Rayleigh Wave Tomography From Ambient Noise In Central And Eastern Chinese Mainland

Central and eastern Chinese mainland (102°E～122°E,22°N～42°N) is affected by three plates:the subduction of West Pacific plate and Philippine plate beneath the Eurasian plate respectively, as well as the collision of India plate towards Eurasian plate. It is a geologically diverse and tectonically active area and alternated with fold belts and ancient blocks, displaying a strong lateral heterogeneity in crust and upper mantle.568earthquakes with magnitude over6.0have occurred in this area showing strong seismic activity in some part. Therefore, high resolution tomography study in central and eastern China helps to study the lateral heterogeneity of the crust and upper mantle, the possible dynamic mechanism and the relations between seismic activity and tectonic environments.Earthquake tomography is an essential method to study the lateral heterogeneity of crust and upper mantel. But owing to its reliance on earthquake, it is hard to obtain high resolution tomographic results in earthquake sparse area using this method. While, ambient noise tomographic method which is on the cutting edge of seismology tomography field at present is independent to earthquake. This new method can retrieve information between two stations by cross-correlate the recorded waveforms at the station pair. This thesis use the independent advantage of ambient noise tomography to construct high resolution maps of crust and upper mantle structure in central and eastern of Chinese mainland.In this study, we choose494evenly distributed broadband stations from China National Seismic Networks and regional networks and7Iris stations in the surrounding areas. We use21months vertical-component waveforms recorded by these stations to retrieve more than120,000dispersion curves. To ensure the reliability of tomography, we select acceptable measurements based on inter-station distance and signal to noise ratio. After selection, the empirical Green functions and earthquake signals alone same paths are compared. The results display that they coincide with each other in general, showing the reliability of empirical Green functions in this study. We also test the relation between stability of dispersion curves and stacking time length. The results show12months data can make sure that dispersion curves under30s are temporal stable. So we suggest that in order to obtain stable dispersion curves above30s to construct temporal velocity variation study, time-series longer than12moths should be used.Finally, we obtain50,000to110,000dispersion curves at different periods. The dispersion curves which meet selecting standards are used to invert for group velocity maps from8s to40s on0.5°×0.5°grids using the ray theoretic tomographic method. The resolution of the result can reach0.5°at most of the study area. The main conclusions are as follows:(1) Rayleigh wave group velocity maps at short periods (8s-20s) show that major basins (e.g., Sichuan basin, Ordos basin, North China basin, Jianghan basin, North Jiangsu basin) are delineated by low velocities. While orogenic belt and uplift areas (e.g., Yanshan fold belt, Shanxi anticlinorium, Qinling fold belt, South China orogenic system) are manifested by high velocities.(2) Rayleigh wave group velocity maps at long periods (25s-40s) display the variation of crust thickness. The whole study area presents a striking bimodal distribution along the Daxinan mountain-Taihang mountain-Wuyi mountain trend gravity lineation with high velocity in the east and low velocity in the west. Above30s, group velocity decrease further from west side of the Liupan mountain-Longmen mountain-Daliang mountain crust thickness mutation belt.(3) North China basin shows remarkable low velocity at group velocity maps below25s. The shape of North China plain fault depression was delineated well by low velocity which coincides with the block distribution of North China plain-Bohaiwan basin since Paleogene. In addition Rayleigh wave across North China basin reduces by30%at8s, shows the considerable influence of Meso-Cenozoic sedimentary in the basin.(4) A relative high velocity is observed in the central part of low velocity Sichuan basin above12s. We consider this phenomenon may reveal an uplifted basement beneath the basin. The average velocity in Sichuan basin is higher than in North China plain at short periods below20s. This may result from the rare sedimentary since Mesozoic and the weathering denudation process since Late Eocene at most part of Sichuan basin.(5) Low velocity marks the shape of Ordos basin at periods8s-10s. While group velocity in southeast part becomes higher than in northwest part at periods12s-20s, reveals the crustal heterogeneity to some extent. The shape of Ordos basin disappears completely at longer period (30s-40s).(6) Qinling orogenic belt, Taihang Mountain and Eqian fold belt present prominent high velocity at short periods below20s. While the high velocity zones disappear at periods above20s and group velocity there does not show differences from the surrounding structures.