocity anomaly in the mantle transition zone. This high-velocity anomaly is the front edge of Pacific slab.(3) The North China basin is a rift style basin. In the basin the velocity anomaly distribution is more complicated. At the depth range of 100km?250km, there is a high-velocity anomaly block. This is the residue existing in the lithosphere delamination.(4) Compare to the lithospheric thickness of old craton, the lithosphere of NCC is thinning. There is alternate distribution between low- and high-velocity anomalies. Beneath the Yanshan Uplift region, there is a high-velocity zone. These show that the North China basin has obviously experienced strong erosion and tectonic alteration, but the destruction region doesn't include the Yanshan Uplift or the destruction of North China basin is more apparent than that of Yanshan Uplift.