Full Waveform Seismic Tomography 101

Prof. Li ZHAO

Peking University

20180323-15:00:00

Room 2829, No.2 Science Building

Started in the final days of the last century as the so-called finite-frequency tomography, full waveform seismic tomography (FWST) is now one of the frontiers in seismological research. As a brand new approach to imaging the Earth structure, FWST has undergone a steady development in the past twenty years, and has been practiced in more and more investigations of the upper-mantle structures in regions such as southern Africa, East Asia, and Europe. Generally speaking, seismic tomography involves three crucial elements: measurement of data or modeling residuals, computation of structural sensitivity kernels, and solution of the inverse problem. For FWST in particular, accurate waveform modeling, such as the normal-mode method for one-dimensional Earth models and finite-difference method (FDM) or spectral element method (SEM) for two- or three-dimensional Earth models, is required to obtain full-wave data measurements and structural sensitivity kernels. In order to use the large amount of data to constrain regional and global structure, additional steps must be taken to improve the computational efficiency. We have developed the strain Green tensor (SGT) database approach in which the strain field from fundamental seismic sources are calculated in the reference Earth model beforehand and stored on disks. The SGT database can then be used in efficient calculations of all synthetic seismograms as well as structural sensitivity kernels of any data. Full waveform approach enables us to use any seismic waves including direct P and S waves, their surface reflections PP, SS, PPP, etc., surface waves, the 410-km and 660-km discontinuity reflections P410P, P660P, the CMB-reflected PcP, ScS, PcPPcP, etc., the CMB-diffracted Pdiff and Sdiff waves, and the core phases such as SKS and all the PKP branches. On the other hand, all these seismic phases can be used to constrain the Earth structures, including the isotropic, anisotropic, elastic, and anelastic velocity perturbations. In this talk, I will review the theoretical development of full waveform seismic tomography with an introduction of its basic formulation and exhibition of numerical examples.