Imaging structure and geometry of Alpine slabs by full waveform inversion of teleseismic body waves

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Seismic body waves from distant earthquakes recorded by the extraordinarily dense and large-aperture AlpArray Seismic Network (AASN) are inverted for the 3D distribution of seismic wave speeds in the lithosphere and sublithospheric mantle beneath the greater Alpine region (activity fields A, B, C and D and research themes 1 and 4). The resulting image is expected to resolve the internal structure and geometry of lithospheric slabs beneath the Alps down to the transition zone. It will give new insights into heavily debated issues such as subduction polarity switches, and provide further detail on possible detachments of the Alpine slabs from the lower crust, on their connection to high-velocity material in the transition zone attributed to Late Cretaceous to Early Paleogene subduction of the Alpine Tethys, and finally on the amount of subducted material accumulated in the transition zone (research theme 1). The 3D image will constitute essential input for geodynamic models of lithospheric deformation and mantle flow as well as plate tectonic reconstructions (research themes 2, 3 and 4).Iterative Gauss-Newton full waveform inversion based on waveform sensitivity kernels and adapted to the teleseismic imaging problem is employed to construct the 3D model of P and S wave speeds beneath the Alpine mountain belt. Numerical simulation of teleseismic wave propagation is performed in a hybrid way where either a 3D spectral element or a 3D discontinuous Galerkin method are used to model wave propagation in the target region. For the rest of the Earth, a fast and efficient solver valid for spherically symmetric earth models is employed. Initial wave speed models are obtained by classical tomographic inversion of automatically determined and quality-ranked travel times of P and S phases. The latter also enter the full waveform inversion as additional constraints to stabilize the inversion against cycle-skipping. Effects of heterogeneities outside the target volume are minimized by inverting linear combinations of the data whose sensitivity is focused on the target region. The approach should permit inversions of body wave records up to periods of a few seconds implying a resolution length of about 20 km.