3D high-resolution imaging of lithospheric VP, VS, and density structure in the Alps using full-waveform inversion of the teleseismic P waves

The convergence between the African and European plates has created the magnificent Alpine chain with a very complex geological structure. This natural laboratory helps researchers to decipher the geotectonic processes imposed on the region. One useful way to understand better the prevailing geodynamics system is to interpret high-resolution crustal and upper-mantle models developed by full wavefield tomographic methods simultaneously. The high density of broadband stations deployed during the AlpArray project allows us to apply Full Waveform Inversion (FWI) on the teleseismic earthquakes recorded in the Alpine region. FWI minimizes the misfit between the entire recorded and simulated seismograms to reconstruct multiparameter models of the Earth’s interior with a resolution close to the wavelength. We used 203 teleseismic earthquakes with 6.8≤MW≤7.4 and 8≤depth≤630 km recorded by 1232 stations including permanent seismological broadband stations and AlpArray temporary seismic network. To model the propagation of the teleseismic wavefields through the target area, we used a hybrid technique that couples a global wavefield computed by AxiSEM in axisymmetric Earth from the source to the boundaries of the study area to regional wavefield propagating through the lithospheric domain computed by SPECFEM3DCartesian. This target-oriented wavefield injection method mitigates the computational cost of the wavefield simulation at the global scale, hence making high-frequency wavefield simulations in the lithospheric target possible (up to the 1Hz period). We use the AK135 velocity model as the initial model and iteratively inverted the band-pass filtered data at 10-30 s periods using the limited-memory BFGS optimization algorithm to obtain a 3D high-resolution elastic V_P, V_S, and density model for the crust and upper mantle of the entire Alpine chain. Our results show that the main documented structures of the Alps have been recovered well in the crust and upper mantle and confirm that a reliable geotechnical interpretation in the Alps depends on the consideration of the geodynamical process on Apennine and Dinaric simultaneously.