Direct surface wave tomography for three dimensional structure based on surface wave traveltimes: Methodology review and applications

Surface wave tomography using dispersion data to obtain isotropic and anisotropic shear wave velocity structures is a very efficient approach to study regional and global tectonics and deformation and to probe high-resolution crustal, upper mantle and near surface structures. Conventional surface wave tomography based on dispersion data usually has two steps, that is, inverting for 2D phase/group velocity maps first and then conducting point-wise inversion to obtain a 1D shear wave velocity model at each geographical grid point, which are then combined to obtain a 3D shear wave velocity model. In this paper, we review in detail the direct surface wave tomography framework based on surface wave dispersion traveltimes, that is, the one-step surface wave tomography. This framework includes direct inversion of 3D isotropic shear wave velocity model using all dispersion measurements at different periods and from all paths (DSurfTomo), direct inversion of both isotropic and azimuthally anisotropic shear wave velocity model (DAzimSurfTomo), and direct inversion of radially anisotropic shear wave velocity model (DRadiSurfTomo).The new direct tomography method computes surface wave ray paths at different periods, thus better considering the ray path bending effect of surface waves in complex media on the precision of tomographic images. We then introduce some applications of the new direct tomography methods, including multi-scale isotropic and anisotropic shear wave velocity tomography in the crust and upper mantle as well as shallow crust. These tomographic studies provide important constraints on regional tectonic evolution, seismogenic structures, shallow fault zone structures, ore deposit structures, and urban subsurface structures. At last, we discuss the dispersion data and model parameterization problems in surface wave tomography, direct surface wave tomography based on finite frequency theory and full waveform inversion, and perspective research of joint tomography problems using surface wave dispersion data and other seismological or geophysical data in the framework of direct surface wave tomography.