Mantle flow pattern from seismic anisotropy above the core-mantle boundary underneath Siberia and North America

Within the DFG Priority Program 2404 'Reconstructing the Deep Dynamics of Planet Earth over Geologic Time' (DeepDyn, https://www.geo.lmu.de/deepdyn/en/) we investigate possible seismic signatures related to deep Earth processes. Specifically, we investigate seismic anisotropy, by measuring shear wave splitting (SWS) of SKS, SKKS, and PKS phases. Thereby, we determine the splitting parameters, the fast polarization direction Φ and the delay time δt, using both the energy-minimization and the rotation-correlation methods. Especially, we search for phase pair discrepancies based on the observation type (null vs. split) between SKS and SKKS phases. Such discrepancies are indications for a lowermost mantle contribution to the splitting signal because these phases propagate along different paths after leaving the core. Besides using own measurements, we complement our database with measurements from Wolf et al., GJI, 2025. In two regions, beneath Siberia and North America, we find laterally varying values for Φ, in the D’’ layer just above the core-mantle boundary. The preferred directions of Φ are thought to be due to the alignment of minerals resulting from shear in a material flow. In the centers of the study regions, where high seismic velocity is present in global seismic tomography models, mainly null measurements are retrieved whereas systematic variations of Φ seem to dominate at the edges of the high seismic velocity anomalies which are often interpreted as remnants of slabs. A preliminary interpretation for our observations may be that the sinking slab material pushes local mantle material aside, inducing a flow pattern which causes an alignment of minerals and thereby seismic anisotropy.