Density Effects of Shallow Structures on the Crustal Shear Wavespeeds in Surface Wave Tomography

Abstract:

In surface wave tomography, shear velocity of the crust and uppermost mantle is routinely inversed from the phase/group velocity dispersion curves with density model empirically scaled according to the medium wavespeed. However, for some certain tectonic formations such as sedimentary basins, this scaling could lead to unrealistic density models if the relation is not properly selected. In this study, we take the Sichuan basin adjacent to the Tibetan plateau as an example to compare the different density profiles scaled from four different empirical relations and demonstrate the density effects on Rayleigh phase velocity dispersion curves. Furthermore, we use analytical equations for 1-D layered models as well as adjoint tomography for 3-D models to investigate the tradeoff between the density and shear wavespeed at various depths. In this way, we conclude that the shallow density structure has a significant effect on the phase velocities at short period so that influence the inversion for shear wavespeed from dispersion curves. In particular, a 25% deviation in the initial density model at shallow depth can give rise to an error up to 5% in the resultant shear velocity at middle/lower crustal depths. Therefore, cautions must be taken in choosing a proper density model at shallow depths when implementing the inversion.