Mapping the 410-km and 660-km discontinuities across the European Alps using noise correlations

The mantle transition zone is delineated by seismic discontinuities at approximately 410-km and 660-km depth. The lateral variations in reflectivity and depth of the two seismic discontinuities reflect changes in mineralogy composition, thermal state, and water content, that is key to understanding the Earth’s dynamics. Traditional imaging methods based on the analysis of earthquake signals, such as seismic tomography and receiver function analysis, are often limited by earthquake occurrence and uncertainties related to the earthquake source parameters. Recent studies demonstrated the feasibility of recovering body waves from noise correlations, providing new prospects for imaging deep Earth [e.g., Poli et al., 2012; Boué et al., 2013]. 

In this study, we map the 410-km and 660-km discontinuities beneath the European Alps using reflected body waves recovered from noise correlations. To that end, we compute noise correlations using four years of continuous recordings from ∼1200 broadband stations in the greater Alpine region. To enhance the signal-to-noise ratio of the body-wave reflection phases, for each station pair, we stack daily noise correlations in selected time spans with a high level of near vertical-incident body waves and less dominant surface waves [Lu et al., 2021]. We further stack noise correlations of station pairs with common/nearby reflection points to obtain local zero-offset reflection waveforms. The retrieved P410P and P660P reflection phases clearly reveal lateral variations of both reflectivity and depth of the two discontinuities in the studied region, providing new constraints in addition to existing results from earthquake tomography and receiver function analysis. Besides, this study also sheds light on the strategies to recover deep reflection phases from noise correlations.

[1] Boué, P., Poli, P., Campillo, M., Pedersen, H., Briand, X., & Roux, P., 2013. Teleseismic correlations of ambient seismic noise for deep global imaging of the Earth, Geophys. J. Int., 194(2), 844-848.

[2] Lu, Y., Pedersen, H.A., Stehly, L., and AlpArray Working Group, 2022. Mapping the seismic noise field in Europe: spatio-temporal variations in wavefield composition and noise source contributions, Geophys. J. Int., 228(1), 171-192.

[3] Poli, P., Campillo, M., Pedersen, H., and L. W. Grp, 2012. Body-wave imaging of Earth’s mantle discontinuities from ambient seismic noise, Science, 338(6110), 1063-1065.