Crustal And Upper-most Mantle Structures From Seismic Receiver Functions And Tomography Of Ambient Noise Cross-coruelations

Two common seismic methods, receiver functions and ambient noise tomography, are used to study the crustal and upper-most mantle structures of the earth. In receiver functions research, we use15seismic stations, crossing Qinling orogen(QO), Weihe graben(WG) and Ordos block(OB), to study the crustal structures by receiver functions(RFs) methods. The results show quite difference in crustal structures and materials of three tectonic units(orogenic belt, extentional basin and stable craton). The average crustal thickness in northern QO is37.8km, and Poisson ratio is0.247, which indicates the increase of felsic materials in QO. In southern OB, the average crustal thickness and Poisson ratio are39.2km and0.265. Comparatively high value of Poisson ratio is related with old crystallized base in lower crust and shallow sediments. The artificial RFs reveal that low-velocity and thick sediment have significant effect on phases of Mohorovicic discontinuity(Moho). As a result of this, the Moho phases in WG are tangled. S-wave velocity(Vs)nversion shows that there are shallow sediment layer with4-8km’s thickness and high velocity zone in middle-lower crust in WG. Complex Moho structure and high velocity zone may be induced by the activities of Weihe faults series.For ambient noise tomography,298stations records in South China are used to retrieve Rayleigh wave propagating between possible station pairs by applying cross-correlations. Waveform and dispersion curve comparisons with earthquake-emitted Rayleigh wave indicate that the ambient noise method is efficient and reliable. Both group and phase velocity images of0.5°×0.5°grid from8to40s are estimated. Then, shear velocities are inversed in each grid. The Rayleigh wave velocity maps in short periods clearly delineate basins and mountains in South China. In long periods, the velocity variances are prominent with high velocity in the east and low velocity in the west, which is related with different Moho depth between east and west. A flat Moho with about30km depth is found in the east of South China, and it turns deeper in the west. Jiangshao suture, the division of Cathaysian block and Yangtze block, is only visible near the surface in lower Yangtze block and emerges as a velocity boundary in upper Yangtze block near Moho. The combination of Cathaysian block and Yangtze block firstly occoured in lower Yangtze block in Neo-Proterozoic. Then CTB thrust over upper Yangtze block in early-Paleozoic. Besides, low velocity zones caused by "Hainan Island mantle plume", partial melting of Paleo-Pacific plate subduction and upwelling of asthenosphere under Tanlu fault are also clearly displayed in shear velocity profiles.