Insights in P- and S-wave relative traveltime tomography from analysing finite-frequency Frechet kernels

Regional body-wave tomography, also called ACH tomography, is the inversion of relative traveltime residuals of teleseismic body waves measured at regional networks. We analyse the characteristics of the finite-frequency Fr,chet kernels for P and S waves for this kind of tomography. Using a simplified geometry enables us to use the complete Green's function in the expression of the Fr,chet kernels and analyse elements, which are usually neglected, like the importance of the near-field terms and the P-wave traveltime sensitivity to shear wave velocity variations. By comparing the kernels of the relative residuals and absolute ones, we show that relative residuals have a reduced sensitivity to heterogeneities of large dimensions, and that this reduction is a generalization of the fact that the average model is not recovered in ACH tomography. This sensitivity reduction affects equally short- and long-period residuals. We show in addition the presence of a sensitivity reduction at large depth for the long-period waves. Kernels and reflectivity impulse responses of the crust are used to analyse if crustal corrections should be made frequency-dependent in finite-frequency regional tomography. We find that in most cases the frequency dependence due to reverberations is substantial, and that in many realistic network configurations ray theory is unlikely to be well appropriate to compute crustal corrections for the long-period waves. We also find that the lateral dimensions of the crust affecting the traveltimes is frequency dependent and reaches, at long periods, 50 km for sedimentary basins and 100 km for Moho depth.