We measure ellipticity of Rayleigh waves from polarization analysis (DOP-E) of oceanic microseismic noise at 23 broad-band seismic stations around the globe. We observe an unexpected, clear seasonal oscillation of Rayleigh ellipticity in specific geographical areas such as Northern Italy, Eastern Asia and Antarctica. We determine that these oscillations are not due to local changes of mechanical parameters in the crust beneath the stations, but they are linked to the seasonal variation of the ambient wavefield composition. In particular, we show that the superposition of overtones of Rayleigh waves impacts on ellipticity measurements, and infer that their fraction in the wavefield strongly increases during summers. This effect is not uniform around the globe, but it is stronger in specific regions relatively far from strong oceanic sources, hinting at generation of overtones during wave propagation. The geological structure beneath the stations can also play an important role, being the amplitude of overtones larger in sedimentary regions. This study highlights the importance of better understanding the composition of the oceanic microseismic field, and its variations during the year, in order to improve the ability of noise-based techniques to explore the Earth interior and its time-dependent variations.

We present the first 3D anisotropic teleseismic P-wave tomography model of the upper mantle covering the entire Central Mediterranean. Compared to isotropic tomography, we find that including the magnitude, azimuth, and, importantly, dip of seismic anisotropy in our inversions simplifies isotropic heterogeneity by reducing the magnitude of slow anomalies while yielding anisotropy patterns that are consistent with regional tectonics. The isotropic component of our preferred tomography model is dominated by numerous fast anomalies associated with retreating, stagnant, and detached slab segments. We also observe relatively slower mantle structure related to slab windows and the opening of back-arc basins. To better understand the complexities in slab geometry and their relationship to surface geological phenomenon, we present a 3D reconstruction of the main Central Mediterranean slabs down to 700 km based on our anisotropic model. P-wave seismic anisotropy is widespread in the Central Mediterranean upper mantle and is strongest at 200-300 km depth. We interpret the anisotropy patterns as the result of asthenospheric material flowing primarily horizontally around the main slabs in response to pressure exerted by their mid-to-late Cenezoic horizontal motion. We also image sub-vertical anisotropy possibly reflecting asthenospheric entrainment by descending lithosphere. Our results highlight the importance of anisotropic P-wave imaging for better constraining regional upper mantle geodynamics.