Mapping the mantle transition zone discontinuities across South-Central Europe using body waves from seismic noise

Imaging the deep Earth structures is conventionally carried out using earthquake recordings. However, the resolving capability of such techniques (e.g., SS precursors and receiver function analysis) is often limited by the uneven spatial distribution of earthquake events and the high complexity of earthquake rupture processes. Recent advances in passive noise interferometry demonstrated the possibilities of recovering body waves from noise correlations, opening up new prospects for imaging the deep earth.

In this study, we map the mantle transition zone (MTZ) discontinuities beneath South- Central Europe using P-wave reflection phases recovered from noise correlations. We analyze up to four years of seismic noise recordings from 900 broadband stations in the study area. By stacking noise correlations in selected summer months, we significantly improved the retrieval of typically low-amplitude body-wave reflection phases in the light of a quiet surface-wave environment and sufficient noise body-wave illumination from deep paths. We obtain reliable P-wave reflections associated with the 410-km and 660-km discontinuities in the period band of 4-10 s. These short-period reflections reveal clear lateral depth variations of the two discontinuities, indicating a complex MTZ arrangement in the greater Alpine region that is related to both present and past tectonic regimes. Our imaging method shows its potential for general applications in studying deep-earth discontinuities.