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

<p><span dir="ltr" role="presentation">The convergence between the African and European plates has created the magnificent Alpine chain with a </span><span dir="ltr" role="presentation">very complex geological structure. This natural laboratory helps researchers to decipher the geotectonic processes</span> <span dir="ltr" role="presentation">imposed on the region.</span> <span dir="ltr" role="presentation">One useful way to understand better the prevailing geodynamics system is to interpret</span> <span dir="ltr" role="presentation">high-resolution crustal and upper-mantle models developed by full wavefield tomographic methods simultaneously. </span><span dir="ltr" role="presentation">The high density of broadband stations deployed during the AlpArray project allows us to apply Full Waveform </span><span dir="ltr" role="presentation">Inversion (FWI) on the teleseismic earthquakes recorded in the Alpine region. FWI minimizes the misfit between</span> <span dir="ltr" role="presentation">the entire recorded and simulated seismograms to reconstruct multiparameter models of the Earth&#8217;s interior with a </span><span dir="ltr" role="presentation">resolution close to the wavelength. We used 203 teleseismic earthquakes with 6.8</span><span dir="ltr" role="presentation">&#8804;</span><span dir="ltr" role="presentation">M</span><span dir="ltr" role="presentation">W</span><span dir="ltr" role="presentation">&#8804;</span><span dir="ltr" role="presentation">7.4 and 8</span><span dir="ltr" role="presentation">&#8804;</span><span dir="ltr" role="presentation">depth</span><span dir="ltr" role="presentation">&#8804;</span><span dir="ltr" role="presentation">630 km recorded by 1232 stations including permanent seismological broadband stations and AlpArray temporary</span> <span dir="ltr" role="presentation">seismic network. To model the propagation of the teleseismic wavefields through the target area, we used a hybrid</span> <span dir="ltr" role="presentation">technique that couples a global wavefield computed by AxiSEM in axisymmetric Earth from the source</span> <span dir="ltr" role="presentation">to the boundaries of the study area to regional wavefield propagating through the lithospheric domain computed </span><span dir="ltr" role="presentation">by SPECFEM3DCartesian.</span> <span dir="ltr" role="presentation">This target-oriented wavefield injection method mitigates the computational cost of </span><span dir="ltr" role="presentation">the wavefield simulation at the global scale, hence making high-frequency wavefield simulations in the lithospheric</span> <span dir="ltr" role="presentation">target possible (up to the 1Hz period).</span> <span dir="ltr" role="presentation">We use the AK135 velocity model as the initial model and iteratively</span> <span dir="ltr" role="presentation">inverted the band-pass filtered data at 10-30 s periods using the limited-memory BFGS optimization algorithm to </span><span dir="ltr" role="presentation">obtain a 3D high-resolution elastic</span> <span dir="ltr" role="presentation">V_P</span><span dir="ltr" role="presentation">,</span> <span dir="ltr" role="presentation">V_S</span><span dir="ltr" role="presentation">, and density model for the crust and upper mantle of the entire Alpine</span> <span dir="ltr" role="presentation">chain.</span> <span dir="ltr" role="presentation">Our results show that the main documented structures of the Alps have been recovered well in the crust</span> <span dir="ltr" role="presentation">and upper mantle and confirm that a reliable geotechnical interpretation in the Alps depends on the consideration</span> <span dir="ltr" role="presentation">of the geodynamical process on Apennine and Dinaric simultaneously.</span></p>