First step towards an integrated geophysical-geological model of the W-Alps: A new Vs model from transdimensional ambient-noise tomography

We have successfully derived a new 3-D high resolution shear wave velocity model of the crust and uppermost mantle of a large part of W-Europe from transdimensional ambient-noise tomography. This model is intended to contribute to the development of the first 3-D crustal-scale integrated geophysical-geological model of the W-Alps to deepen understanding of orogenesis and its relationship to mantle dynamics.

We used an exceptional dataset of 4 years of vertical-component, daily seismic noise records (2015 - 2019) of more than 950 permanent broadband seismic stations located in and around the Greater Alpine region, complemented by 490 temporary stations from the AlpArray sea-land seismic network and 110 stations from Cifalps dense deployments.

We firstly performed a 2-D data-driven transdimensional travel time inversion for group velocity maps from 4 to 150 s (Bodin & Sambridge, 2009). The data noise level was treated as a parameter of the inversion problem, and determined within a Hierarchical Bayes method. We used Fast Marching Eikonal solver (Rawlinson & Sambridge, 2005) jointly with the reversible jump algorithm to update raypath geometry during inversion. In the inversion of group velocity maps for shear-wave velocity, we set up a new formulation of the approach proposed by Lu et al (2018) by including group velocity uncertainties. Posterior probability distributions on Vs and interfaces were estimated by exploring a set of 130 millions synthetic 4-layer 1-D Vs models that allow for low-velocity zones. The obtained probabilistic model was refined using a linearized inversion. For the ocean-bottom seismometers of the Ligurian-Provencal basin, we applied a specific processing to clean daily noise signals from instrumental and oceanic noises (Crawford & Webb, 2000) and adapted the inversion for Vs to include the water column.

Our Vs model evidences strong variations of the crustal structure along strike, particulary in the subduction complex. The European crust includes lower crustal low-velocity zones and a Moho jump of ~8-12 km beneath the W-boundary of the external crystalline massifs. We observe a deep LVZ structure (50 - 80 km) in the prolongation of the European continental subduction beneath the Ivrea body. The striking fit between the receiver functions ccp migrated section across the Cifalps profile and this new Vs model validate its reliability.