LATERAL VARIATIONS OF P-N WAVE VELOCITY IN NORTHWESTERN ITALY

The P-n arrival times recorded from seismic networks operating through out northwestern Italy and surrounding regions were inverted to map th e structural variations of the uppermost mantle over the area and to e stimate the crustal static delays at each station. By means of careful data selection a quality data set was obtained removing statistical o utliers, poorly recorded events, and scantily sampled stations. Moreov er, synthetic data were used to evaluate the resolution power of the a vailable data set and the adopted iterative inversion technique. The a greement 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 s ide of the western Alps indicate the presence of high velocity and den sity, lower crust rocks of the: so-called ''Ivrea Body.'' The high P-n velocity underlying the Ligurian Sea could be related to a high-veloc ity structure existing in the upper mantle. The shape of the velocity anomalies matches not only the major tectonic features, but also the B ouguer gravity anomalies of the area.