Imaging of crust and mantle structures with teleseismic reflected waves

Abstract

In this thesis, we developed and applied seismic imaging methods based on teleseismic reflected waves to study discontinuities in the crust and mantle. Specifically, we further developed virtual deep seismic sounding (VDSS) to probe the Moho and we used SS precursors to study mantle transition zone discontinuities. To extend the applicability of VDSS, we developed a source deconvolution scheme to remove complex source signatures of shallow earthquakes. Application of VDSS to data from the North China craton reveals significant lateral variations in crustal thickness. Assuming that the crust was originally uniform, the thick crust beneath the Ordos plateau supports the hypothesis of lower crustal foundering as the mechanism of reactivation of the eastern North China craton. Application of VDSS to the entire western United States suggests significant, but highly variable, mantle contribution to surface topography. Although thermal anomalies are the main contribution to high elevation of the western US cordillera, our results suggest that petrological heterogeneities or dynamic forces must also play a role in mantle buoyancy. To improve mantle transition zone imaging, we developed an array processing technique to remove random and signal-generated noise that contaminating SS precursors. Application of our scheme to data that sample the Central Pacific greatly improves both the travel time picks and amplitude measurements of SS precursors. The observed changes in reflectivity over distance yield density contrast across 410-km and 660-km discontinuities that are well below those in the PREM model. The density and Vs contrasts across the 410-km discontinuity are consistent with upper mantle composition containing 45% olivine, which is intermediate between piclogite and pyrolite models of mantle composition. Lateral variations in density and Vs contrasts across the 660-km discontinuity are observed, perhaps indicating compositional heterogeneities at the base of the mantle transition zone.