Teleseismic tomography of the southern Tyrrhenian subduction zone: New results from seafloor and land recordings

Abstract

Teleseismic traveltime data, recorded by temporary ocean bottom seismographs deployed in Tyrrhenian Sea around the Aeolian Islands (Tyrrhenian Deep‐sea Experiment (TYDE)), have been used for the first time in Italy to refine the 3‐D model for the deep P wave velocity structure of the southern Tyrrhenian subduction zone. The arrival times of 35 teleseisms have been combined with those recorded by the Italian National Network. In order to obtain a more complete azimuthal coverage of teleseismic rays, 80 events recorded by land stations from 1990 to 2002 have been included in the data set. In total, 2904 P and 314 PKPdf phases, 1300 recorded by ocean bottom instruments, have been collected. The upper mantle structure is reconstructed down to 500 km by a nonlinear inversion of the relative residuals computed with respect to the reference 1‐D velocity model ak135. The obtained tomographic model has a higher resolution than those previously published thanks to the recordings of TYDE seafloor stations. Tomographic results confirm the presence of the Tyrrhenian slab imaged as a high‐velocity body extending from the uppermost mantle down to the bottom velocity model with dip 70–75° NW. The model better defines the geometry of the seismogenic part of the slab. Its lateral extension is about 200 km in the depth interval 150–300 km, where most of the deep seismicity is concentrated. At uppermost mantle depths the fast structure has smaller lateral dimensions (about 100 km). The inversion also points out a wide well‐resolved low‐velocity zone completely surrounding the steeply dipping fast structure from the lower crust down to about 300 km. This feature suggests the presence of a three‐dimensional circulation of asthenospheric flow around the Ionian slab caused by retreat and roll‐back of the slab. Our results are in agreement with recent laboratory experiments, mantle anisotropy studies, geochemical and isotopic analyses, and modeling based on residual topography.