Teleseismic Full‐Waveform Tomography Based on a 3D FK‐LTSOS Hybrid Method for Local Imaging

Teleseismic waveforms contain abundant interpretable information about Earth's properties. They can be used to explore the refined structure of Earth's interior, especially in the regions with imbalanced spatial distribution of seismic activity. However, it's technically infeasible to numerically simulate high‐frequency (>1 Hz) teleseismic wave propagation within a whole domain iteratively in full‐waveform inversion due to its vast computational costs. We develop a 3D FK‐LTSOS (Frequency‐Wavenumber, Layered Time‐Space Optimized Symplectic) hybrid method and then apply it to teleseismic full‐waveform tomography to tackle this computational challenge. The 3D FK‐LTSOS hybrid method combines the semi‐analytical solution computed by the FK method rapidly in a 1D background model and the numerical solution calculated by the 3D LTSOS method accurately in 3D localized heterogeneous media with topography to simulate teleseismic wave propagation efficiently and accurately. The comparison of synthetic seismograms shows its accuracy and stability when simulating wave propagation in the topographic model. Based on this hybrid method, the teleseismic full‐waveform tomographic method is developed to efficiently resolve the detailed structure of the local research domain utilizing high‐frequency teleseismic data. The essential contents of full‐waveform tomography are presented, including misfit function, Fréchet kernels, smoothing strategy, and nonlinear conjugate gradient method. Synthetic data application for spherical anomaly models with planer surface and Gaussian topography, and observed data application for the crust‐upper mantle structure beneath eastern Tibet confirm the validity of the teleseismic full‐waveform tomography and demonstrate that our tomographic method can image the localized structures speedily from full‐waveform information. Plain Language Summary Teleseismic full‐waveform tomography is implemented by iteratively solving seismic wave equations, which can be computationally prohibitive in high‐frequency and large‐scale cases. The hybrid method significantly reduces the computational cost of teleseismic wavefield simulations by combining two or more methods. The 3D FK‐LTSOS hybrid method combines the semi‐analytical solution in 1D background models computed by the frequency‐wavenumber (FK) method and the numerical solution in 3D localized media calculated by the layered time‐space optimized symplectic (LTSOS) method. Based on this hybrid method, we f