Wave Equation-based Seismic Tomography Method And Their Applications

Wave equation-based seismic tomography method has been seen as the new generation imaging tool to investigate interior structures and dynamic evolution of the Earth.In this thesis,the adjoint method,Born approximation and Green function are used to derive a kind of joint inversion method of wave equation-based seismic tomography and earthquake location.Due to two wavefields,namely the regular wavefield and adjoint wavefield,should be solved when computing the sensitivity kernels of seismic tomography,3-D nearly analytic central difference method with properties of high efficiency,low numerical dispersion,low storage cost,and good parallelism is proposed to solve wave equations.The numerical method has fourth-order accuracies in both time and space with only a three-point stencil in each axis direction.Theoretical properties including the stability criterion and the numerical dispersion relation are analyzed in detail.Numerical simulations of wave propagating in different media are also conducted.GPU/CPU heterogeneous computing tests show the numerical method can achieve high speedups comparing with serial computing by only one CPU.Then the 3-D nearly analytic central difference method is used to calculate both the waveform and the traveltime sensitivity kernels for different phases of seismic waves,including direct wave,Moho-reflected wave,multi-reflection wave and the head wave in a crust over mantle model.All the sensitivity kernels,including waveform kernels and traveltime kernels,are mainly distributed within the first Fresnel zone.For later phases,the sensitivity kernels are also including the isochron of scatterers or reflecting points in addition to the first Fresnel zone.It is one of the reasons that wave equation-based seismic tomography can naturally deal with multiple path problems.With respect to real application,crustal structures of Beijing and its surrounding regions are studied with wave equation-based traveltime seismic tomography method.The tomographic results reveal strong heterogeneous of the crust of studying area,which is also showing relationship with the occurrence of earthquakes.Low-velocity anomalies discovered below the hypocenter of Tangshan earthquake may indicate the existence of fluid materials.The occurrence of Tangshan earthquake may be due to the weakening effect on seismogenic layer of fluid materials,and its aftershocks may becaused by the diffusion and movement of these fluids.In seismic tomography study of the 1992 Landers earthquake area in Southern California,Moho-reflected phases and wave equation-based tomography method are used to obtain the whole crustal tomographic images.Checkerboard resolution tests imply the Moho-reflected phases can significantly improve the resolution of tomographic results in the lower crust of studying area.The high resolution inverted results show strong relationship between surface geology and crustal heterogeneities in the upper crust.Earthquakes in this area mainly occurred in high-velocity and low Poisson's ratio region or near the boundaries of high-and low-velocity anomalies.In the lower crust,pronounced low-velocity anomalies are recovered in the southeastern segment of the San Andreas Fault close to the Salton Trough,which may indicate the existence of crustal fluids coming from magma intrusion.The high-velocity anomalies on the north may be the boundaries this fluid-filled area.An anomaly with high Vp and low Vs discovered in the lower crust below the hypocenter of the Big Bear earthquake may be formed by the influence of the upwelling of partial melt asthenospheric material driven by the relict Frallon subduction.