End of subduction in northern Apennines confirmed by observations of quasi-Love waves from the great 2004 Sumatra-Andaman earthquake

Surface waves from the great Sumatra-Andaman earthquakes of 2004 and 2005 that cross Italy south of ∼44°N display Love-to-Rayleigh scattered waves (quasi-Love phases) diagnostic of sharp lateral gradients in the anisotropic properties of Earth's upper mantle. Surface waves that traverse Italy further north lack this distinctive phase, documenting a change in the upper mantle fabric that is corroborated by a shift in the fast polarization of shear wave birefringence. These observations suggest that orogen-parallel asthenospheric extension behind the retreating Apennines slab has limited geographical expression. We hypothesize that subduction rollback currently terminates at 44°N, while the upper mantle flow pattern further to the north has been recently rearranged.