The federation
Intranet
ISTerre seminar
Study of the mantle transition zone by combining earthquake data and seismic noise data: application to the Alps and Massif Central
Friday 9 October 2026 - 10h00
Piel Pawlowski - ISTerre---
The mantle transition zone plays a crucial role in mantle dynamics, particularly regarding mantle convection. Long studied via seismology, this zone is primarily characterized by sharp wave velocity jumps resulting from phase changes in olivine, a mineral abundant in the upper mantle. However, the limited sensitivity and resolution of so-called "global" methods make in-depth study challenging. Investigating these discontinuities at a local scale is difficult due to both their depth and their complexity, specifically their topography, which varies significantly with mantle composition and temperature. Receiver functions, a particularly robust method, have yielded significant insights into this zone and its spatial heterogeneity. In this manuscript, we propose combining this method with a newer observable: body-wave reflections detected in ambient noise correlations. To better constrain the mantle transition zone and understand these two observables, we model them using realistic mantle composition profiles, testing our methods against complex velocity gradients. These synthetic data reveal significant differences in how the two methods respond to compositional changes, particularly regarding broad velocity gradients, where reflected wave amplitudes drop drastically. Observing reflections from the mantle transition zone is now possible thanks to a better understanding of the microseismic wavefield, which requires selecting favorable sources located at great distances from the observation points. We therefore investigated the biases between the theoretically reconstructed Green’s function and synthetic cross-correlations, which depend on these specific source distributions. We demonstrated that such biases are small in Europe, but that noise conditions for this imagery technique can’t be applied anywhere due to source localization. Finally, we jointly compared and interpreted observables from both methods at a European scale, revealing interesting similarities alongside notable differences in key tectonic regions. The observed amplitudes align well with our synthetic modeling and reveal a distinct trend beneath the southwestern Alps, suggesting the presence of water. Signals resulting from conversions and reflections at the 410 km and 660 km discontinuities become more complex in certain areas, appearing sometimes in both methods and sometimes in only one. Meanwhile, measurement variations beneath the Massif Central remain slight but indicate a deepening of both discontinuities, linked either to actual topography or to velocity variations in the upper mantle. This work thus offers new possibilities for interpreting the composition and topography of the transition zone in Europe.
Organizing team : Ondes et structures
Amphithéâtre Killian, Maison des Géosciences, 38400 Saint Martin d'Hères