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

Abstract

<p><span>We have successfully derived a new </span><span>3-D</span><span> high resolution shear wave velocity model of the crust and uppermost mantle of a large part of W-Europe from transdimensional</span><span><strong> </strong></span><span>ambient-noise tomography. This model is intended to contribute to the development of the first </span><span>3-D</span><span> crustal-scale integrated geophysical-geological model of the W-Alps to deepen understanding of orogenesis and its relationship to mantle dynamics. </span></p><p><span>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.</span></p><p><span>We firstly performed a </span><span>2-D</span><span> 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</span><span> approach proposed by Lu et al (2018) by including group velocity uncertainties. Posterior probability distributions on </span><span>Vs</span><span> and interfaces were estimated by exploring a set of 130 millions synthetic </span><span>4-</span><span>layer </span><span>1-D Vs</span><span> models that allow for </span><span>low-velocity zones</span><span><em>.</em></span><span> The obtained probabilistic model was refined using a linearized inversion</span><span><em>. </em></span><span>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 </span><span>&</span><span> Webb, 2000) and adapted the inversion for Vs to include the water column.</span></p><p>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<em> </em>structure (50 - 80 km) in the prolongation<em> </em>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.</p>