SKS splitting measurements in the Apenninic‐Tyrrhenian domain (Italy) and their relation with lithospheric subduction and mantle convection

Abstract

We present a compilation of singular shear wave splitting measurements in the Italian region. We used the method of Silver and Chan [1991] to determine the splitting parameters of fast direction and delay time from broadband SKS waveforms. The stations analyzed were mainly deployed in temporary experiments; a few are permanent MedNet broadband stations. The splitting parameters reveal the presence of strong seismic anisotropy in the mantle beneath Italy. The fast polarization directions define two anisotropic domains: a Tyrrhenian domain with E‐W prevalent fast directions and an Apennines‐Adriatic domain with NW‐SE to NNW‐SSE prevalent fast directions. The stations along the boundary between these two main domains show fast polarization directions that vary for earthquakes coming from different back azimuths. This variability is a sign of a complex upper mantle structure (also shown by seismic tomography) but allows us to constrain the depth of the anisotropic layer in the 100–250 km range, at least in the boundary zone between the two domains. Data at station VSL (Sardinia) suggest the presence of depth‐dependent anisotropic structure, where the shallower signature is consistent with the Tyrrhenian domain results and the deeper one is possibly related to the presence of a subhorizontal slab laying on the 670‐km discontinuity. We argue that the complex pattern of fast directions is strictly related to the upper mantle structure and is caused by asthenospheric flows induced by slab rollback and contemporary deformation of the overriding Tyrrhenian plate.