Adjoint-State Surface Wave Tomography for Azimuthally Anisotropic Media: Eikonal Equation-Based Methods and Incorporation of Surface Topography

Surface waves contain critical information on seismic azimuthal anisotropy, which is directly related to underground geological dynamics. However, despite seismic azimuthal anisotropy, the topographic variation may also contribute to the azimuthal dependence of surface wave speed. To our knowledge, most surface wave traveltime tomography methods ignore the topographic variation in forward modeling. Furthermore, the theoretical propagation path is also calculated in isotropic media with flat surface. Undoubtably, inaccurate forward simulations could introduce artefacts to imaging results.  

To address these problems, we develop a novel surface wave tomography method which tracks the surface wave propagation path in anisotropic media and incorporates the topographic variation. An elliptically anisotropic eikonal equation is used to describe the traveltime field of surface wave propagation, and sensitivity kernels with respect to shear wave velocity and azimuthal anisotropy are derived using the adjoint-state method. This new tomography method is tested and verified in Po Basin and adjacent regions, including the central Alps and northern Apennines.