High-resolution imaging of the deep structure of Italy through SKS anisotropy tomography

Abstract

Seismic anisotropy is a fundamental key to gain knowledge of the mantle dynamics and structure. The image of seismic anisotropy over the upper mantle can be obtained with several methods, including surface waves and SKS splitting measurements. Taken together, these anisotropic measurements contribute to extensively catch anisotropy at different depths, yielding insights into the structure and dynamics of the crust and upper mantle. Nevertheless, mantle images resulting from surface waves result in poor lateral resolution, while the nearly vertically propagating SKS waves, when interpreted in a ray-based framework, results in little or no depth resolution, not allowing to easily image the distribution of the anisotropy through depth. Though the anisotropic seismic nature of the upper mantle is well established by a wealth of observational research, most of common teleseismic body-wave tomography studies neglect P- and S-wave anisotropy, thus producing artefacts in tomographic models in terms of amplitude and localization of heterogeneities. To overcome this problem different tomographic methods have been implemented to invert SKS splitting observations for anisotropic structures, most of which based on finite-frequency sensitivity kernels that relate elastic model perturbations to splitting observations. In this study we adopted the tomographic method relying on the inversion of the splitting intensity, a measure of the amount of energy on the transverse component of the waveform. Since is linearly related to the elastic perturbations of the medium through the 3-D sensitivity kernels, SI can therefore be easily inverted, providing the basis for a better interpretation of shear wave splitting measurements. In this study, we first compute the splitting intensity (SI) and splitting parameters using teleseismic shear-wave recorded at 824 available permanent and temporary stations in Italy and surrounding regions. Then, the dataset of SI has been used as an input for the tomographic inversion. The results obtained show changes of the anisotropic properties with depth, especially for the strength of anisotropy. A progressive depth-increase in anisotropy intensity has been recovered over Italy, affecting the bulge of the Alps and Apennines chain and the southern Tyrrhenian subduction system. On the contrary, weaker anisotropy characterizes the transition zone from the Apenninic to Alps domain beneath the Po plain and the Adriatic domain. The anisotropic tomography models obtained in this study allowed us to recover for the first time a new 3D-imaging of seismic anisotropy of Italy down to the deeper layers, allowing to better understand the dynamic of asthenospheric mantle flow and its relation with subducting plate, as well as the rheology of the continental lithosphere.