Travel time residuals and velocity models beneath Italy

Jeffreys-Bullen and relative P-wave travel-time residuals have been analysed at more than 70 Italian seismic stations. 2362 teleseisms with 16 756 P-wave observations have been selected on the basis of their impulsive onsets rather than on their uniform distance and azimuthal coverage, in the time interval 1962-1982. Station corrections have been computed considering the static effect and two cosine terms with appropriate phase shifts, to look at the possible signature of the upper-mantle anisotropy. Particular attention has been paid to reducing the influence of the crustal velocity inhomogeneities, which has been taken into account by using all available information from seismic refraction data. The results provide a further confirmation of the complex velocity pattern existing beneath Italy. The Alps are characterized by early arrivals of teleseismic P waves, which in the western region are closely related to the presence of the Ivrea body. A 3-D inversion of these data has been performed using the Aki-Christofferson-Husebye block model, allowing us to quantify the degree of lateral velocity variation, which is higher than 5% between many adjacent lithospheric blocks. The low-velocity anomalies extend in the northern Apennines toward the Adriatic Sea. In southern Italy high velocities are partially explained by the presence of a Benioff zone dipping beneath the Tyrrhenian Sea, rather than by the geometry of the lithosphere-asthenosphere boundary.