Changes in anisotropy with depth revealed by splitting intensity tomography beneath the Alps and surrounding regions

Complex tectonics and strong heterogeneity due to thickened crust, subducting lithosphere, and the movement of the surrounding asthenosphere can be well described by seismic anisotropy, a good indicator for active and past tectonic events. Most of methodologies adopted so far to reconstruct anisotropy have a poor depth resolution. To overcome this problem we are using splitting intensity, which is related to the energy on the transverse component of the waveform and is linearly related to the mediums elastic perturbations through 3D finite-frequency sensitivity kernels. Here, we have paid special attention to three regions: the Western Alpine orogen; the Upper Rhine Graben and the still active oceanic subduction in Southern Tyrrhenian region. We used 822 stations in the Central Mediterranean to compute 12480 splitting intensity measurements, afterwards they were inverted for depth dependent anisotropy. The 3D anisotropy models show a complex pattern in the shallower parts (60-100 km depth), becoming more aligned parallel to the slabs in the deeper parts (100-350 km depth) and influenced only by strong mantle flows. In the Upper Rhine Graben we are finally able to appoint an anisotropy pattern of NNW-SSE oriented fast polarisation directions, which are parallel to faults in the graben structure, to the lithosphere and a lower layer with orientations pointing NE-SW, to asthenospheric flow. While between 100 and 250 km depth the strength of anisotropy is very small. In the Western Alps we see complex shallow anisotropy pattern and possible mantle flow around the Alpine slab. Beneath the southern Tyrrhenian subduction system looking at the anisotropy tomography images we are able to identify circular mantle flow directions around the edge of the slab (beneath the Sicily Channel) and possible break-offs in the continuity of the slab.