Seismic studies of 3-D elastic and anelastic structure of crust and upper mantle in Western China

Western China is a region with significant geological heterogeneity, especially because of the Tibetan Plateau, the largest and highest plateau in the world, which was created by the Cenozoic Indian-Eurasian continental collision. It is seismically very active even in some populous regions. Therefore, it is important to study the crustal and upper mantle structure beneath western China to understand the tectonics of the region and provide constraint for earthquake hazards. There are two important properties of Earth’s media: elastic and anelastic. Elastic properties, mainly seismic velocities, are studied by travel times of seismic waves. Anelastic properties are generally studied by attenuation of seismic waves retrieved from seismic amplitudes. We use different seismic datasets, including surface-wave dispersion from ambient noise and earthquakes, teleseismic receiver functions, and body-wave travel times, as well as joint inversions of these datasets utilizing neighborhood searching algorithm. We propose a generalized H-κ method with harmonic correction on receiver functions to provide a better estimation of the crustal thickness (H) and Vp/Vs ratio (κ). Our improved joint inversion results of S-wave velocities and crustal Vp/Vs ratios reveal mid-crustal low velocity, high Vp/Vs zones (possibly due to the presence of partial melt) in northern and southern Tibet, as well as in SE Tibet (in mid-lower crust), but not in central Tibet, which may suggest different characteristics of Lhasa Block in central Tibet. Uppermost mantle P and S velocities suggest that the subducted Indian mantle lithosphere is torn into at least 4 pieces that subduct at different angles and have different northern limits. This observation, when compared to seismicity and strain rates (from GPS), suggests coupled lithospheric deformation in southern Tibet. Notably, the lateral extent of potential megathrust earthquakes may be limited by the segment boundaries. The anelastic structure of the Earth, in particular seismic attenuation, is more useful in characterizing the temperature and fluid content as well as predicting earthquake ground motion. We have done simulations on the major factors that affect seismic amplitudes, such as scattering and focusing/defocusing. Strong amplification is observed within the major sedimentary basins, which sustains along the paths that pass through the thickest sediments and indicates that the source-to-basin direction of seismic waves affects their amplitudes. Internal scattering can generate strong coda which may interfere with the surface wave and make its amplitude difficult to measure. These factors need to be addressed with caution before the extraction of intrinsic attenuation structure in western China. Besides development of different seismic methods and models, this study makes considerable progress in answering two questions: Where earthquakes (especially large ones) are more likely to occur in western China, and what effects these earthquakes will have. Chapters 1-4 focus on the first issue by providing new seismic observations that can be compared with other geological constraints. This part includes the development of three seismic methods, namely generalized H-κ stacking method with harmonic corrections (Chapter 1), joint inversion of surface-wave dispersions and receiver functions with P-velocity model (Chapter 2), and joint inversion of dispersions, receiver functions, and Pn station delay time (Chapter 3), as well as comparison between seismic and geological observations and their implications (Chapter 4). Chapter 5 focuses on the second issue by numerically simulating the amplitudes of seismic waves, aiming at retrieving the attenuation structure in western China and predicting the amplitudes of future earthquakes