Lateral variations of Pn wave velocity in northwestern Italy

The P-n arrival times recorded from seismic networks operating throughout northwestern Italy and surrounding regions were inverted to map the structural variations of the uppermost mantle over the area and to estimate the crustal static delays at each station. By means of careful data selection a quality data set was obtained removing statistical outliers, poorly recorded events, and scantily sampled stations. Moreover, synthetic data were used to evaluate the resolution power of the available data set and the adopted iterative inversion technique. The agreement between synthetic and calculated models is more satisfactory where the path coverage of the rays is quite complete. A low-velocity zone was found beneath the western and northwestern side of the Alps; it may be related to an increase in depth of the Moho, as supported by other geophysical data. The high P, velocities found in the eastern side of the western Alps indicate the presence of high velocity and density, lower crust rocks of the: so-called ''Ivrea Body.'' The high P-n velocity underlying the Ligurian Sea could be related to a high-velocity structure existing in the upper mantle. The shape of the velocity anomalies matches not only the major tectonic features, but also the Bouguer gravity anomalies of the area.