The Alpine chain has historically been one of the main natural laboratories to study orogenesis due to its outstanding outcrop conditions. Despite countless investigations, the deep structures imaged by geophysical methods and related tectonic processes remain controversial, due to the high complexity of the chain but also from the lack of high-resolution structural information at depth. To fill this gap, the European AlpArray consortium deployed 628 broadband stations, spaced less than 52 km apart, across the whole chain, and 30 ocean-bottom seismometers in the Ligurian basin, hence providing a unique opportunity for a step change in the 3D imaging of the Alpine lithosphere and asthenosphere. The LisAlps project proposes to apply Full Waveform Inversion (FWI) on the teleseismic data recorded during AlpArray to build:
(1) a new reference high-resolution multi-parametric (1500x700 km) model of the alpine lithosphere and asthenosphere down to 700km depth from the entire network and a catalogue of ~300 teleseismic earthquakes (periods: 5s-20s);
(2) a high-resolution model of the lithosphere in the western Alps around the structurally-complex Ligurian knot.
Through this imaging, we want to: [a] decipher the debated geometry and petrology of the continental subduction between the European and Adriatic plates, [b] sharpen the location of the seismicity in the western Alps, in particular the deep events beneath the Po plain, [c] extend at greater depths with an increased resolution the imaging of the continental subduction to test the lateral and downdip continuity of the slabs between the western and central Alps and refine the slab structure in the transition zone between the Alps and the Apennines, [d] image the interaction between opposite verging Alpine and Apennine slabs and assess the possible role of such interaction in the compression within Northern Apennines, [e] understand the compressional reactivation and ensuing Oligo-Miocene inversion of the northern Ligurian margin, [f] Solve the origin of the high-rate microseismicity and occasional strong earthquakes (e.g., 1887 Mw6.9 event) over the North Ligurian domain for geohazard assessment in the coastal areas of the French Riviera and Liguria. Answering these questions raise at least three methodological challenges. Unlike other methods used in seismology, FWI exploits both the amplitude and phase of all the arrivals recorded in a continuous time window. This ability provides a wavelength-scale resolution and the sensitivity to several classes of physical properties such as P and S wavespeeds, density, attenuation and anisotropy. However, the coarse sampling of the sources and stations in earthquake seismology can generate aliasing artefacts. To mitigate them, we propose to assess compress sensing via sparsity-promoting regularization. A second challenge is the estimation of “second-order” parameters as anisotropy and attenuation, which are two important proxy for several Earth's properties such as temperature, composition (serpentinization, melt), state of stress and asthenospheric mantle flows. A third challenge is the non linearity of the inversion induced by fitting oscillating signals. To tackle this issue, we propose to assess a new “upside down” FWI paradigm, which reconstructs “data-assimilated” wavefields that fit the observations through a relaxation of the wave equation before estimating the Earth's parameters from these wavefields by minimizing the wave equation errors (a linear problem). The better data-assimilated wavefields mimic the true unknown wavefields, the more the parameter estimation is fast and robust. We believe that the full down-top/top-down illumination of the lithosphere provided by the specific teleseismic setup should foster accurate data-assimilated wavefield reconstruction. The proof of concept of the approaches we propose has shown promising results. We propose to assess them against the real case studies scheduled during LisAlps.
Web site : https://www.geoazur.fr/LISALPS
Fundings: ANR AAPG 2019, 360k€
Dates: Janvier 2021-Décembre 2024