High-resolution surface wave tomography in oceanic and continental regions: Simultaneous inversion for shear wave velocity, azimuthal anisotropy and attenuation.

摘要

The primary goal of this thesis is to understand the structure, dynamics, deformation and evolution of the Earth's lithosphere and mantle in both oceanic and continental settings by surface wave tomography. In chapter 1, we revised 2-D Born-approximation sensitivity kernels of surface waves for the global case to that for a regional case. We found that the kernels can accurately predict the perturbation of the wavefield. Based on the 2-D sensitivity kernels, we developed a surface wave tomography method and tested the inversion method by using synthesized data obtained from numerical simulations. We found the method can almost completely recover the input checkerboard structure when the size of anomalies is larger than one wavelength. In chapter 2, we applied the tomography method developed in chapter 1 to fundamental mode Rayleigh waves in southern California. Two-dimensional phase velocities are used to invert for three-dimensional S-wave velocities of the upper mantle. The pattern of velocity anomalies indicates that there is active small-scale convection in the asthenosphere beneath southern California and that the dominant form of convection is 3-D lithospheric drips and asthenospheric upwellings, rather than 2-D sheets or slabs. Azimuthal anisotropy is obtained in a joint inversion including lateral variations of phase velocities. The strength of anisotropy is ∼1.7% at periods shorter than 67s and decreases to ∼1% at longer periods. In chapter 3, we invert Rayleigh waves recorded at ocean-bottom seismometers (OBSs) in very young (less than 10 Ma) seafloor for shear wave velocity (Vs) and attenuation (Qmu). A high velocity lid with negative gradient in the uppermost mantle overlying a low velocity zone is observed. Our Q mu models exhibit a sharp change over the depth range of 40 to 60 km with higher values above 40 km. The change of Qmu and S-wave velocity with depth is partly due to the sharp change of water content in the upper mantle as a result of the extraction of water by large fractional melting above ∼65 km. ∼1% partial melting in the low velocity zone is required to satisfy the minimum value of shear wave velocity.